The members of each phylum have certain common characteristics. The largest phylum, Arthropoda, includes animals having a segmented body, bilateral symmetry, paired, jointed appendages, usually terminating in claws, a chitinized exoskeleton secreted by the epidermis and molted periodically, striated muscles, a tubular alimentary canal, a ventral nervous system, and a dorsal heart.
Within a phylum there are smaller groups of animals called classes, which also have common characteristics that distinguish them from other groups. This is evident by merely recalling the common names of representative animals in the different arthropod classes, such as crayfish, crabs, lobsters, and sowbugs (Crustacea); centipedes (Chilopoda); millipedes (Diplopoda); spiders, mites, ticks, scorpions, and harvestmen (Arachnida); insects (Hexapoda); and several others; each group brings to mind a distinct image.
Some of the arthropod classes contain economically important pests, and will therefore be briefly characterized in the present section merely to present in one place an over-all view of the phylum. They will be discussed further, in detail depending on their importance, in appropriate chapters and sections of this book. Within a class there are groups of animals called orders, with common characteristics that distinguish them from other groups. Again, the common names of representative groups of animals give a good idea of what is meant by the term. For example, in the class Arachnida are the spiders (Order Araneida) and related orders: scorpions, whip scorpions, pseudoscorpions, sun spiders, harvestmen, and the ticks and mites (figure 35). In the class Hexapoda are such important orders as silverfish, cockroaches, crickets and their allies, termites, thrips, lice, bugs, butterflies and moths, beetles, flies, fleas, and the bees, wasps, and ants. Each order has clearly evident similarities among its families, and each is different in important ways from the other orders.
In description, one may employ adjectives formed from such words as thorax, abdomen, and pleuron--thoracic ganglia, abdominal appendages, and pleural regions.
Centipedes in the United States cause a wound that may be painful, but is seldom serious (see under "Centipedes" in chapter 9). The larger species of Scolopendra in tropical countries may reach 30 cm in length, and the amount of venom issuing from their claws is very painful and even dangerous to man.
Arachnids (figure 207, chapter 9) have 6 pairs of cephalothoracic appendages. The first pair, the chelicerae, are either 2- or 3-segmented, and may bear pincers for holding and crushing prey. However, they may also serve as venom-bearing fangs, intromittent organs for transferring sperm, or spinnerets. The second pair of appendages are 6-segmented, and are called pedipalps. These may also serve as intromittent organs, as pincers, for crushing food, as sensory organs, or for other functions.
Arachnids have 4 pairs of walking legs, usually 7-segmented. Each leg is composed of a coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus (figure 207, chapter 9). Most arachnids are carnivorous, but because they have no mandibles, they commonly predigest their food in the mouth and then suck the juices into the stomach. Some species crush their food with their chelicerae. The black widow spider cannot crush its prey, but pierces it and injects a digestive fluid, then sucks out the digested food. Spiders breathe by means of "book lungs" (from 1 to 4 pairs) or tracheae, or sometimes both. Arachnids are oviparous, except for the scorpions and certain mites. There are 11 orders of living arachnids, of which 7 will be considered of more or less importance as household pests, although some are harmless and are of interest only because they may be confused with bona fide pests.
In the spiders, the female pedipalps (figure 207 [palpus], chapter 9) are short and leglike, but in the males they have become modified to form copulatory organs. The last segment is enlarged and knoblike, and is used for transfer of sperm. The spiders are the only arachnids in which the chelicerae are provided with venom glands that open at the tip of the fang. In some spiders, these glands are located entirely within the chelicerae, and in others they extend backward into the first section of the cephalothorax, as diagrammatically illustrated in figure 208, chapter 9.
The organs of respiration consist of leaf-like external book lungs, or book lungs and tracheae. Most spiders have 1 pair of book lungs, but tarantulas have 2 pairs. They open on the ventral side of the abdomen as slit-like clefts. If a tracheal system is present, the spiracle is usually located just in front of the spinnerets.
Most spiders spin silken webs. The webs are spun from as many as 6 spinnerets, which are transformed vestigial legs and are located ventrally near the tip of the abdomen (figure 207, chapter 9). Webs may serve as nests or as traps for prey. A spider's silk must serve the dual purpose of transporting her to any part of the web, and also of trapping insect prey. She may have to secrete 2 kinds of web, dry and sticky, supplied by different glands in her abdomen. (Other glands supply the strong webbing for her egg sac.) For example, in the orbweaver's web, the central "lookout," where she spends most of her time, and also the "spokes" that radiate to the outer rim of the web, consist of dry webbing over which she can run rapidly, upside-down, aided by hooks, toothed claws, and bristles at the ends of her legs. She avoids the sticky threads that join the spokes and are for the purpose of catching the insect prey. She can perceive the slightest disturbance of any part of the web, her highly developed tactile sense compensating for her poor eyesight. After reaching her prey, the spider quickly envelops it in fresh threads, and bites it with her sharp upper jaws, injecting a venom that quickly kills it (von Frisch, 1960).
After the prey dies, the threads of the web entangling it are severed, and the prey is transported to the central "lookout" and hung up by a short thread. To consume her victim, the spider bites it, meanwhile secreting a digestive fluid, then sucks up the dissolved contents together with the digestive fluid. Dissolved muscles and intestines of the prey are imbibed within a few hours, and the indigestible remnants are dropped out of the web (von Frisch, 1960).
Webbing may also be used for transportation when it is borne aloft by air currents. Gertsch (1949) made the interesting observation that although spiders were wingless, a young spider (spiderling) could float its threads on the breeze, often reaching tremendous heights and sailing for long distances, thus colonizing new areas at a rate not possible even for winged insects. He stated that the rigging of ships 320 km from the nearest land had been "showered with tiny aeronauts riding on silken streamers."
Some spiders do not spin true webs, but live in burrows (trapdoor spiders) or crevices (wolf spiders).
The courtship and mating of spiders is so strange and complicated as to be almost beyond belief. They are described in great detail in W. J. Gertsch's fascinating book, American Spiders (Gertsch, 1949). Soon after he becomes mature, the male spider transmits some seminal fluid to a delicate web which he has spun. He then collects the sperm and stores it in bulbs of his pedipalps (palpi) until the opportunity for mating arises. Herms et al. (1935) observed the sperm-induction procedure in the mating of black widow spiders and the courtship and copulatory behavior of that species, lasting over 2 hours. This was described in detail by McGrone and Levi (1964). Most bizarre is the sperm-induction sequence practiced by tarantulas. It is so complicated and replete with ritual that it sometimes requires 3 or 4 hours (Gertsch, 1949).
Spiders are predators, mostly on insects ensnared in their webs. A few species are very venomous. In the United States, the bite of the black widow spider and the brown recluse spider or related species may be very painful, and can lead to serious general systemic disturbances, or even death. The aid of a physician should be obtained as soon as possible.
Ticks are the largest members of the Acarina They are all bloodsuckers and, when fully engorged, some species attain a length of 1 cm and a few are considerably larger. They are serious pests of domestic animals, and are the only vectors of some severe diseases of man and animals.
Scorpions breathe by means of 4 pairs of book lungs in the ventral blood sinus. Paired coxal glands, opening in the coxae of the third pair of legs, provide for excretion. They do not lay eggs, but give birth to active young. These climb upon the back of the mother and remain fastened by their pincers until after the first molt, nourished by yolk material stored in their bodies. All scorpions are venomous, but most species are not dangerous. In Arizona, however, there is a species that has caused more deaths over a 36-year period than all other venomous animals of that state, including rattlesnakes (see chapter 9, Opening paragraphs).
There are 2 suborders. In the Uropygi, there are 2 glands that open one on each side of the anus and discharge a stream of fluid when the animal is irritated. This fluid has the odor of vinegar, which suggested to the Spanish-speaking settlers of the Southwest the name vinagrón (from the Spanish vinagre), later modified to "vinegarroon" in English. The last abdominal segment bears a flagellum, which is very long with some species. The body length may range from only 2 mm to as much as 6.5 cm, as in Mastigoproctus giganteus (Lucas) (figure 37), which occurs in the southern United States from coast to coast. (The whip in this species can be twice as long as the body proper.) Mastigoproctus giganteus gets water from its prey and by drinking from a moist substrate which is located by the sensitive, antenniform forelegs. The whip scorpion probably does not have a protective epicuticular wax layer, such as that possessed by insects and many other arthropods, to protect it from desiccation. At 26° C (79° F), it readily loses water to the air at relative humidities up to 95% (Crawford and Cloudsley-Thompson, 1971).
In the suborder Amblypygi, the abdomen bears neither odoriferous glands nor a whip. In the United States the principal genus is Tarantula, although it is not a tarantula, which is a large spider. Tarantula is about 2 cm long, and has been reported from California, Texas, and Florida (Storer and Usinger, 1957).
The harvestmen easily lose their legs or parts of their legs, and cannot regenerate them. They drink frequently, and must have water readily available. They are harmless, and should not be confused with true spiders.
Respiration in hexapods usually takes place by means of a system of tubes called tracheae, that divide and subdivide internally until they terminate in the delicate network of tracheoles that reaches every organ, tissue, and cell of the body. Hexapods have a simple, open circulatory system.
The modern tendency is to divide the class Hexapoda into 3 subclasses. Subclass Myrientomata contains 1 order (Protura), subclass Oligoentomata contains 1 order (Collembola), and the subclass Insecta (Euentomata) contains the remaining 23 orders. This modern classification is based on the evidence that the 3 subclasses represent distinct evolutionary lines (Belkin, 1972). However, since the present work has primarily economic rather than taxonomic objectives, for the sake of convenience all orders of hexapods will henceforth be referred to as "insects" Like-wise, for the sake of brevity and convenience, the older concept of all Hexapoda being divided into 2 subclasses, Apterygota (primitively wingless insects without metamorphosis) and Pterygota (winged insects, with some secondarily wingless)will be retained.
The procuticle comprises the bulk of the cuticle, and is immediately above the epidermis. It is essentially a hydrophilic chitin-protein complex containing a considerable quantity of water. The outer layer of the cuticle is called the epicuticle This consists of a proteinaceous cuticulin about 1 µ in thickness, containing no chitin, and covered with a layer of lipid, usually a hard wax, averaging about 0.25 1u in thickness. This wax, and particularly 1 or more packed and oriented monolayers of it at the cuticulin-wax interface, is the principal barrier to water loss. After some insects molt, dermal glands secrete a fluid that spreads over the surface of the wax layer to forma "cement" that appears to be similar to shellac (Beament, 1955). Until somewhat recently, it was not known how the wax was brought to the surface of the cuticulin. Arising from the epidermal cells and extending outward through the procuticle of most insects, and sometimes into (but not completely penetrating) the overlying cuticulin are minute ducts called pore canals. By means of the electron microscope, it was shown that these canals terminated in wax canals that branched out and reached the outer surface of the cuticulin, depositing a liquid wax similar in appearance to the liquid crystalline phases of lipid water systems (Locke 1965, 1974; Gluud, 1968). By means of sorptive dusts, a sufficient amount of wax can be removed from the solidified epicuticular wax layer to cause a lethal rate of water loss, providing means of controlling some insects by nonchemical methods. (See chapter 2, under "Dust Desiccants.")
Setae (bristles or hairs) are sensilla that arise from pits in the cuticle surface in a sort of ball-and-socket arrangement (figure 40, d). Setae are formed by 2 cells: a sensillum-forming cell (trichogen) and a socket-forming cell (tormogen) (figure 40, a, i, k) and, of course, whatever cells that give rise to the sensory nerve(s). In the pit, the cuticle (articular membrane) is relatively thin, and so is the cuticle of the seta, etc., particularly in the case of some of the chemoreceptors. Cuticles of olfactory sense hairs are perforated by many minute pores (Slifer et al., 1959; Steinbrecht and Müller, 1971). Cytoplasmic filaments, arising in the trichogen and tormogen cells, pass up through the cuticle to enter the seta, forming a cytoplasm continuum in close contact with chemicals that may be deposited on the surface of the cuticle, and thereby forming one of the pathways for entry of insecticides through the integument.
The insect cuticle does not readily decompose, but retains its original form after the insect dies, which makes possible the preservation of insects for hundreds of years with their original lifelike appearance. Thus, the mounting and preservation of insects are comparatively easy.
An insect's exoskeleton cannot be readily dissolved away by strong acids, alkalis, or solvents, but fortunately for man's attempts to control insects by means of contact poisons, it can be penetrated by solvents, surface-active materials, vapors, and gases. In fact, it can be penetrated by organic compounds of all kinds, even when they are solids and are not dissolved in a liquid solvent. Fortunately, the rate of penetration of organic insecticides into insect cuticle is much greater than into mammalian skin. Even inorganic insecticides can penetrate the cuticles of some insects, such as cockroaches, at a lethal rate. The spiracles which are openings for the tracheae, form a channel of entry for liquids of low surface tension, and for vapors and gases.
Figures 41 and 42 show that insects are divided by constrictions into ring-like plates called segments Thin, flexible portions of the cuticula called conjunctivae connect the segments. These conjunctivae may be folded into the body of the insect, allowing for expansion between segments much in the manner of an expanding accordion. Other constrictions or infoldings of the cuticle called sutures run in various directions on the surface of the exoskeleton. The internal infoldings often provide points for muscle attachments. The areas bounded by sutures are called sclerites. The shape and position of the sclerites are often used in the description of species in some insect groups.
In addition, the hairs, spines, setae, ridges, and other exoskeleton structures are used in taxonomy. External structures may be merely projections of the cuticle, or they may be associated with the underlying body structures, such as the sense organs.
The dorsal or upper face of each segment is called the tergum the ventral or lower face is called the sternum and each lateral face is called a pleuron. Each of these faces is made up of 1 or more sclerites. The dorsal part of a segment may be called a tergiteand the ventral part a sternite - terms usually restricted to the abdomen (figure 42).
If reference is made to the thorax, the term notum is generally employed instead of tergum. Thus, one refers to the dorsal part of the first segment of the thorax as the pronotum but the corresponding part on the ventral area as the prosternum.
Besides the 2 compound eyes, an adult insect usually has 3 simple eyes or ocelli, but may have 2,1, or none, depending upon the species. Simple eyes have only a single facet. The larvae of some orders of insects, such as the flies, beetles, butterflies, and ants, never have compound eyes. They may have from 1 to 6 or more simple eyes on each side of the head. Sometimes, several simple eyes form a cluster. Nymphs of bugs, grasshoppers, and dragonflies may have compound eyes.
Insects cannot change the focus of their eyes (accommodation), and they have no protection equivalent to the eyelids and eyelashes of vertebrates. Protection is afforded by the cuticle that covers the entire insect body, and which continues in an unbroken sheet over the eyes. Over the eye surface, however, the cuticle is thin and transparent, admits and refracts light, and is curved to form a lens.
Antennae may be used as tactile organs, or may possess organs for smelling or hearing. In general, they are used as sensory aids for locating food and finding mates, and it appears that the ants use their antennae to communicate with others of their species.
Chewing Mouthparts. Examples of insects with chewing mouthparts are silverfish, cockroaches, grasshoppers, crickets, termites, earwigs, psocids, biting lice (Mallophaga), dragonflies, scorpionflies, beetles, beetles, ants, sawflies, woodwasps and larvae of moths, butterflies, and primitive flies.
The mouthparts of the cockroach are of a fairly generalized type, and may be used as an example of the chewing mouthparts (figures 44 and 45). The upper lip (labrum) and lower lip (labium) oppose each other in a vertical plane. The labrum is a sclerite of the head, but the labium is a true mouthpart appendage. The actual biting and chewing appendages are the mandibles and maxillae (figure 45). They operate in a horizontal plane, i.e., from side to side. The mandibles are the first pair of "jaws" that can be seen when the labrum and labium are removed. They are strong, heavily sclerotized, often darkened, and possess teeth and grinding surfaces. They are operated by 2 sets of muscles, one set that closes them against each other and another set that pulls them apart.
The second pair of "jaws" are the maxillae. As shown in the figure, a maxilla is made up of a number of parts. The part modified for tearing food apart is called the lacinia. The maxillae bear segmented sensory palps provided with tactile hairs and also organs of smell and taste. In cockroaches, chemoreceptors involved in feeding (taste) are confined to the mouth. The Hemiptera, Lepidoptera, and Diptera also have chemoreceptors on the tarsi, while the Hymenoptera have similar receptors on the antennae as well as on the mouthparts (Frings and Frings, 1949).
The lower lip or labium performs a function similar to the lower lip of vertebrates. In addition, it bears a pair of sensory palps whose function is similar to those of the maxillary palps. As might be surmised from the paired palps, the labium was formed by the fusion of primitively separate appendages, the "second maxillae."
The epipharynx is a sensory organ attached to the inner surface of the labrum that is believed to contain end-organs of taste and is somewhat analogous to the palate of higher animals. It is of interest principally because it becomes modified into highly specialized structures in some of the sucking insects.
The hypopharynx is a tonguelike organ that forms the floor of the mouth cavity. The salivary duct opens in a pocket between the hypopharynx and the labium. The mouth opens between the hypopharynx and labrum.
Various Types of Sucking Mouthparts. Chewing mouthparts are the most primitive type, from which the various sucking types have evolved to form various interesting structural modifications. Sucking mouthparts are thus thought to be homologous with chewing mouthparts, even though they usually differ greatly both in appearance and function. The following types of sucking mouthparts among adult insects are distinguished by Metcalf et al. (1962).
There are usually 2 claws on the tarsus, and there are frequently pads (pulvilli) beneath the claws that offer greater purchase against smooth objects. The ability of the pulvilli to cling to smooth surfaces is usually enhanced by many small hairs which sometimes, as in the case of the house fly, exude a sticky substance. Between the claws, there may be another type of pad or lobe called the arolium which has considerable suction force, and enables the insect to cling to objects with little or no aid from the claws. When the arolium is "hairlike," as in flies, it is called an empodium. Among the thrips (Thysanoptera), the claws are very small or may be entirely absent, and the tarsus terminates in a large bladder like arolium. Among the scale insects, sucking lice, and some biting lice, there is only a single claw on the tarsus. The claws and the associated structures at the tip of the tarsus are presently considered to be a separate segment, the pretarsus.
Depending on the mode of life to which an insect has become adapted, great variation can be seen in the morphology of the legs of different insect species. A rapidly running insect like the tiger beetle (Cicindelidae) will have long, slender legs, while an insect that leaps, such as a grasshopper, flea, or flea beetle (Haltica) will have enlarged femora or coxae, at least on the hindlegs. The legs of insects that dig into soil or wood are usually short and stout, and may be armed with spurs. The forelegs may be specialized for the grasping of prey, as in the case of the praying mantis and ambush bugs. Aquatic insects often have legs adapted for swimming. Arboreal insects, such as the cerambycid beetles, may have greatly expanded and hairy tarsi and strongly developed claws and pulvilli to afford the maximum grip.
Most insects can flex their wings against the top of the body while they are resting, and also fold their wings if they are broad. It is usually the broader hindwings that are folded, and if they are too long, the tips may be folded under so as to be protected by the forewings. Dragonflies and damselflies (order Odonata) and mayflies (order Ephemerida) lack the mechanism for flexing their wings, which are therefore held outstretched laterally or held vertically over the body. This is considered to be a primitive condition, but some of the higher insects, such as butterflies, have secondarily lost the ability to flex their wings.
Typically, there are 2 pairs of wings: one pair on the mesothorax and another pair on the metathorax. The posterior pair, in the case of the true flies (Diptera), is replaced by vestigial appendages called halteres, which are believed by some entomologists to function as balancing organs. In the male scale insects, the hind pair of wings is replaced by 2 spines, 1 on each side, which hook into the weakly developed pair of wings of the mesothorax, giving the forewings added rigidity. Among the Coleoptera, the mesothoracic wings (elytra singular elytron) are horny and shieldlike. When in flight, most beetles hold their elytra to each side like the wings of a glider, depending on the membranous second pair of wings for propulsion. The second pair of wings may be longer than the first pair, but are folded away under the elytra when the beetles are not flying.
The exoskeleton and musculature of the thorax are highly developed to suit the needs of the great muscular activity involved in flight. This is most conspicuous among Odonata, Diptera, and aculeate Hymenoptera, and these insects are among the most active of the insect fliers. The wings of flies and bees move so rapidly that they become invisible.
The wingbeat frequency of one species of midge (Forcypomyia) has been recorded at 133,-080 beats per minute, or 0.00045 second per wingbeat.
An examination of the wing pads of a nymph of the lower winged insects or of the chrysalis of Lepidoptera will reveal that an insect's wing is simply a double-layered hollow sac folded out from the body wall, in the lumen of which are nerves, tracheae, and body fluid. The double-layered nature of the wings may also be easily demonstrated by placing small, adult beetles in a damp location until the wings have absorbed enough water to become bloated, thus revealing their saclike construction. The color of the wings of Lepidoptera depends on the color and arrangement of myriads of shinglelike "scales" attached to their wings (figure 50, right). The shingled arrangement of the scales strengthens an otherwise membranous structure that would be inadequate for rapid flight. Each scale is often strengthened by minute corrugations (figure 50, right) that diffract light rays and result in the brilliant color and iridescence usually noted in butterflies. Each scale has a pedicel at its base which serves as an anchor point. The pedicel is enlarged in the middle, and forms the "ball" that fits, in a fixed position, into the "socket" cavity in the cuticle of the wing membrane (figure 50, right).
Adult insects usually do not have any abdominal appendages, except for those called genitalia, used in reproduction (figure 41, ovipositor). The genitalia are discussed in this chapter, under the heading, "Reproductive System." Cerci and caudal filaments are borne on the eleventh segment of some insect species (figure 42, and figure 324, chapter 9). If 11 abdominal segments are present, the respiratory spiracles normally occur on the first to eighth segments. In many species of insects, a fusion of posterior abdominal segments takes place, and it may be impossible to recognize more than 5 or 6 segments.
The larvae of some insects have fleshy, unjointed projections of the abdomen, known as prolegs (figure 185, C, D, chapter 7), which are used as legs and are considerable aids to locomotion for some of the more elongate forms. The larvae of Lepidoptera never have more than 5 pairs of prolegs, but those of certain Hymenoptera, the sawflies (Tenthredinidae), may have 6 to 8 pairs.
Orthopterans, such as grasshoppers, locusts, katydids, and crickets, that produce stridulatory sounds, have well-developed, thin, membranous tympanic organs (figure 41, tympanum) for detecting these sounds. Receptive tympanic sensilla are located on the front tibiae or the first tergite of the abdomen. Insects that lack such complex tympanic organs, such as cockroaches, can detect air movements by means of sensilla that may cover the cerci. The cerci of the American cockroach have a partly auditory function (Pumphrey and Rawdon-Smith, 1936a, b).
The alimentary canal is divided into 3 regions: the fore-intestine, the mid-intestine, and the hind-intestine. The fore- and mid-intestines are connected by the cardiac valve, and the mid- and hind-intestines are connected by the pyloric value. The fore- and hind-intestines are divided into several more or less distinguishable parts, which are well illustrated by the dissected cockroach shown in figure 51. The parts of the cockroach's fore-intestine are the pharynx (which begins at the mouth), the esophagus, the crop and the proventriculus, or gizzard. The gizzard occurs in insects that eat hard substances, and it appears to have a grinding and straining function like the gizzard of birds. The crop serves as a food reservoir, and is absent in some insects, or may be represented as a side sac, or diverticulum. The pharynx is a well-muscled pump in insects having sucking mouthparts.
The mid-intestine or ventriculus is not differentiated into regions, but a variable number of evaginations of variable size and shape and in various locations arise from it. These are called enteric or gastric caeca; and digestive enzymes may be produced in them, or they may represent additional digestive surface.
In the hind-intestine of the cockroach, the ileum, the colon, and the rectum may be distinguished. Malpighian tubules discharge into the anterior end of the hind-intestine at the pyloric region. The tubules vary in number and length, but the point at which they are attached to the alimentary canal can always be used as a means of establishing the point of juncture between the mid-intestine and hind-intestine. The functions of the Malpighian tubules are primarily to remove the waste products of metabolism (water and uric acid) from the blood stream, empty them into the hind-intestine, and adjust the ionic concentration of the blood (homeostasis).
The length and complexity of the alimentary tract of an insect species are usually related to the kind of food the insect eats. Long and complex systems are found among the herbivorous insects, while those of the entomophagous species and the insects feeding on concentrated food are relatively short and simple.
The "heart" of an insect, stated in simplest terms, is a flexible dorsal tube (figure 51), closed at the posterior end and open at the anterior end, with small, valvelike openings or incurrent ostia (sing., ostium) located along the sides of successive chambers. Blood enters the heart through the ostia as the alary muscles dilate the heart chamber, but the ostia close when circular muscles of the chamber contract, and their valvelike arrangement prevents the blood from returning to the hemocoeles or posterior chambers. Because the contractions start at the posterior chamber of the heart and pass forward, the blood is pushed anteriorly, passing through the aorta, a constricted portion of the anterior part of the heart. The blood is pumped from the aorta into a series of cavities or sinuses and veins throughout the insect. The blood bathes the "brain," and from there it passes posteriorly through the hemocoeles, bathing the internal organs and passing into the appendages and wings. The circulation of the blood is aided by the contraction of a ventral diaphragm, and often by accessory hearts at the bases of legs and wings. One function of the blood is to carry digested food materials from the alimentary canal to all tissues of the body. In contact with the alimentary canal, the blood picks up the products of digestion by osmosis. While the blood is bathing the network of Malpighian tubules, waste products are removed. Insect blood has no red blood corpuscles and plays no part in oxygen transport, although the plasma of chironomid larvae does contain hemoglobin which may aid in respiration.
In the greater number of cases, both oxygen and carbon dioxide are exchanged directly with the external environment through the tracheal system.
In common with all arthropods, the nervous system in the body of insects is located ventrally (figure 51). Primitively paired ganglia in each segment are usually fused together, as are also the double cords connecting the ganglia. In the head, above the esophagus, lies the supraesophageal ganglion or brain, linked to the smaller subesophageal ganglion by paired connectives which pass around the esophagus.
Since so much of the nervous activity of insects is on a simple receptor-effector basis, the brain does not serve as an important coordinating center as it does among the vertebrates. If the brain of an insect is removed, it may continue to live for a considerable period, and can carry on many reflex activities. If a bee is captured and confined in a transparent container (e.g., a Mason jar) when returning to the hive with a load of nectar or pollen, it will die from stress in 2 or 3 hours (Pence et al., 1963). However, the excised abdomen will live for 2 or 3 days, and has been found to be an excellent subject to monitor physiologic response to injected drugs or insecticides, free of psychogenic infuences, by means of a recently developed biosonic analyzer (Pence, 1969; Pence and Lomax, 1971).
The spiracles (figure 52) are the external openings of the air tubes, known as tracheae, which divide and subdivide as they penetrate to all portions of the body. The tracheae eventually divide into microscopic tubes, usually less than 1 µ in diameter, called tracheoles. These reach every cell in the body. The cells obtain oxygen and give off the waste gases of metabolism by diffusion through the walls of the tracheoles.
By microscopic examination of insect tissue, the presence of the tracheae can always be determined by the fact that they are lined with cuticula in fine, spirally arranged thickenings called taenidia, which prevent the collapse of the delicate tracheal tubes.
Many insects, for example, parasitic insects spending their larval lives in the bodies of their hosts, respire through their cuticles, even though they may have a tracheal system. Some insects may live for long periods without oxygen. Wigglesworth (1939) reported an oestrid larva remaining alive 17 days immersed in oil. Specimens of California red scale, Aonidiella aurantii (Maskell), have lived as long as 40 days after being placed on glass slides, covered with refined spray oil, and then sealed with glass cover slips. The majority survived under these conditions for 10 days or more. If the cover slips were not placed on the scale insects after they were bathed in oil, they became desiccated and died much more rapidly; none of them survived for a period longer than 10 days (Ebeling, 1945).
The genitalia are the external organs of reproduction, with all their appendages. In the females they may be modified into a highly specialized exserted or retractile egg-laying organ or ovipositor. It is not present in all orders of insects, but when present it may be used to deposit eggs into the ground, into plant tissues, or into the bodies of animals. The most remarkable modifications of the ovipositor may be found in the Hymenoptera. In the hymenopterous parasites, it is greatly elongated and slender. The eggs are then compressed and stretched to an extreme degree in their passage through the very narrow channel of the bristlelike ovipositor (Fulton, 1933). In the stinging Hymenoptera (ants, wasps, and bees), the ovipositor is modified into a stinger. The venom is ejected at the tip of the stinger, and the eggs are ejected from the opening of the genital chamber: which lies at the base of the ovipositor (Snodgrass, 1935).
The female internal reproductive organs (figure 51, and figure 53, right) consist primarily of a pair of ovaries. Each ovary normally consists of several ovarioles within which the eggs are developed. From the ovaries issue a pair of oviducts, which are usually united and lead to the vagina. There is almost always a saclike pouch called the seminal receptacle or spermatheca (figure 53), which receives and stores the sperm. Since some insects mate only once, the spermatheca performs an important function. In some insects (e.g., Lepidoptera), there is also a bursa copulatrix which receives the sperm first. The colleterial glands secrete a viscid material used to "cement" eggs together, waterproof them, attach them to objects, or to form envelopes that may encapsulate a number of eggs.
. The number of eggs laid by an insect may vary from a few to many thousand. The queen honey bee may lay 2,000 to 3,000 eggs per day for weeks at a time, and the termite queen can lay millions of eggs. Not all insects lay eggs. Insects which lay eggs are said to be oviparous. Their eggs may be fertilized, as they pass through the vagina, by sperm stored in the spermatheca. If the eggs produced by the female bear viable offspring although the eggs are not fertilized, reproduction is said to be parthenogenetic. The developing embryo within the egg is nourished by means of the yolk, as in the case of birds. Unlike the birds, however, insects do not incubate the eggs and, in fact, rarely care for the eggs in any way after they are laid. They usually lay eggs in, on, or near food that will be consumed by the hatched young.
Many species of insects give birth to active, living young. Such insects are said to be viviparous. Among such species, the production of young may also be by parthenogenesis as, for example, among the aphids. The term ovoviviparous refers to the type of reproduction in which eggs with well-developed shells hatch within the body of the female and active young are then produced. The most recent tendency is to consider that the term "ovoviviparous" is superfluous, because all insects lay eggs, whether these hatch within the body or, as is more often the case, they are laid by the female and hatch outside.
The males of ants, bees, and social wasps are produced from unfertilized eggs and the females and workers from fertilized eggs. The males of some parasitic Hymenoptera and aleyrodids are also produced from unfertilized eggs.
In a diagram of the male and female reproductive organs, the structural analogy can be seen (figure 53). Comparable to the ovaries are the male testes in which sperm is produced, leading to the vasa deferentia which unite to form the ejaculatory duct. Accessory glands secrete fluid with which the sperms are mixed. The external genitalia consist functionally of the genital claspers and the aedeagus, which includes the penis.
The external genitalia of the male, are used extensively in the description of species because they often show more constant and more distinct differences than any other structural characters to be found on the insect.
Insects have a large number of muscles. The external skeleton allows for a large area of attachment for muscles compared to the bony internal skeleton of vertebrates. In man, there are said to be 696 muscles, but Lyonet found over 4,000 in the caterpillar of the goat moth, Cossus ligniperda.
The muscles of insects are soft, but quite strong, and they are cross-striated, alternate light and dark bands crossing the fibers. There are skeletal and visceral muscles. In the latter, the transverse striations are not so distinct as in the skeletal muscles.
Insect muscles are very strong in relation to the insect's body weight. Insects are able to lift from 15 to 25 times their body weight. Their muscles contract and relax with phenomenal rapidity, as illustrated by the great speed of wing vibrations. Insect wing muscle is the most active tissue known, and consumes the most fuel and oxygen per unit of protein. The insect tracheal system facilitates the diffusion of oxygen and carbon dioxide through muscle tissue several thousand times faster than diffusion in the liquid phase (Weis-Fogh, 1964).
In some of the lower forms of insects, there is no change in form; in others, there are varying degrees of change, up to such striking transformations as may be observed between a grub and an adult beetle, a caterpillar and a butterfly, or a maggot and an adult fly.
Development Without Metamorphosis
The insects of the 3 orders lowest in the scale of evolution, the Protura, Thysanura, and Collembola, do not go through any change in form after hatching, any more than do the vertebrates. Their development is without metamorphosis (ametabolous development) (figure 54). A young silverfish or springtail, for example, appears so much like the adult that one would immediately infer that the newly hatched young and the adult are the same species. The young insect increases in size, but its shape changes very little. The 3 orders in which no metamorphosis occurs comprise the subclass Apterygota, the insects of which have no wings and are descended from wingless ancestry.
Since no insect has visible wings immediately after birth, any insect that develops wings must undergo some degree of metamorphosis during its life history. Therefore, metamorphosis may be found among all the insects of the subclass Pterygota. These are insects that have wings in the adult stage or, if the wings are absent, they were lost during the course of evolution from winged ancestry. Cockroaches, grasshoppers, earwigs, termites, lice, thrips, bugs, aphids, and scales are representative of the insects that undergo gradual metamorphosis (paurometabolous development) (figure 55). Since the immature stages, commonly called nymphs (see the next paragraph), require about the same kind of habitat and food as the adults, internal anatomical changes during the life history of these insects are not on the same scale as for insects for which food and habitat change drastically. The most conspicuous change other than an increase in size during the development of insects with gradual metamorphosis is the difference in the appearance of the externally developed wing buds. Such insects are placed in the Exopterygota (Division A), of the subclass Pterygota later in this chapter.
European entomologists have not been so adamant as those in the United States in using only the term "nymph" for the immature stages of the paurometabolous insects. Some refer to them as "larvae," or may use the term "nymph" for some insects (e.g., cockroaches and earwigs) and the term "larva" for others (e.g., thrips and scale insects). Some use the term "larva" for early instars and "nymph" for later instars of the immature forms of the same paurometabolous insects, such as the termites. European entomologists did the pioneer work on termite caste development, and used the term "larva" to refer to termite nymphs before they acquire wing buds and the term "nymph" for individuals succeeding the apterous nymphs and possessing external wing buds. For the sake of standardization, American entomologists are using the same terminology for termites.
Three orders of aquatic insects, the Ephemerida (mayflies), Plecoptera (stoneflies), and Odonata (dragonflies), are placed by some authors in a group called the Hemimetabola, a group having incomplete metamorphosis (hemimetabolous development). These insects undergo radical changes in habitat during their life histories (figure 56).
The nymphs (naiads) and adults of insects with incomplete metamorphosis live in entirely different habitats. The aquatic nymphs have tracheal anal or rectal gills; their legs are modified for clinging, climbing, or burrowing; their bodies are modified for swimming; and their mouthparts are modified for taking food in the water. Special adaptations are required to enable the adults to escape from the last nymphal skin. The adults, being aerial, require different methods for capturing food (dragonflies and damselflies) or they may take little or no food, but inflate the alimentary canal with air to aid in flight (mayflies and most stoneflies) (Matheson, 1951).
Obviously, the morphological changes dependent upon a change from aquatic to terrestrial existence must of necessity be greater than those which accompany the changes occurring among insects with gradual metamorphosis, which undergo no change in habitat or food habits. The 3 orders involved are placed in a separate division of the Exopterygota later in this chapter, but this is not meant to indicate their phylogenetic relationship to other insects. An arrangement of orders according to modern concepts of phylogenetic relationships may be found in Fundamentals of Entomology, Part 1 (Belkin, 1972).
The higher orders of insects, including the 4 that have the greatest number of described species, the Coleoptera (beetles), Lepidoptera (butterflies, moths), Hymenoptera (ants, wasps, bees), and Diptera (flies, mosquitoes), undergo what is known as complete, complex, or indirect metamorphosis (figure 186, chapter 7), and are placed in a group called the Holometabola (having holometabolous development). The young of insects with complete metamorphosis are known as larvae, and are so different from the adult that no one unfamiliar with their life history would suspect them of being merely a different stage in the development of the same individual. For example, what is there about the habitat, food habits, and appearance of a codling moth larva, the "worm" of the wormy apple, that would lead an uninformed person to suspect that it is an immature stage of the codling moth? For such radical transformations, the insects with complete metamorphosis have a life stage called the pupa, that is not possessed by any of the groups already discussed. The pupal stage is a quiescent, transient one, in which there is usually little locomotory movement and no feeding. The larva generally pupates in a place where the pupa will be hidden or inconspicuous--the pupa, being motionless, is also defenseless. A protection for the pupa is often spun, or prepared in some other way, by the larva. On the other hand, some dipterous pupae in the Culicidae and Chironomidae are very active, as evidenced by the familiar mosquito "wigglers."
All activity during the pupal stage is concentrated on the complex chemical changes associated with simultaneously breaking down (histolysis) and building up (histogenesis) tissues before the insect can emerge from the pupal skin, transformed for its radically new form and life habits. For example, in the pupa of a moth, sexual organs develop, chewing mouthparts are transformed into coiled, siphoning mouthparts, wings develop, compound eyes and antennae are formed, and in general the insect whose simple, cylindrical larval form was specialized for feeding and storing energy is transformed into a highly active and sensitive organism, specialized for flying about, securing food, finding a mate, and locating a place for oviposition.
One of the advantages of complete metamorphosis is that the larvae are specialized in habits and form for feeding, growing, and storing energy, and the adults are specialized for mating, reproducing, and effecting a dispersal of the species.
The means of locomotion, sense organs, and reproductive organs that the larva did not require are now essential to the adult. In addition, the pupae may provide a means of passing through seasons of adverse weather conditions or periods of food shortage.
Insects with complete metamorphosis develop their wing buds and the buds for all other adult structures internally in the larval instars. For this reason, they are placed in the division Endopterygota of the subclass Pterygota in the synopsis of orders at the end of this chapter. The buds often become external in the pupal stage, but are hidden under the pupal sheath in the case of obtect (smooth) pupae, when they are not seen until the adult stage.
Neither the pupa nor the adult ever grows. The layman often supposes that a small house fly is a young fly that has not yet completed its growth. However, differences in the sizes of adults merely indicate variations in the amount of growth attained by the larvae. This, in turn, was largely dependent upon the food supply. The change of form from larva to adult never fails to excite the wonder and imagination of all people; the development of mammals, including man, appears so simple and uneventful by comparison. In fairness to the mammals, however, it should be pointed out that their greatest changes in form occur during embryonic development--changes which, in fact, recapitulate the greater part of their long and eventful phylogenetic development. The morphologic changes of the higher insects are more spectacular because they occur during the postembryonic development of the animal.
The Terms "lnstar" and "Stage"
Now that some understanding has been gained of the meaning of "metamorphosis," the distinction between the terms instar and stage can be more readily understood. The cuticle of an insect allows for only limited growth; for further growth, the insect must split and shed its cuticle, and this process is called molting or ecdysis. Before the old cuticle is shed, a new cuticle is already forming, and will allow for another limited increase in body size. This process usually continues until the insect becomes an adult, although in a few insects such as the proturans (Protura), springtails (Collembola), and silverfish (Thysanura), molting continues even after the adult stage is reached.
An instar is the form in which an insect appears after each molt. The period between any 2 molts is called a stadium (plural, stadia). The life stages of an insect are the periods of its life which are radically different from each other in appearance and usually also in behavior and activity, particularly so in the case of holometabolous insects. For example, a cockroach has 3 stages: egg, nymph, and adult; a moth has 4 stages: egg, larva, pupa, and adult.
Differences Between Nymphs and Larvae
Since the naiads of aquatic insects with incomplete metamorphosis are of comparatively little interest to the majority of entomologists, the accompanying tabulation gives only the factors which differentiate nymphs and larvae (p. 112).
Some Adult Morphological Specializations
In many of the holometabolous insects, the adults do little feeding; the energy supply for their relatively brief adult life is carried over from the larval stage in the fat bodies. Some females (e.g., mosquitoes and fleas) must feed in order to develop eggs. However, adult specializations are chiefly for reproduction and distribution of the species. In the higher insects, one form of specialization is sexual dimorphism. In some insects, the 2 sexes are so different that they could easily be mistaken for separate species. In others, the male may be partially or completely eliminated, and reproduction is by parthenogenesis (e.g., in aphids). In some insects, the normal adult morphological features are absent; characters of the immature stage are carried over to the sexually mature and reproducing individuals. This phenomenon, called neoteny is frequently associated with parthenogenetic reproduction and viviparity (rearing active young instead of eggs), and is restricted to the female.
In social insects, different castes perform particular functions in the colony. Information on castes and their functions can be found elsewhere n this book in discussions of the biologies of termites, ants, wasps, and bees.
In insects, diapause most often occurs in the gg or pupal stages, although it may occur in other stages of development. It may be caused by hanges in temperature, moisture, food, water, or oxygen. Diapause may be triggered by a engthening (vernal) photoperiod or by a shortening (autumnal) photoperiod, or may be imposed by such internal factors as heredity, enzymes, or hormones. Some believe that diapause ay be brought about by autointoxication or byhe accumulation of some chemical, somewhat like muscular fatigue in higher animals. Diapause that enables an insect to resist cold is called hibernation; if it enables an insect to resist heat or drought, it is called aestivation.
The orders are subdivided into families, the families into genera (sing. genus), the genera into species, and the species sometimes are subdivided into subspecies or varieties. The accompanying classification of man in comparison to that of one of his external parasites, the head louse, Pediculus humanus capitis De Geer, will illustrate the significance and the relative magnitude of the various categories universally accepted in both plant and animal classification. However, only a relatively few species of insects have been divided into taxonomically distinct subspecies.
Binomial nomenclature gives to each species of plant or animal a name of 2 terms, of which the first is the genus, and should always begin with a capital letter, and the second is the species, and should always begin with a small letter. Both terms are italicized in scientific literature, and are accordingly underscored (underlined) in manuscripts. It is a system of nomenclature first standardized by the great Swedish naturalist Linnaeus about the middle of the eighteenth century. In binomial nomenclature, the name for the human species is Homo sapiens, and the name for the head louse is Pediculus humanus. There are two subspecies or varieties of Pediculus humanus: P. h. capitis, the head louse; and P. h. humanus, the body louse.
Some people are inclined to think of scientific names as pedantic gestures, or of having perhaps some vague academic significance of small concern to anyone besides the taxonomist. The common names of most animals, especially insects, are highly variable, even within a relatively limited area, and may mean nothing to people in other areas within the same country, to say nothing of those in other parts of the world. Likewise, common names may give little or no indication of the taxonomic relationship of the insects another. Therefore, only the scientific have a universal significance--referring same plant or animal the world over.
In the Entomological Society of America, there is a Committee on Common Names of Insects. This committee has developed a list of approved common names of insects and a few other arthropods, and also their scientific names, according to the latest concepts of taxonomists in each insect group. This list is periodically revised, expanded, and sent to members of the society. By following the recommendations of the committee, American and Canadian entomologists are standardizing nomenclature in their published papers.
The dictionary is not always a reliable authority on common names of insects. One finds such words as "bedbug," "housefly," and "honeybee." These should be written as 2 words--"bed bug,'' "house fly," and "honey bee." The bed bug is; true bug (Hemiptera), the house fly is a true fly (Diptera), and the honey bee is a true bee (a superfamily in the order Hymenoptera). On the other hand, the words "doodlebug," "dragonfly," and "silverfish" are properly written as 1 word because the doodlebug is not a bug, the dragonfly is not a fly, and the silverfish is not a fish. The words "bedbug," "housefly," and "honeybee" would be equivalent to the word "Johnsmith."
Rules of Binomial Nomenclature
Before the time of Linnaeus, the name of the insect was a short Latin description. It can be imagined how cluttered with names entomological literature would be if such a system of nomenclature had been retained. Linnaeus suggested that the name of an animal (or plant) be composed of the compounded names proposed for the genus and for the species of the animal in question. These were always Latin names, for Latin was the universal language among scholars of that time. Latin has an additional value for use in "scientific names," in that its form and usage are now stereotyped, and can be used with unchanged meaning by people of all languages
and in all parts of the world. We hear much nowadays about the desirability of a universal language. A universal language has been used for centuries by biologists in the naming of plants and animals, and if this had not been the case, the chaotic condition of biological literature would be difficult to imagine. Certainly, natural science would have been greatly retarded. Chemists are also in world-wide agreement as to chemical nomenclature, having adopted a universal system at a conference held at Geneva, Switzerland, in 1892. (While Linnaeus used only Latin, latinized forms of Greek and other languages were later often used in binomial nomenclature. Many generic names are more recently of Greek derivation.)
Linnaeus grouped the insects into 7 orders: Coleoptera, Hemiptera, Lepidoptera, Neuroptera, Hymenoptera, Diptera, and Aptera. He described and named a surprisingly large number of species, as a perusal of general entomological literature will show. In 834 pages, Linnaeus described 4,379 animals, including 1,937 insects. By now, about a million species of insects have been named and described, and about 6 or 7 thousand new species are being added in the world's taxonomic literature each year. There are probably at least as many species of insects still unnamed and undescribed.
Linnaeus perfected the binomial system of classification, and used it in his famous general treatise on animals, called Systema Naturae. The tenth edition of this publication, which appeared in 1758, is universally accepted as the starting point in binomial nomenclature. Any specific name proposed before the date of publication of the tenth edition of Systema Naturae is not considered valid. In all the naming of species subsequent to 1758, however, the rules of priority are strictly followed. If a species is described and named, and subsequent research reveals that someone else had already described and named the same insect, the later name becomes invalid. Taxonomists are no more infallible than any other specialists, and the constant revision of initially presumed taxonomic relationships of species, genera, and families of insects to one another is a task that continues apace with the description of new species.
For complete information on a scientific name, the name of the "author," the researcher who first described and named the species, follows the name of the species, but his name is not underscored or written in italics. In most entomological literature, the author's name is abbreviated. However, the modern tendency is to write out the author's name completely, since there are now too many names to abbreviate without duplication or confusion. Likewise, beginning students and those not familiar with a given insect group are not likely to know the meaning of the majority of the abbreviations. However, an abbreviation for Linnaeus or Fabricius is acceptable.
The house fly may be taken as an example to illustrate the correct form of binomial nomenclature. It is one of the many species which was named by Linnaeus. The scientific name of the house fly is Musca domestica Linnaeus (or Musca domestica Linn., or L.). One knows from the scientific name that Linnaeus described the house fly as the species domestica and that he placed in the genus Musca. Let us take another example, the codling moth of apples, pears, and walnuts, another well-known cosmopolitan insect pest. The scientific name of this insect is Laspeyresia pomonella (Linnaeus). The parentheses around "Linnaeus" show that he named the codling moth pomonella, but that he placed it in some other genus (Tinea). Later, a specialist in this particular group of the Lepidoptera found that the codling moth showed greater taxonomic relationships to insects which belong in the genus Laspeyresia, a genus that had not yet been recognized in Linnaeus' time. Since then, the species pomonella has actually been placed in 3 successive genera, Cydia, Carpocapsa, and Laspeyresia, the latter being currently recognized by entomologists and approved by the Entomological Society of America.
Relation of Scientific Names to Entomological Investigation
The first step in investigating an insect problem is to establish the identity (scientific name) of the insect in question. If the name of the insect is not known to the investigator, or if he is not absolutely certain of its identity, he may send specimens to the United States National Museum for identification, or if he knows to what group (family or genus) the insect belongs, he may send specimens directly to a taxonomist who specializes in that particular group of insects. When he is certain of the scientific name of the insect, the investigator proceeds to examine the entomological literature for information on it.
The literature may reveal to an investigator much that is already known about an insect, such as its economic importance, geographic distribution, hosts, life history, seasonal history, habits, tropisms and ecological relationships, climatic or other physical factors favorable or unfavorable to its abundance, natural enemies, and previous investigations on control measures. The obtaining of all this wealth of information is predicated on the correct identification of the insect species and the ability to use the world's entomological literature on the basis of a standardized system of nomenclature. No matter in what country the literature originated, the insect should always have the same scientific name, for insects are named according to universally accepted standards of nomenclature. Without a universally accepted system of classification and naming of insects, no other phase of entomological investigation could be systematically pursued.
Some Characteristics of the Phylum Arthropoda
As shown in the various categories of classification, the phylum is the most inclusive of the subdivisions of the animal kingdom. In the Arthropoda, the various classes (subdivisions of a phylum) share these characteristics in common: bilateral symmetry; segmented bodies; paired, jointed appendages; protective cuticle; a dorsal blood vessel; and a ventral nervous system. Some, but not all, of these characteristics are possessed by some groups of animals farther down in the scale of evolution. For example, the members of the phylum Annelida, to which the earthworms belong, possess bilateral symmetry; segmented bodies; protective cuticle; a dorsal blood vessel; and a double ventral nerve cord, segmented into a series of ganglia. However, they do not have jointed appendages. No other phylum of animals possesses all of the characteristics of Arthropoda. Within the phylum Arthropoda, however, there are some points of difference which relegate different groups of animals to different classes. The class Hexapoda, for example, is the only group of arthropods that has 3 body regions, 3 pairs of legs, and wings (as adults).
Use of Keys for Identification of Insects
When an insect taxonomist or systematist is given a large group of insects to separate into their respective orders, families, genera, and species, he first attempts to sort out the species he recognizes from earlier experience. For aid in determining the relationships of the insects of whose identification he is not certain, the taxonomist will often use "keys," by means of which the insects can be traced through a successive series of morphological characters. The keys are so arranged as to facilitate the determination of scientific names and systematic relationships, and have been prepared for all the categories of insect classification. They are based on the work of a large number of specialists in the various groups of insects, and are, of course, subject to revision as new information is gained regarding the taxonomic relationships of the various groups. The use of a key depends on some knowledge of the structural characteristics and terminology of the group of animals (or plants) for which it has been worked out.
The accompanying simple and admittedly incomplete key was designed to enable students to "key out" the principal orders of adult insects for a course in economic entomology. The orders Protura and Collembola are included among the insects, although modern taxonomists consider them to belong to different subclasses. In this simplified key the diplurans (Diplura) are considered to be in the order Thysanura and the cockroaches are considered to be in the order Orthoptera, although the cockroaches are placed in Dictyoptera in chapter 6. Belkin (1972) has combined keys for adult and immature insects.
1. Wings usually present ................2 wings absent ...........................17 2. Mouthparts formed for biting ..........3 Mouthparts formed for sucking ..........13 3. Tip of abdomen with paired, forcepslike appendages (earwigs) .................Derm ap'tera Tip of abdomen without paired, forcepslike appendages ......................................4 4. Forewings membranous or leathery, not horny in texture ....................................5 Forewings horny, shieldlike, forming cover for hindwings (beetles) ..............Cole op'tera 5. Forewings leathery, hindwings folded fanlike (grasshoppers, crickets, cockroaches) Orth op'tera Forewings membranous, hindwings not folded fanlike ......................................6 6. Abdomen usually constricted at base, often with a stinger or specialized ovipositor (ants, bees, wasps) ..........................Hymen op'tera Abdomen not constricted at base, without a stinger or specialized ovipositor ............7 7. Antennae short, inconspicuous ................8 Antennae long or conspicuous ...................9 8. Hindwings smaller than forewings; abdomen with 2 or 3 threadlike filaments (mayflies) .........................Ephem er'ida Hindwings not smaller than forewings, abdomen without long, threadlike filaments (dragonflies, damselflies) ...........Odo na'ta 9. Tarsi with less than 5 segments .............10 Tarsi 5-segmented .............................11 10. Hindwings broader than forewings, folded in repose (stoneflies) .....................Plec op'tera Hindwings same width as forewings, not folded (termites) ................................Is op'tera 11. wings naked or very slightly hairy ........12 wings densely clothed with hair (caddisflies) ..........................Trich op'tera 12. Head prolonged into beak, with mouthparts at tip (scorpionflies) ......................Mec op'tera Head not prolonged or beaklike (lacewings, ant-lions) ..................................Neur op'tera 13. Wings densely clothed with scales; mouthparts coiled below head (butterflies, moths) .................................Lepid op'tera Wings not clothed with scales; mouthparts not coiled below head .................................14 14. Mouthparts sunken into head capsule, asymmetrical; usually very tiny insects; wings with hair fringe (thrips) ..................Thysan op'tera Mouthparts external in repose, symmetrical . . 15 15. Wings usually 4; mouthparts forming a jointed beak ..............................................16 Wings 2; mouthparts styletlike or spongelike, not forming a jointed beak (flies) .... Dip'tera 16. Basal portion of forewings thickened, leathery, or horny; distal portion membranous (true bugs) ..............................Hem ip'tera Forewings with same consistency throughout (aphids, scale insects, leafhoppers, cicadas, psyllids) .......................Hom op'tera 17. Abdomen without conspicuous appendages .. 18 Abdomen with conspicuous "tails" or "spring" 19 18. Body flattened dorsoventrally; prothorax small, ringlike (lice) ...........................Ano plur'a Body flattened laterally; prothorax large and hoodlike (fleas) ......................Siphon ap'tera 19. Abdomen 10- or 1l-segmented, terminating in forceps (pincers) or long filaments (silverfish, bristletails) ............................Thysa nu'ra Abdomen with not more than 6 segments, often terminating in a springing apparatus (springtails) ...........................Collem'bo laMost arthropod orders in the key may be keyed out to important urban pest species by illustrated keys in manuals for students in university courses in medical entomology, such as the one by Furman and Catts (1970).
Mouthparts. Piercing-sucking, retracted into a pouch.
Miscellaneous. Very small, slender, white insects. Abdomen 12-segmented or more, with a pair of appendages on the first 3 segments; tracheae may be present or absent; no visible antennae; blind. They are principally found in leaf mold. Proturans crawl only on the second and third pair of legs, holding the first pair in front of and above the head as tactile organs (figure 57).
Mouthparts. Chewing type, sunken into the head.
Miscellaneous. Very small to minute; compound eyes; eyes are grouped, ocelli-like. Antennae short, rarely have more than 5 or 6 segments; 6 abdominal segments, the first usually provided with a collophore (suckerlike ventral tube), the fourth usually bearing a pair of appendages that constitute the springing organ or furcula and the third bearing a pair of appendages that operate to act as a catch to hold the furcula when it is at rest; tracheal system usually absent; Malpighian tubules absent. Adults molt many times after reaching sexual maturity.
The Collembola (figures 336 and 337, chapter 10) are sometimes very abundant, and some species are injurious to plants. They may be found in mushroom beds, maple sap buckets, on the surface and in the soil of fields and woodlands wherever there is shade, and moving about on the surface of water in swimming pools, but they can be found in greatest abundance under boards or logs, where they find both darkness and moisture. They are nocturnal. The characteristic of their habits for which they are best known is their ability to spring into the air by means of their furcula, which is doubled under the abdomen and held in place by the "catch," then released like a spring.
Mouthparts. Chewing, partially retracted.
Miscellaneous. Small, narrow, rarely over 10 mm in length, unpigmented because of their secluded and subterranean habits. Antennae long, filiform, with beadlike segments; eyes and ocelli absent; abdomen 11-segmented Diplurans may have long cerci with beadlike segments as in the antennae, and without the median caudal filament that characterizes the Thysanura, or they may have a pair of forceps or pincers (Japyx). Molting continues after sexual maturity is reached.
Miscellaneous. Small insects, with elongated, flattened, naked, or scaly bodies; abdomen with 10 or 11 segments; 3 long, styliform, many-segmented abdominal appendages consisting of a pair of cerci and a median caudal filament, and bearing 2 to 8 pairs of ventral appendages (styli); antennae long and filiform; compound eyes present in some species, degenerate in others, absent in some. The Thysanura have an unusually large number of molts, both before and after maturity; they may pass through 45 to 60 molts before death. (See chapter 8.)
This order of universally distributed insects is represented by the silverfish (figure 206, chapter 8), which are common household pests and are injurious to book bindings, wallpaper, and similar articles containing starch or glue size.
Miscellaneous. For this brief and elementary treatment of orders, the older concept of the Orthoptera is retained, although there are good grounds for the splitting of the order into a number of other orders; in subsequent chapters, the cockroaches and mantids are considered as a distinct order (Dictyoptera) (see chapter 6, under "Cockroaches"), as are also the walkingsticks (Phasmida) and grylloblattids (Grylloblattodea).
Among the Orthoptera are found some of the largest of all insects. A grasshopper from Venezuela is as long. as 17 cm, and walkingsticks attain a length of 25 cm in Africa and South America. While the forewings (tegmina) of most Orthoptera are very drab, the hindwings are often brightly colored. The katydids (figure 58) and crickets are "singing" insects, the males being able to make sounds of more or less musical quality by means of stridulating structures. The locusts (family Acrididae), since times of earliest historic record, have caused widespread agricultural losses and even occasional famine. The migratory locust, Locusta migratoria L., occurs over vast areas of the Eastern Hemisphere. It is normally confined to its breeding grounds, but occasionally, according to Uvarov (1928), crowding results in the production of a migratory phase. During this phase, the locusts sweep over vast areas in incredible numbers, often destroying practically all the vegetation. The Rocky Mountain grasshopper, Melanoplus spretus (Walsh), has done great damage in the Great Plains region of the United States and Canada. A general lessening of the severity of its attacks is believed to have resulted from cultivation in its usual breeding grounds, which results in the destruction of the egg cases (oöthecae) in the soil.
Cockroaches are predominantly tropical and subtropical insects, but some species have been disseminated into temperate regions, being able to survive in homes, bakeries, restaurants, and similar protected places where food is available. They are very objectionable because they frequent dark, filthy places, have a repugnant odor, and may be carriers of disease.
Other Orthoptera are noteworthy because of their interesting forms and habits. The praying mantis, so called because of the manner in which it holds its enlarged forelegs, uses these forelegs for capturing and holding its prey. The walkingsticks (figure 59) closely resemble the twigs of leaves of their environment, and afford some of the most striking examples of protective mimicry.
Miscellaneous. Social and polymorphic species, living in colonies composed of winged and wingless reproductive forms together with numerous wingless, sterile soldiers and workers. Abdomen wide where it joins the thorax, and sometimes ending in a pair of very short cerci.
Termites, sometimes called "white ants," can sometimes be seen in large numbers in logs in forests, in wood lying in contact with the ground, or in the timbers of buildings. The subterranean species of the family Rhinotermitidae are very destructive, either to wooden structures that are in contact with the ground or that can be reached by means of shelter tubes built by the termites from soil and body secretions. Building timbers are often so badly infested with termites that, while the exterior may appear to be sound, the interior may be a mass of tunnels and excrement. The insects will frequently not be seen until spring, when great swarms of winged reproductives appear. The latter are often mistaken for "flying ants."
A termite colony is an example of regimentation par excellence. The colony consists of a number of castes. Only a few of the eggs develop into individuals that can reproduce; the remainder devote themselves to the care of the "kings" and "queens" and their myriads of offspring (figure 60). The wings of the reproductive individuals are used for only a single flight, after which they break off near their bases. Mating then takes place. Some of the eggs develop into wingless castes known as soldiers and workers. The soldiers have massive heads with greatly enlarged mandibles, or have other modifications for the defense of the colony.
The digestive tracts of some termites contain protozoa possessing enzymes that break down the cellulose into products that termites can assimilate. If these protozoa are removed, the termites will eventually die of starvation. Certain species that have no intestinal fauna to aid in digestion feed on fungi cultivated in their nests.
Miscellaneous. The earwigs are easily recognized by their short, leathery forewings and the large, forcepslike cerci at the end of the abdomen. The European earwig, Forficula auricularia L. (figure 356, chapter 12), is a widely distributed species that breeds in garbage dumps, manure piles, lawn cuttings, leaf piles, and other kinds of debris, and has but 1 generation per year. Earwigs feed on dead or living animal matter, but also on many kinds of plants. They often become household pests. The female may brood (stay with and care for) her eggs, a rare phenomenon in nonsocial insects.
Wings. Males usually winged, but some are wingless or have vestigial wings. When winged, they have 2 pairs that are membranous, with reduced venation. The females are wingless. Mouthparts. Chewing.
Miscellaneous. The embiids are solitary or gregarious insects, living in silken tunnels; cerci 2-segmented, usually asymmetrical in the male. These insects (figure 61) comprise a small order of fewer than 150 species. They live under stones and other objects, the silk of their tunnels being spun by glands located on the swollen first segments of the tarsi of the forelegs. Their webbed galleries are the most conspicuous indicators of their presence. Embiids generally shun the light of day, and leave their tunnels only at night; however, the males are commonly collected at lights (Ross, 1965). Their food is thought to be chiefly dead and decayed vegetable matter. The males may eat animal food.
Miscellaneous. Small or minute in size; antennae relatively long; prothorax small; cerci absent; tarsi 2- or 3-segmented.
One group of psocids, all formerly included in the family Atropidae, includes the booklice, which are found in stored books, especially in dark, damp areas. The booklice have no wings or ocelli, and have more than 13 segments in the antennae (figures 334, 335, chapter 10). Other psocids live on the bark of trees and on foliage. Molds and fungi are eaten by these species, and none is known to be of economic importance. Psocids sometimes become a household nuisance by breeding in cereals and starchy materials. They sometimes occur in great numbers in a house.
Wings. Four narrow, membranous wings, nearly veinless, fringed with hairs; some species are wingless.
Mouthparts. Rasping-sucking; asymmetrical, there being only a single mandible.
Miscellaneous. Tarsi 1- or 2-segmented, terminating in a protrusible bladder.
The suborder Terebrantia includes those species of the order in which the abdomen of the female terminates in a sawlike ovipositor. This kind of ovipositor is used for inserting her kidneyshaped (reniform) eggs into the epidermis of leaves or fruit. These species are generally quite active, the greenhouse thrips (figure 346, chapter 11) being a conspicuous exception, and include the most important pests. Males may be absent, but even when present, reproduction may occur by parthenogenesis.
The suborder Tubulifera includes large, inactive species, in which the female has no ovipositor, the eggs being laid on the plant surface. The last segment of the abdomen of both sexes is tubular, as contrasted with the coneshaped terminal abdominal segment of the female Terebrantia or the bluntly rounded terminal segments of male Terebrantia. The Tubulifera may often be found in the flower heads of the Compositae, under bark, in galls, in moss and turf, or under leaves, where they may prey on small insects and mites or feed on plant exudates or on dead and decaying vegetable material. A few species are injurious to cultivated plants.
Mouthparts. Greatly modified; adapted for chewing in suborder Mallophaga, for piercing-sucking in suborder Anoplura.
Miscellaneous. These are small, flattened, wingless insects, living as external parasites of birds and mammals; legs short, with tarsi adapted for clinging to the host; antennae short, 3- to 5-segmented; eyes reduced or absent.
The order Phthiraptera may be divided into 2 suborders, the Mallophaga (biting lice) and the Anoplura (sucking lice) (figure 294, chapter 9). Some authors, however, regard the Mallophaga as a distinct order. Both orders exist continuously on the bodies of warmblooded animals. They glue their eggs to the hairs or feathers of their hosts. The biting lice subsist on bits of hairs or feathers, skin scales, or the dried blood from scabs, and some species can obtain blood by puncturing the bases of young feathers. The great majority of biting lice occur on birds, but 3 families are restricted to mammals, attacking, among other hosts, cattle, horses, sheep, goats, dogs, and cats. Nearly all species infesting mammals have only 1 tarsal claw which, with modification of the end of the tibia, forms a grasping organ for holding on to the hairs of its host. The prothorax is free and well developed. The species on birds have 2 tarsal claws. Compared with sucking lice, biting lice are of little importance.
The members of the suborder Anoplura are all parasitic on mammals, and their mouthparts are modified for sucking, but are completely withdrawn into the head when not in use. Their tarsi have only 1 segment that terminates in a single claw, much in the manner of the biting lice occurring on mammals. The sucking lice can be distinguished from biting lice not only by the difference in mouthparts, but also because the head of the sucking lice is narrower and more pointed, and the thoracic segments are fused. The head and body lice of man, Pediculus humanus var. capitis and P. humanus var. humanus (plate VIII, 2, and figure 294, chapter 9), the "cooties" of World War I fame, are carriers of typhus and trench fever, and have caused great losses of human life, especially in times of war and pestilence. In World War II, the timely development of DDT, a remarkably effective insecticide against lice, reduced casualties attributable to louse-borne diseases to a very low number. The crab louse (figure 295, chapter 9) is the only other louse parasite of humans, but different species of lice attack cattle, horses, dogs, hogs, and goats.
Wings. Four, or secondarily wingless (2 wings in male Coccoidea).
Mouthparts. Piercing-sucking, with palpi lacking. The labium is modified into a dorsally grooved sheath which is usually jointed; within it lie needlelike stylets, the modified mandibles and maxillae.
Miscellaneous. Antennae, with rare exceptions, are 2- to 10-segmented; tarsi are 1- to 3-segmented, with 1 (Coccoidea) or 2 claws, with or without arolia (cushionlike pads); abdomen with few to 10 segments; cerci absent.
This suborder is often given order status (Hemiptera), and comprises a reasonably well-defined group of insects, characterized primarily by the forewings (hemelytra) being thickened in the basal half, while the apical half is thin and membranous (figures 363 and 364, chapter 12). The wings lie flat on the back when at rest, and the membranous apical halves of the forewings overlap. The tarsi are usually 3-segmented. The beak is attached well forward.
The aquatic bugs of this suborder have short antennae that can be concealed in grooves on the underside or back of the head. The ones most likely to attract attention are the giant water bugs or electric-light bugs (Belostomatidae). These are very large forms that feed on insects, tadpoles, and small fish. The females of 2 genera lay their eggs on the backs of the males. The females secrete a waterproof glue with which the eggs are attached. The males carry the eggs about until they hatch.
The terrestrial Heteroptera have conspicuous antennae that are longer than the head and extend in front of it. The predaceous species include the semi-aquatic water striders, the assassin bugs, bed bugs, damsel bugs, ambush bugs, some stink bugs, and some Iygaeids and mirids.
The plant-infesting bugs include the stink bugs, leaf bugs, lace bugs, chinch bugs and squash bugs. Among the stink bugs is the harlequin bug, Murgantia histrionica (Hahn), a very destructive pest of cruciferous plants in the southern states. The tarnished plant bug, Lygus pratensis (L.), a representative of the family Miridae, is a serious pest of fruits and field crops. Best known of the family Lygaeidae is the very destructive chinch bug, Blissus leucopterus (Say), which annually causes millions of dollars of damage to corn and wheat crops. Among the Coreidae, the well-known squash bug, Anasa tristis (De Geer), is a serious pest of cucurbitaceous plants, and the boxelder bug, Leptocoris trivittatus (Say) (plate VIII, 8), infests box elder, ash, maple, and many fruit trees, and also becomes a nuisance by hibernating in houses or invading them in large numbers on warm winter days (see chapter 12, under "Bugs (Hemiptera)"). In the Cimicidae is the bed bug, Cimex lectularius L. (plate VIII, 4).
The suborder Homoptera is characterized by the wings, when present, being membranous, or sometimes with a somewhat leathery texture, but never with the basally thickened forewings (hemelytra) which characterize the Heteroptera. The hindwings are shorter and wider than the forewings. When these insects are at rest, they usually hold their wings rooflike over the abdomen. The beak of the Homoptera is attached ventrally near the back of the head, and often appears to arise from between the front coxae. All Homoptera are plant feeders, and the excrement of many species becomes practically pure sugar. It is known as "honey-dew," and can support the growth of the familiar sooty-mold fungus which blackens the foliage and fruit of plants upon which the honeydew-producing species appear.
The aphids or "plantlice" are Homoptera that are economically important on the majority of agricultural plant crops. Their bodies may be naked, or may be covered with a cottony or mealy wax, as with the woolly apple aphis and mealy plum aphis. The mealy-bugs (figure 347, chapter 11) nearly always have this type of body covering.
The armored scale insects are so named from the separate "scale" that the members of the family Diaspididae secrete to form a protective "armor" that covers the body completely on its upper surface (figure 349, chapter 11). An analogous term applies to the unarmored scales (figure 348) of the family Coccidae, even though they do not secrete a separate scale or armor. However, the adult females have a leathery or hard exoskeleton that is either smooth, roughened, naked, or covered with a thin layer of wax. In both families, metamorphosis is incomplete in females and complete in males, which have 2 wings. The mature females are unable to move.
In the majority of the citrus-growing regions of the world, the Homoptera are more important pests than all other insect orders combined, the scale insects usually being the worst offenders, often attacking other fruit trees as well. The aphids, leafhoppers, and whiteflies (Aleyrodidae) (figure 350) are also important plant pests the world over.
Mouthparts. Those of adults vestigial or absent; those of the nymphal aquatic forms (naiads) chewing.
Miscellaneous. Antennae tiny and setaceous; adults with 2 or 3 very long, slender, many-segmented "tails." Forelegs longer. Naiads aquatic, with paired tracheal gills on sides and back of abdomen; 2 or 3 slender "tails."
Mayflies are the only pterygotes with a subimago. When the last nymphal instar is ready to transform, it contains within its skin a fully developed adult, almost sexually mature. It rises to the surface of the water and out flies a mayfly, the subimago. It alights, rests for a day, then molts, and thereafter exists as a true sexually mature adult.
The delicate, gauzy-winged, long-tailed adults of the mayflies (figure 324, chapter 9) may swarm in large numbers around lights or near their breeding grounds. At the right season, one may observe these swarms, which are actually mating flights or "dances," near the breeding grounds. The swarm, consisting only of males, rises and falls until a receptive female approaches. She is then seized by a male and the pair depart. The "dance" of the males continues, with females being seized as soon as they enter the swarm. They lay their eggs soon after mating.
The scientific name of the order is derived from the fact that the mayfly adult lives one of the briefest (or most ephemeral) existences known in the winged state among insects, a period of 4 days or less, depending upon the species. Because the adults rarely if ever consume food, their intestinal tract is modified so as to contain large amounts of air for buoyancy. It no longer functions as a digestive tract.
Mouthparts. Chewing. In naiads, the labium of the mouthparts is modified into a prehensile organ used in capturing prey, and folding like a mask over the face when not in use. Miscellaneous. Antennae very small, setaceous; compound eyes large; abdomen long and slender; pleural sclerites slanted backwards so the wing attachments are behind the leg attachments; naiads aquatic, respiring by means of rectal or caudal gills. Adults and naiads predaceous. The Odonata are divided into the Anisoptera (dragonflies) and the Zygoptera (damselflies).
These 2 well-defined suborders differ in a number of morphological and behavioral characteristics. The dragonflies (figure 56, this chapter) are strong fliers. Their hindwings are broader, at the base, than are the forewings. The wings are not folded when at rest, but are held in a horizontal position at right angles to the sides of the body. The damselflies are feeble fliers. The 2 pairs of wings are of the same size and shape, and both are narrow at the base. When at rest, the wings are held parallel to the abdomen, or are slightly uptilted. The eyes of the dragonflies do not project from the side of the head, whereas the eyes of the damselflies do project, and are constricted at the base.
Dragonflies lay their eggs on water or on aquatic plants, whereas damselflies deposit them in the stems of aquatic plants. Dragonfly naiads respire through tracheal gills inside the rectum, and are propelled by the forcible ejection of water from the anus. Damselfly naiads respire by means of 3 leaflike tracheal gills that project from the end of the abdomen.
Mouthparts. Chewing, though weakly developed and frequently vestigial in adult.
Miscellaneous. Antennae long, filiform, with 25 to 100 segments; abdomen of adult bears cerci that are usually long and many-segmented; immature forms (naiads) aquatic, with tufted tracheal gills commonly present, though variably located.
The drab-colored stoneflies may be seen resting on stones, trees, and bushes, or flying for short distances over the water near swift streams with stony bottoms. The naiads may be found under stones or trash, in streams or along their margins. Their cast skins are a common sight on stones and shrubs projecting over a stream or lake shore. They are of value to man as a native food for fish.
Mouthparts. Chewing, though secondarily adapted in many larvae for sucking the blood of other insects.
Miscellaneous. Tarsi 5-segmented; larvae predaceous; the aquatic forms usually possess abdominal gills; some pupate in earthen cells, but most species spin silken cocoons in which the pupa lies freely.
The family Corydalidae includes the dobsonflies, the largest of the Neuroptera. They have a large, square prothorax, long antennae, and the male has very long mandibles. The larvae, sometimes called hellgrammites, are aquatic and are well known to fishermen, especially those fishing for bass or trout. They live under water, usually under stones in swift currents, and are predatory on other insect life. Corydalus cornutus (L.) (figure 62) occurs in eastern North America and is the largest species. It has a wingspread of 10 or 12.5 cm, and the male has very long mandibles. The larva has paired lateral filaments on the abdomen and short tracheal tufts at the bases of the first 7 of these filaments. The family Corydalidae is represented in California by only a single rare species in the genus Corydalis, but there are 6 species in Protochauloides and 2 in Dysmichohermes (Chandler, 1956).
The family Chrysopidae contains the lacewings. The green lacewing, Chrysopa californica Coquillett, (figure 63), is a beautiful insect, with gauzy green wings folded rooflike over the body and with golden eyes. The female places her eggs on the ends of hairlike stalks about 12 mm in length. This may be an adaptation to keep them out of reach of the cannibalistic larvae. The larvae are thysanuriform (alligatorlike), hairy, and with large, sicklelike mandibles. They are often important as predators of aphids.
The larvae of the Myrmeleontidae (antlions or doodlebugs) dig conical pitfalls in sand to trap insects, mainly ants, which they devour as food. Only their mouthparts protrude at the bottom of these pits. After their victims are drained of their body juices, they are cast far beyond the borders of the pit.
Mouthparts. Chewing, situated at the end of a beak or snout that is prolonged ventrally into a conspicuous structure that characterizes the adult Mecoptera.
Miscellaneous. Predaceous insects; small to medium-sized and slender; antennae long, filiform, many-segmented; legs long, slender; compound eyes large, widely separated.
The common name of these insects, "scorpionfly," is derived from the shape of the upturned terminal segments of the males of the family Panorpidae, which resemble the corresponding part of a scorpion. The larvae and adults of the panorpids are scavengers on animal food, and are found in moist locations, usually near streams. The larvae resemble caterpillars. The members of the family Boreidae are wingless, and the abdomen of the male terminates in large claspers.
Mouthparts. Modified by reduction for chewing in the adult, the mandibles being absent, with palpi well developed.
Miscellaneous. Antennae long, filiform; legs long, tarsi 5-segmented. Larvae aquatic, usually living in cases and breathing by means of abdominal gills.
The caddisflies are medium-sized, mothlike insects, with hairy, brownish, or dull-colored wings. They are found by streams and lakes, often in large numbers. They attract popular interest primarily because of the cases which the larvae build around their bodies for protection. What appears to be a mass of small sticks and pebbles at the bottom of a quiet pool may, upon further investigation, be found to be in motion, with the head, thorax, and legs of the tiny larva protruding from this protective case (figure 325, D, chapter 9). The cases of certain species may be attached to stones. The larvae of one trichopterous family (Limnephilidae) build the familiar "log-cabin" type of structure out of tiny sticks.
Mouthparts. Siphoning in adults; chewing in larvae.
Miscellaneous. Body of adult covered with scales and hairs; compound eyes large; antennae variable, often serrate, hooked, or knobbed. Larvae (caterpillars) cylindrical in shape, with 3 pairs of thoracic legs and 2 to 5 pairs of prolegs on the abdomen; prolegs terminate in series of tiny hooklets (crochets); pupae with appendages usually fastened to body, often in silken cocoons, although butterflies in general, and some moths, do not construct cocoons.
This order is among the largest in numbers of species, and is also one of the most destructive of all insect orders. Among the destructive species are the codling moth, oriental fruit moth, cutworms, corn earworm, pink bollworm, European corn borer, tomato hornworm, webworms, leaf rollers, tussock moths, gypsy and browntail moths, flour and meal moths, and clothes moths.
Nearly all of the larvae feed on plants or plant products, the clothes moths (chapter 8) and the greater wax moth (chapter 12) being exceptions. The silkworm moth is a species of considerable value to man.
The general characteristics of the larvae and adults (figure 185, chapter 7) of the Lepidoptera are well known to everyone, and further description for the sake of identification is unnecessary. However, certain features connected with the structure and habits of these insects are not so well understood. The patterns and colors that constitute the chief attraction of the Lepidoptera to the majority of people are understood by only a few. The arrangement of the hairs or scales (figure 50) that usually cover the wings and all other parts of the body accounts for their colors and color patterns. The scales vary in structure from simple hairs to broad, striated plates of various sizes and shapes. The Lepidoptera are also highly specialized in other ways. The coiled proboscis is probably the most highly specialized of all types of insect mouthparts.
The Lepidoptera are generally divided into 2 suborders, the Jugate and the Frenate. In the Jugate, the 2 wings on each side are united by the jugum, a small lobe at the base of the forewing. The fore- and hind-wings are similar in venation. In the Frenate, the hindwing is smaller than the forewing, and has a reduced venation. The 2 wings on each side are united by the frenulum, a spine or group of spines arising at the humeral angle of the hindwing, or simply by the expanded humeral angle of the hindwing. The suborder Jugate contains only 3 rare families. The Frenate can be divided into a series of superfamilies on the basis of differences in wing venation, the presence or absence of a frenulum, and other taxonomic features. These superfamilies are divided into the Macrolepidoptera and the Microlepidoptera, the latter made up of many families of small moths, and including nearly all the species discussed as pests in this book: the limacodids (slug caterpillars) and megalopygids (flannel moths) in chapter 9; the pyralids (snout moths), olethreutids (codling moth), tortricids (leaf rollers), cosmopterygids (the pink scavenger caterpillar), gelechiids (Angoumois grain moth), oecophorids (the white-shouldered and brown house moths) in chapter 7; and the tineids (clothes moths) in chapter 8.
The Macrolepidoptera include the large moths, butterflies, and skippers. The moths have large bodies, and the wings, when at rest, are held horizontally, roof-like, or wrapped about the body. Their antennae are varied in structure; they may be filamentous or feathery, but are rarely clubshaped at the tip. They are mostly night-fliers. The pupae are often protected by cocoons, which the larvae spin of silk produced by modified salivary glands.
The butterflies have slender bodies, and hold their wings vertically when at rest. They have no special structure to hold the fore- and hindwings together, this usually being accomplished by an expansion of the hindwing near its base. They have threadlike antennae that are clubshaped at the tip. Butterflies (and skippers) have no ocelli, while the moths often have two. They are primarily day-fliers. Their larvae are usually not so hairy as those of some moths, but they may possess spines. Most butterfly pupae are without cocoons, and some are attached to twigs, leaves, and other objects by the cremaster, a stout spine or hooked area at the posterior end of the pupal abdomen.
The skippers (family Hesperiidae) are so called because of their manner of flying. They dart suddenly from place to place. In contrast to the butterflies, the wing area is small when compared with body size. The larvae are usually naked, and have a constriction just behind their large heads.
An advantage found by the amateur collector of Lepidoptera is that the majority of species are so distinct that they can be identified by means of illustrations in a number of books that may be purchased in bookstores, and are obtainable in most libraries.
Mouthparts. Chewing. Miscellaneous. The adults are heavily sclerotized or rarely larviform; larvae wormlike, usually with 3 pairs of thoracic legs and not more than 1 pair of prolegs; pupae with appendages free, rarely in cocoons.
At least as far as the number of described species is concerned, the order Coleoptera is the largest of the insect orders. The order contains about 350,000 described species, more than a fourth of all the described species of animals (Arnett, 1968). In common with Hemiptera and Lepidoptera, the order Coleoptera contains a large number of species that are injurious to agricultural crops, but among the beetles are also found many beneficial predators, such as the lady beetles and ground beetles. Among the injurious species may be mentioned the June beetles and their larvae (white grubs), Colorado potato beetle, Japanese beetle, alfalfa weevil, cucumber beetles, flea beetles, Mexican bean beetle, bean and pea weevils, plum curculio, granary weevils, rice weevils, and mealworms. Among the forest pests are the bark beetles and flatheaded and roundheaded borers; and among the pests of cured lumber are the powderpost beetles.
The Coleoptera commonly have been divided into 2 suborders, the Adephaga and the Polyphaga. All the commonly found Adephaga are predaceous, including 2 terrestrial families: tiger beetles (Cicindelidae) and ground beetles (Carabidae); and 2 families of aquatic beetles: diving beetles (Dytiscidae) and whirligig beetles (Gyrinidae). The latter are so called because of their strange gyrations when they circle round and round one another as they swim or skate on the water. The Adephaga are distinguished from the other Coleoptera by the separation of the pronotum from the propleuron (lateral portion of the prothorax) by a distinct suture (notopleural suture), and by the first visible abdominal segment being deeply cleft by the hind coxal cavities. Sclerites and sutures are often used in the taxonomy of Coleoptera because the sclerites are fitted together with remarkable exactness and precision. The larvae of the Adephaga have 6 segments in each leg and 2 claws at the end of the leg.
The Polyphaga are divided into a number of superfamilies. All have the first ventral segment of the abdomen in a single piece, not cleft by the hind coxal cavities, and lack the notopleural suture. The legs of the larvae have 5 or fewer segments, and always end in a single claw. The division of the suborder into its superfamilies is based mainly on the nature of the antennae and on tarsal segmentation. The following list of superfamilies is based on a system of classification of the Polyphaga adopted by Metcalf et al. (1962). It is adequate only in a general way, or for beginning students or amateurs. Whereas only 7 superfamilies are recognized here, some authorities may recognize as many as 18, including 112 families (Borror and DeLong, 1971).
Mouthparts. Vestigial or lacking.
Miscellaneous. The adult female is larviform, and lacks legs, wings, eyes, antennae, and mouthparts.
The Strepsiptera comprise a very small order of 5 families, and are primarily of interest because of the peculiarities of their biology. They were formerly considered as a family (Stylopidae) of the order Coleoptera. Some taxonomists believe they should be returned to that order. The larvae are endoparasitic on bees, wasps, and a few Hemiptera and Orthoptera. The female develops and remains within the host, and the males fly about, seeking a mate. Both males and females extrude the anterior portions of their bodies between the abdominal segments of their hosts, and can be seen if an infested host is carefully examined. A few free-living female Strepsiptera have been found, but their life histories are unknown.
Mouthparts. Chewing or chewing-lapping.
Miscellaneous. Ovipositor is always present in females, and is modified for sawing, piercing, or stinging; larvae usually legless (except Symphyta); pupae with appendages free, commonly in cocoons; many species are social.
Although currently the order Hymenoptera is third in size from the standpoint of the number of species which have been described and named, probably great numbers of inconspicuous species, especially among parasites, have escaped the attention of taxonomists, so it is likely that there are more species of this order in existence than of any other. Some authors consider the Hymenoptera to be the most highly evolved of the insect orders because of the high degree of development and specialization of social organization found in this group, including the care of the young. The striking ability of some Hymenoptera to learn through experience has been taken by some scientists to indicate that the rudiments of intelligence may be found among these insects. On the other hand, some authorities consider flies and fleas as having reached a higher evolutionary status than the Hymenoptera, basing their opinions on the greater degree of morphological specialization in these orders.
The social organization of the Hymenoptera has long excited wonder and admiration, and has been the subject of much investigation. In colonies or nests of Hymenoptera, as with the termites, the reproductive function has been limited to a few specialized individuals, the kings and queens, but unlike the termites, the workers of the Hymenoptera are exclusively females. The males are therefore called drones, their only function being the fertilization of the queen. The caste formation and development of various Hymenoptera, particularly of ants, wasps, and bees, are discussed elsewhere in this book in considerable detail.
Among the insects, the Hymenoptera are distinguished by many species having a highly specialized modification of the ovipositor into an organ known as the stinger. Only the females can sting, and the pain of the sting is caused mostly by the reaction that the victim has to the venom injected into the wound made by the stinger. The order Hymenoptera has been divided into 2 suborders, the Symphyta (Chalastogastra) and Apocrita (Clistogastra).
This suborder contains at least 12 families, including the Tenthredinidae (sawflies) and Siricidae (woodwasps) discussed below. (See figure 138, chapter 5.) The anterior segments of the abdomen are as broad as the following segments, and are joined to the thorax throughout their entire width. The trochanters are 2- segmented. The ovipositors are sawlike (sawflies) or awl-like (woodwasps) for cutting plant tissues in oviposition. The larvae are caterpillarlike, but can be differentiated from lepidopterous larvae in that they have 6 to 8 pairs of prolegs, while the lepidopterous larvae never have more than 5. Moreover, the prolegs of the sawfly larvae never possess hooklets (crochets) like those of lepidopterous larvae. The sawfly larvae have a single pair of ocelli, while caterpillars have several on each side of the head.
In this suborder, the second abdominal segment is constricted into a slender petiole connecting the thorax to the abdomen. The ovipositor may be specialized for boring into plant tissue, placing eggs within other insects as done by the parasites (figure 64), or it may be modified into a stinger associated with venom glands. The larvae are always legless and grublike, having a reduced head and mouthparts, antennae and palpi with at most 1 segment, and usually lack ocelli.
The Apocrita may be divided into 3 groups: (1) the ants, with the petiole or pedicel (a slender stalk) of the abdomen with 1 or 2 swellings or nodes; (2) the wasps or wasplike species, with the hind tarsus usually slender and cylindrical and no nodes on the abdomen; and (3) the bees, with branched or plumose hairs on the thorax, and with broader and more hairy bodies than the ants and wasps. Nearly all are winged.
Among the Hymenoptera in general, there are few agricultural pests, and the majority of these are Symphyta (sawflies). In the Apocrita nearly all species are either beneficial to man or harmless. The honey bee, of course, is of great value as a producer of honey and beeswax, as well as being an important pollinator of fruit crops, many of which would have no commercial value without insectborne pollination. Ants, wasps, and bees can be pests because of their venomous stings. Ants may be injurious because of their ability to keep predators and parasites away from injurious insects, or to care for them in other ways. In return, they feed on the "honeydew" of certain injurious species that they protect, such as soft scales, aphids, mealybugs, and whiteflies. Ants are occasionally phytophagous, and damage plants. The Apocrita, however, are predominantly beneficial. Hymenopterous parasites are the principal entomophagous members of the insect world. They are among the "wasplike" species of the suborder, and include the families Ichneumonidae, Braconidae, Evaniidae, Chalcididae, and Proctotrupidae, as well as others that are not so well known.
Mouthparts. Sponging or piercing-sucking.
Miscellaneous. Head, thorax, and abdomen are very distinct (i.e., neck and petiole very narrow); larvae legless, usually maggotlike, with head greatly reduced and mouthparts composed of highly modified mandibles and maxillae which form the "mouthhooks" in higher forms; pupae with appendages commonly enclosed in a puparium or case in higher forms.
The Diptera differ from the other orders of insects in that the species with wings have only a single pair, The vestigial second pair (halteres) are believed by some entomologists to be balancing organs. A haltere may be seen on the asilid shown in figure 65. The few species in other orders that have only a single pair of wings do not possess halteres. In the Diptera, all but a few species have halteres, even if they have lost the front pair of wings. Since only the forewings are functional and are attached to the mesothorax, this segment makes up the larger part of the thorax. The thorax is highly developed, and among the insects, certain flies are the most rapid fliers. Many species also show a marked specialization of the abdomen, with a great reduction in the number of segments.
Flies are not conspicuous agricultural pests. Among the more economically important Diptera are the fruit flies, seed midges, and the Hessian fly. The Diptera rank first, however, as vectors (carriers) of diseases affecting man and animals. Mosquitoes are vectors of the causative organisms of malaria, yellow fever, dengue, filariasis, and encephalitis. Horse flies can transmit filariasis and anthrax. The tsetse fly is the vector of sleeping sickness. Various diseases are transmitted by Diptera of the family Psychodidae (Phlebotomus spp.), others by the black flies (Simulium spp.), and the ceratopogonids. The house fly is a mechanical carrier of the causative organisms of typhoid, dysentery, cholera, poliomyelitis, anthrax, yaws, conjunctivitis, and the eggs of several cestodes and nematodes. House flies, flesh flies, and others may cause myiasis, the invasion by fly maggots of the organs, tissues, and cavities of man and animals.
The Diptera are divided into 3 suborders: Nematocera, Brachycera, and Cyclorrhapha (Furman and Catts, 1970).
In the Nematocera and Brachycera, formerly considered to be superfamilies of the suborder Orthorrhapha, the adult flies usually emerge from the pupal skin through a T-shaped or straight split down the back. The pupae are generally naked. In the suborder Cyclorrhapha (circular-seamed flies), the larva pupates in the last larval skin, and the adult emerges from the anterior end, pushing off a circular lid.
The Cyclorrhapha do not have mouthparts with a biting mechanism. Most species collect their liquid food via the "pseudotracheae" of the broad labellar disk (figure 46, J, this chapter), but in some species (horn flies, stable flies, tsetse flies), the labium forms a Piercing organ (figure 46. F. G) (Snodgrass, 1944).
Mouthparts. Piercing-sucking, with 2 pairs of palpi.
Miscellaneous. Bodies small and laterally compressed; hind legs enlarged for jumping; coxae elongate; tarsi 5-segmented; adults are external parasites on warmblooded animals; larvae are slender and cylindrical, without legs or eyes, but with well-developed heads; the pupae have their appendages free, and are enclosed in cocoons.
When examined with a hand lens, fleas are easily distinguished from other insects (figure 303, A, chapter 9). They have a laterally compressed body that is, in itself, almost infallible as a taxonomic character. In addition, the hard, polished body contains many hair and short spines that are regularly arranged and directed backward. Fleas have no compound eyes, and often lack even simple ones They lay their eggs upon their hosts, as do the lice, but the eggs are not fastened and will drop off, hatching on the ground or floor. The larvae feed on dead animal or vegetable matter and adult flea excrement. They pupate in a silken cocoon, to which particles of dirt usually adhere.
There are only 5 families in this small but important order, and 2 of them are of special importance. The Pulicidae (common fleas) includes the human flea, Pulex irritans L., and the fleas of cats and dogs, Ctenocephalides felis (Bouché) and C. canis (Curtis). Fleas are most likely to infest houses where pets are kept.
These insects are vectors of bubonic plague. The chief vector is the oriental rat flea, Xenopsylla cheopis (Rothschild), which feeds mainly on rats, but also attacks man. The sticktight flea, Echidnophaga gallinacea (Westwood), is an important pest of poultry that also attacks many other animals. The chigoe flea, Tunga penetrans (L.) (see chapter 9, under "Fleas"), is considered not to have been permanently established in the United States, but is found in many tropical and subtropical regions throughout the world.