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Essays 6

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How is the excretory system of insects involved in the control of water balance?

Malpighian tubules lie loose in the haemolymph and move about the body cavity in most insects (some Tenebrio have a crypto nephridial arrangement to enhance water conservation with tubules closely associated with the rectum). The Malpighian tubules produce primary urine - it can be iso-osmotic or dilute but not concentrated.

Potassium is the driving force, and is actively pumped from microvilli to tubule lumen. This creates a pull on the basement membrane side where K+ enters from the haemolymph. Water follows the K+. Then the Malpighian tubule contents (primary urine) enter the hind gut.

The hind gut is very effective at absorbing water especially in insects like Tenebrio etc. It can transport water from the rectal lumen into the haemolymph even against a concentration gradient of 3X in locusts. It is lined with cuticle with pores of 7 nm in locusts, and this may act as a sieve against large molecules so protecting the rectal epithelium.

Rectal pads. These are large aggregations of epithelial cells with lots of mitochondria. The epithelial cells with their mitochondria are folded into stacks creating intercellular spaces. The stacks are rich in ATPase which is used in ion transportation. Movement of water from the rectal lumen to the haemolymph appears to occur without transport of ions - this is physiologically impossible, so how is it done?
1) There may be active secretion of ions from epithelial cells and/or haemolymph into the intercellular spaces raising concentration.
2) This would cause water (and possibly small molecules) to move from the rectal lumen to the intercellular spaces. These spaces are all interconnected so there could be a general flow into the infundibular space.
3) When sufficient pressure is created in the infundibular space the valve would open allowing the contents (water) to pass into haemolymph.
4) The ions can be recycled into the epithelial cells to start the process again.
So the active transport of ions into the intercellular spaces causes movement of water from the lumen into intercellular spaces, then a general flow to the infundibular space where pressure build up causes passage into the haemolymph - all powered by the large number of mitochondria.

The rate at which the fluid is lost depends on the rate of fluid secretion by the Malpighian tubules and the absorption in the rectum. By regulating these two processes insects maintain water balance.

A locust feeding on wet food produces faeces with a water content of 80%, but a water content of only 20% when feeding on dry food.

In Tenebrous all water from the rectal lumen returns to the haemopymph.

Hormones. Neurosecretory cells in the brain regulate the fluid produced by the Malpighian tubules. Generally feeding stimulus, e.g. stretch receptors - trigger neurosecretory cells to release diuretic hormone promoting water loss. The diuretic hormone causes an increase in Cl- permeability of the Malpighian tubules, decreasing the already negative potential of the tubules which is maintained during fast fluid secretion.

Anti-diuretic hormones may also exist to increase water uptake from the rectal lumen, or perhaps the absence of diuretic hormone is enough as the Malpighian tubules then secrete at very low levels in the absence of diuretic hormone.

Why is the cultivation of crops inevitably associated with pests?

It is estimated that without insect pests world food production would be increased by about a third. Thus in spite of all control measures we still lose a third of our crops to insects. Each insect is fairly small and eats only enough to fill its gut, but even so it only takes one insect to blemish a whole apple or transmit a disease to kill a tree. The most noticeable damage is done by large populations of insects. For example one hectare of oats may be home to 22 million fly larvae and 222 million black bean aphids can happily live on one hectare of beet. In the wild it is rare or never that such outbreaks occur. So why do they occur in crops?

1) By monocultural, or near monocultural cultivation man make an unapparent plant apparent. The theory of plant apparency was developed by Feeny, Rhodes and Cates. It states that unapparent plants (most wild relatives of crops) are relatively difficult to find, not available throughout the year, and invest little in chemical and physical defence. Their defence is that they won't be found. They breed rapidly and produce lots of seeds. Man has taken these plants and selectively bred them for seed, tuber, fruit etc. production, and densely planted them over miles of countryside while eliminating most other vegetation. What could be more apparent? In the wild if one plant was discovered it may have been badly eaten and died, but others would have lived to reproduce. With crop varieties the pest insect is surrounded by a sea of food, the next plant is the same as the one it is on, and so on. The situation might not be so bad as an insect cannot eat so much, but insects can breed.

2) Certain pest species breed enormously fast (r-seleceted) under favourable conditions. One aphid can give birth to 40 others in a few days, and some morphs are parthenogenic so don't even have to waste time on sex. When one plant gets crowded or begins to wilt they move. In a crop this move is just to the next plant. In the wild mortality is high during host transference. The winter moth larva balloons on a thread if it hatches before budburst. This is one of the factors that produces highest mortality. But if they are in an apple orchard in Nova Scotia the next tree in any direction is as far as they need to go. Studies showed that infections of sticky banded and unbanded trees were the same, i.e. had no relation to the number of eggs on a tree. Insects lay lots of eggs, often near or on the food plant. So the larva must colonise the food plant to survive and reproduce, but again if they are not food limited they have a much higher survival rate and may swamp predators and parasites.

3) Many crops are planted in the same field again and again. This is perfect for building up insect numbers as they may overwinter in the soil or stubble, then hatch and have to find food. How convenient of man to have planted some right next to the eggs. Watercress growers in Japan reported that the diamondback moth had become resistant to Bacillus thuringiensis (a bacterial insecticide). They had been growing watercress (a very fast maturing crop) in glasshouses continuously for 3 - 4 years, and in that time Bt had been applied 40 - 50 times! The moth was under such strong selection pressure no wonder resistance was the outcome.

4) Pest control. Insecticides etc. are never 100% effective, so there will always be a few escapees (see the diamondback moth above), and the escapees may have some mutation that makes them resistant, so they breed and the next generation are all resistant. The insecticide may have killed all the insects or even all arthropods, so now the new resistant generation are surrounded by a sea of food and most of their predators, e.g. spiders, carabid and staphylinid beetles are either dead or not yet adults. The pest control has led to pest outbreak such as has happened to rice leaf hoppers.

5) Many crops are introduced with their pest but without the pest's predator. For example the winter moth in apple orchards in Nova Scotia. The cotton cushiony scale insect in California. Both of these had happy endings for man, but for a while the insects came close to ruining an industry. Sometimes success is not so easy as just introducing a predator. For example tea in Sri Lanka is an introduced crop. A tortrix moth became a pest, s a parasitoid was introduced with marvellous success. Then a boring beetle became a pest on the crop, so dieldrin was used and was a greta success in reducing beetle damage, but unfortunately it facilitated outbreaks of two caterpillars which multiplied to become even greater pests that the beetle had been. Spraying of dieldrin was stopped, the damage lessened, and a new method of control is being searched for.

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.
3) 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.

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