The insect class comprises the most diverse group of animals on the earth and constitutes more than half of all described animal species. Insect species also make up close to 90 percent (800,000 of 900,000) of all arthropod species.
The most distinctive attributes of insects are their (generally) small size, wings and metamorphosis. Feeding habits are vastly different, depending on the species of insect. Some eat leaves, flowers, seeds, fruits, bark, or roots from plants; others feed on the nectar or collect the pollen from flowers; while some prey on other insects.
Insects are widely distributed around the world, more so than any other group of animals; they are everywhere that green plants grow. Although they are mainly terrestrial, there are also insects that live in aquatic habitats (both fresh water and marine).
Like all arthropods, insects have an exoskeleton to support muscles and protect their body. Touch receptors are found on the antennae, legs, and feet. Insect bodies are composed of three distinct regions: The head, the thorax, and the abdomen.
The head is where the insect’s mouth, eyes (if they have them), and antennae are located. The mouth generally has an upper lip, two pairs of jaws, a lower lip and a tongue-like hypopharynx. The jaws work from side to side, not up and down like most other animals. However this mouth structure is not true of many insects, notably butterflies, which instead have a tube-like proboscis (see photo, at left) that is used for sucking up nectars.
An insect’s thorax is divided into three sections, each with a pair of legs underneath. The spiracles, which are tiny openings in the body used for respiration, are often on the middle and last segment of the thorax. Wings are located on the thorax of flying insects. The abdomen is made of plates that form ring-like segments, and mostly consists of organs used for reproduction.
All insects begin as eggs. The development from egg to adult—called metamorphosis—varies greatly, depending on the species. There are four main types of metamorphosis: anamorphosis, ametamorphosis, simple metamorphosis, and complete metamorphosis. Each represents a distinct cycle of development from the insect’s egg stage into adulthood. Most insects go through a larval stage early in their development. During this period they may or may not look like the full-grown adult.
Insects increase in size by molting their outer layer. This occurs regularly in the larval stage because growth occurs very quickly.
Once they become adults, insects take part in the complex ritual of reproduction. Most insects differ from other animals in that egg fertilization does not happen during mating. Sperm is stored by the females until their eggs are ready, at which time they are fertilized. However, unfertilized eggs can sometimes produce young in certain species. This is called parthenogenesis.
Certain insects live together as cooperating members of a social colony, in which different members of the group provide certain services in a caste-like system. In these colonies, there is often one queen that lays eggs while workers, drones and soldiers keep everything else running smoothly. All termites and ants, as well as certain species of bees and wasps are social insects.
Although insects are generally considered pests by humans, they play an important role in the health of ecosystems.
Insects provide a major food source for many vertebrates, including bats, birds, and frogs, as well as for invertebrates, including other insects. Learn more:
When an insect population decreases, there are major effects on other animals; there is often a decline in certain animal populations, and sometimes a growth in population of other insects that are considered pests.
Insects also play a major role in plant reproduction by pollinating certain plants. Some species of bees, butterflies, moths, beetles, flies, and wasps contribute by delivering pollen on the same plant or on another plant of the same species, which stimulates seed production. Close to 65 percent of all flowering plants use insects as pollinators.
The carnation (Dianthus caryophyllus) is pollinated exclusively by members of order Lepidoptera (butterflies, moths, and skippers). Yucca plants (family Agavaceae) are only pollinated by yucca moths (genus Tegeticula). The star of Bethlehem orchid (Angraecum sesquipedale) depends on a type of hawkmoth (Xanthopan morganii) for pollination.