Opportunistic predators

Opportunistic predation is known in many species of quite diverse sorts of insects. The adults of some species of plant-eating grasshoppers and crickets are notorious carnivores whenever they get the chance. Often they are cannibals, and commonly scavenge on dead bodies of both vertebrates and invertebrates.

Another related and widespread, but usually overlooked behaviour of young arthropods (insects, crustaceans) is that of eating the shell of the egg they have just hatched from. However, a recent study of the caterpillars of a butterfly which feed on brassica plants in Brazil has shown how important a supplement this is for animals which otherwise eat nothing but plant material. And highlighting this is a twist to the behaviour that occurs if these flush-feeding caterpillars hatch on old, poor-quality leaves in the field, or in large batches of asynchronously hatching eggs in the laboratory. The first caterpillars to hatch do not stop at eating their own shell but quickly turn to cannibalising neighbouring eggs.

The shell of an insect's egg is pure animal protein, and in the case of these young caterpillars, amounts to a meal equal to 50 per cent of the neonate's weight. So the Brazilian scientists reared batches of these caterpillars on fresh leaves of kale without letting them first eat their eggshells, and contrasted their survival and growth with matched batches of caterpillars that were left to eat their eggshell before being given kale leaves. Many of those thus deprived did not survive beyond this first stage out of the egg, whereas most of those eating their eggshells did. What is more, the benefit of doing so flowed on. They weighed more, grew faster and became bigger butterflies which laid more eggs, than their less fortunate fellows. Clearly this initial meal of animal protein is of enormous value to these herbivores.

A related behaviour also widespread among arthropods, is that of eating their cast skins. All arthropods have to shed their external skeleton to grow, and they do this several times between hatching from the egg and metamorphosing to an adult. This, too, is a behaviour largely dismissed as being of any nutritional benefit to the animal. But a recent study in Oxford changed this perception. The work was done with colonies of the ubiquitous American cockroach maintained for 20 years at the University. This animal is not a herbivore, but a highly successful and adaptable omnivore - they will eat just about anything organic! Yet in nature they mostly have to get by on food that does not contain enough protein. To help overcome this shortage they have a large gut fauna of bacteria, and also carry specialised endosymbiotic bacteria that recycle the roaches' waste uric acid to usable amino acids.

The growing nymphs always eat their cast skin after each moult while only some of the newly emerged adult insects do so. Adult females, however, do so significantly more often than do the males.

The scientist who did this study found that the cast skins consisted of up to 87 per cent nitrogen that would be lost to the animals each time they moulted. By eating them, however, he found that they recycled between 60 and 70 per cent of this nitrogen. To see if the amount of protein in their diet influenced the frequency of this behaviour, he did two things. He raised one lot of cockroaches after killing their endosymbiont bacteria with antibiotics. Then he fed them, and a separate lot still possessing their endosymbionts, on a range of synthetic diets that differed only in the amount of protein they contained.

The results were fairly clear-cut. On the lowest nitrogen diet 90 per cent of the animals died before they became adults. All those that did survive, however, both male and female, ate all of their cast skins. By way of contrast none of the adults raised on a high nitrogen diet ate all of their cast skin. A third of them ate part of it, but 50 per cent of the females and more than 60 per cent of the males did not touch them. Those without endosymbionts ate significantly more skins than those not so deprived.

So, if you are a cockroach not getting enough protein in your food, or lack bacteria to help you gain more, you will attempt to counter this by recycling more of the otherwise wasted nitrogen in your cast skin. And if you are a female which has to produce young you will be even more inclined to do so, with or without the help of symbiotic bacteria. If, on the other hand, you have access to a nitrogen-rich diet, you will be much less likely to bother.

Small sap-sucking insects called thrips have been found to pierce and feed on mites' eggs whenever they find them on the surface of a leaf. Mostly they do this when they are fast-growing immatures. When they gain access to this supplement of animal protein they grow much more quickly, and far more of them survive to maturity. When adult female thrips are able to eat these mite eggs they live much longer and lay many more eggs than those confined to a pure diet of plant sap.

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