Sunday, October 21, 2001		Nature

THE survival of the young is best assured by laying eggs directly on food. Many insects have evolved ingenious methods of achieving this goal. Insects whose larvae feed on large mammals have a greater problem as most mammals have developed ways of removing parasitic eggs from their fur. Grooming, washing and wallowing in mud are all habits that make it difficult for insect eggs to survive for long on a prospective host.

A South American botfly has evolved a roundabout solution to the problem. Instead of laying her eggs directly on a host, the female botfly catches a bloodsucking fly and sticks her eggs along its abdomen. The bloodsucker then carries the eggs with it to its next meal. As it feeds, the eggs hatch instantly and the botfly larvae burrow into the host’s skin.

Certain insects have such efficient adaptations that they not only lay their eggs near food, but use the same material to provide shelter as well. Champions among them must be leaf-rolling weevils and the gall wasps.

The female birch-leaf roller makes two neat, S-shaped cuts across a birch-leaf, one on each side of the central rib. With her legs, the weevil rolls half the birch leaf into a cone, then wraps the other half round the cone, in the other direction. Upon completing the structure, she goes inside and lays a number of eggs in slits on the leaf’s inner surface. Finally, she tucks a fold over the end of the leaf cone to seal the eggs in safely and provide food and protection for the young when they hatch.

Gall wasps save themselves the trouble of building by getting plants to do it for them. They lay their eggs in plant tissues, often at the tip of a shoot or within a leaf bud. When an egg hatches, the larva begins eating the material around it, stimulating a growth of abnormal tissue.

In time, the larvae are surrounded by a mass of large cells—a gall. On its inner surface, the gall produces tissues rich in starch, sugar and protein that the larvae feed upon throughout their growth.

The parent predators give their offspring a good start in life by depositing their eggs on the right plants. Ladybirds like aphids and they place their eggs on the underside of leaves close to aphid colonies. When the slate-blue larvae hatch, they head upwards, for up is where the flowers and the aphids are. During their three weeks of life, the larvae devour hundreds of aphids, piercing the skins and sucking them dry. Then they pupate and re-emerge as adult beetles, which also feed on aphids.

The small, soft-bodied slug-like larva of the hover fly is also a predator of aphids. Like the ladybird, the egg from which it emerged is deposited close to aphids. It can detect light but has poor eyesight and, therefore, must find its prey by smell. Moving across a leaf on its flattened underside, it glides along on saliva. It grabs an aphid with its mouth, hooks and rears up, holding the unfortunate fly high in the air. All soft parts inside the prey are sucked out, leaving the shrivelled exoskeleton behind. And having scoffed one aphid, it goes on to eat many, many more before pupating and changing, by metamorphosis, into the nectar-sipping adult.

Lacewings, both adult and the larvae, attack aphids, and the larvae of certain species of green lacewings, having finished their meal, show curious behaviour : they take the skin of their victim and place it on their back. The debris seems to function as a shield against predators, but not all lacewings have them. By the time an individual has consumed 100 or so aphids, it has a considerable load to carry. After pupation, adult green lacewings discontinue the debris-carrying habit, but their adult brown lacewing cousins might go on to consume an average of 315 aphids during their short lifetime. Lacewings, hoveflies and ladybirds are clearly a gardener’s friends and are important insects in the biological control of pests.

Larger animals also contrive to surround themselves with their food supply. The spiny moloch lizard sits beside ant trails licking up to 20-30 per minute and consumes, in an average meal, about 1800 ants. Aardvarks, aardwolfs and bat-eared foxes in Africa, ant-eaters in South America and echidnas in Australia have to work a little harder. They are all great consumers of ants and termites, but they must dig into insects’ nests until they reach the bonanza.

This feature was published on October 14, 2001