Discuss the role of true flies (Diptera) in the health of humans and their domestic animals.
Diptera parasitise vertebrates as larvae, and bite and transmit disease as adults.
Myiasis is the parasitism of the tissues of animals by larvae of Cyclorapha, these include the Muscoidea some of which suck blood. The Congo floor maggot, another bloodsucker is found only in humans. The screw worm fly which can enter through the smallest skin puncture then feed on the flesh, literally eating the animal alive. The point of control is the mating stage as the female mates only once. So sterile males are released in such great numbers as to swamp the normal males. This eliminated the fly from Libya where it was accidentally introduced in sheep. In America the fly is kept out of some areas by this technique. Strike in sheep is caused by the adult laying eggs in the soiled area around the anus. The maggots hatch and just eat into the flesh. These are the same maggots that are used medicinally to clean wounds. Bot flies of humans, sheep, cattle and horses. These enter through the skin, nasal passages etc. and lay their eggs in the nostrils in the case of the nasal bot fly. The larvae grow in the sinuses and are sneezed out. Sheep do not thrive in the presence of bot flies neither do cattle. The warble fly spoils leather by emerging at the side of the spine.
Ectoparasites in diptera are few in number. They include the deer fly of birds and the sheep ked which does not look like a fly.
Mosquito borne diseases kill millions of humans each year and infect many more. The diseases include malaria where the female mosquito transmits plasmodium spp. whilst taking a blood meal. Fliaraisis which involves the transmission of nematodes that cause elephantiasis. Yellow fever which is caused by a virus as is dengue.
Sleeping sickness is caused when certain species of tsetse (Glossina spp.) transmit trypanosomes.
Many other species of Diptera cause or transmit diseases. As far as humans are concerned malaria is probably the biggest killer among dipteran caused diseases. It was thought that DDT would eliminate the mosquitoes but they have proved resistant to most insecticides. Some new treatment for malaria is badly needed as there is some resistance to just about all the current medicines. Malaria not only kills but debilitates first. Some lessening of the effect would surely help the countries where it occurs as it puts a great strain on the health budget. There is also the spectre of the spread of malaria into the more prosperous nations caused by climate change.
Yellow fever and sleeping sickness both rely on having a reservoir of disease in the wild hosts - primates for yellow fever, and just about any vertebrate for trypanosomes. This makes elimination of the disease virtually impossible. It also means that previously cleared areas can become reinfested. The presence of the tsetse fly has limited the colonisation of many parts of Africa by cattle farmers. Others lose a proportion of their cattle each year, in cattle the disease is called nagana or ngana. Sleeping sickness kills only a few thousand humans each year, so now is no longer seen as a real danger.
Filariasis affects humans and one type can cause heart worm in dogs. There are about one hundred million cases of elephantiasis in the world. It is a debilitating disease transmitted by Culex quinquefasciatus which lives in and around human dwellings, so it should be possible, with education, to limit the amount of stagnant water left lying around. River blindness is caused by filarial nematodes transmitted by black flies (Simulium) it does not kill, it just blind making people unable to work.
Most dipteran caused diseases occur in the developing world, they have been eradicated or occur only sporadically in the developed world. So the people affected can least afford to pay for treatment. And the dissemination of information on lifestyle changes to limit the spread of disease is perhaps more difficult in these countries. This, plus the ling time before drugs are approved limits the years a company will have a patent on a new treatment. So research into new drugs or control methods has been decreasing while the pests and vectors have been developing resistance to the drugs and treatments currently available.
Describe the role of the insect gut and its symbionts in overcoming the biochemical barriers to herbivory.
Plants, apart from pollen, seeds and nectar, are not very nutritious, though they are plentiful. The cell walls are fairly indigestible as they contain cellulose, hemicellulose and lignins; proteins and lipids are low. Also most plants have allelochemicals usually both constitutive and induceable. Many insects have modified mouthparts to deal with the physical barriers to herbivory. The biochemical barriers are mainly overcome in the insect's gut.
The gut pH in most insects is quite high. In caterpillars (gut pH 8.8) which eat large quantities of leaf with minimal chewing this high pH further breaks down the leaf parts.
Cellulose digesters. To digest cellulose requires both exo- and endo-glucanases.
1) Hind gut flagellate protozoa are found in some termites. The termites chew up the wood and the protozoa break down the cellulose. The lining of the hind gut of insects is cuticular, and this is lost when moulting. So termites would lose their symbionts if it were not for the habit of feeding from the anus and faeces of nest mates. This may be one of the causes of eusociality in Isoptera. Roaches (non-social) retain their hind gut protozoa by anti-peristaltic movements of the gut prior to moulting to bring glucose into the mid gut for absorption.
2) The higher termites, wood-boring beetles, crane flies and cockroaches have bacteria, these operate in a similar way to the hind gut protozoa.
3) Fungal enzymes break down cellulose and are taken in along with the wood by some beetles, e.g. Ambrosia beetles, and also by wood wasp larvae. In the fungus growing termites the enzymes are ingested when the termites eat the fungal fruiting bodies.
1, 2 and 3 are all examples symbiotic relationships enabling cellulose digestion. Nasutermes spp. and one cockroach species have their own cellulases so can digest cellulose without symbionts.
Aphids tap straight into the phloem of the plant. Phloem is rich in water and sugars, but relatively low in nitrogen. So the aphid must process a lot of phloem to meet its nitrogen requirements. The water is quickly excreted by by- passing most of the mid gut, as the fore gut has a connection to the rear part of the hind gut so excess water takes this route.
Dealing with allelochemicals.
Sequestration. Some insects actually depend on the so-called defensive chemicals for their survival. They do not digest them, but sequester them and use them to protect themselves against predation, e.g. the cinabar moth eats ragwort which contains cyanide. The antibiotic properties of some allelochemicals may also provide protection against pathogens.
Mixed function oxidases (MFO) are memberane-bound enzymes that detoxify a wide variety of allelochemicals. MFO are usually found in the fat body or the mid gut. Their characteristics are:
1) They catalyse oxidative reactions resulting in polar products that are easily excreted.
2) They are non-specific, accepting many chemical substrates.
3) They are easily induced by exposure to novel toxins.
MFO are especially valuable to polyphagous insects as they eliminate the need to maintain a wide variety of specific protective enzymes. The detoxification is as follows:
1) Primary degradation in which a toxic molecule receives a chemical group, e.g OH which makes it water soluble.
2) Conjugation with sugars, amino acids, sulfates, phosphates etc, bound for excretion.
For example nicotine becomes cotinine and is excreted.
MFO activity can appear within minutes of exposure to novel chemicals, e.g. the armyworm (Spodoptera eridania) chews a new leaf, waits a few minutes, then starts to eat the leaf. The few minutes' wait is all that is required to induce MFO activity. As the hours pass the caterpillar becomes increasingly efficient at digesting the food source. MFO activity varies among species, and even among individuals within species. Lepidopteran larvae that are polyphagous have higher MFO activity than mono- or oligophagous larvae. So generalists are better adapted. MFO has preadapted many insects for developing resistance to pesticides, e.g. DDT and kelthane.