Sunday, February 6, 2000		Nature

Masters of disguise
By Nutan Skukla

DECEPTION and disguise is an art used not only by humans, but also by other creatures, including insects. The larvae of the North American lacewing fly, which lives among and feed on the woolly alder aphid is one such master of disguise.

Aphids are a group of tiny insects (bugs), also known as plant lice, which are a serious threat to farmers and gardeners as they deform plants by sucking their sap. They excrete copious quantities of sugary honeydew, a solution rich in carbohydrates, which is consumed by many organisms, including ants. Aphids are not attacked by ants, instead they are ‘milked’ for the honeydew, for which they are given security against predators by ants.

To fool these ‘security guards’ the larvae of the North American lacewing fly cover their bodies with the waxy ‘wool’ which they remove from the aphids. Lacewing larvae are then mistaken by the ants for aphids and left alone. But if a human investigator strips the lacewing larva of its wool it is immediately attacked by the ants and removed from the aphid colony.

The ants guard their aphids, attacking and driving off other insects that are parasitical. In some cases, the size of the aphid colony, and the growth and maturation of the aphids, are controlled by their ant caretakers. Aphids may also be kept in the ant colony over the winter and carried to new food plants when they begin to grow again in the spring. Some ant colony queens carry a honeydew insect with them in their mandibles during the nuptial flight, when they leave the parent colony, mate and find a new colony.

There are many species of aphids: There may be upto 5000 million (500 crore) of them feeding from vegetation in just one hectare of the countryside. In most species, sexually reproducing males and females occur chiefly in autumn. In spring and summer reproduction is by parthenogenesis which means there is no mating, females producing daughters genetically indexical to themselves. The rate of reproduction is so rapid that a single female can theoretically give rise to six hundred thousand million offspring in a season. An aphid weighs about I mg and a spring female therefore has the potential of producing six hundred thousand kilograms of offspring by the end of the year, all of them identical to herself.

From the evolutionary point of view, aphids produced in this way are all the same individual. Hence when a predator attacks and eats an aphid, it is merely removing a tiny piece from one gigantic individual; it does about as much damage as a caterpillar does when it chews the leaves of a large tree. In the evolutionary sense, and this is what matters when we are speaking about the adaptive significance or anti-predator strategies, individual aphids are rarely if ever killed by predators: parts of the individual always survive and reproduce and the effect of predators on the total genetic unit is minimal. Aphids are therefore a special case. It now becomes clear why their more conventional anti-predator strategies are so poorly developed: They can afford to be eaten.

Besides the above mentioned anti-predator strategies different species have adopted varied strategies. Many aphids are green and are presumed to be camouflaged against a leafy background; others are black and easy to see, or whitish, and do not appear camouflaged. A few are coloured. One species ‘Aphis nerii’ is bright yellow-orange.It feeds on Oleander and other related plants rich in poisonous compounds known as cardenolides which the aphids store, thereby making themselves unpalatable to predators. Such secondary defences as aphids do have involve both escape and intimidation. Most species face downwards and since insect predators tend to walk up a stem or leaf they meet the aphids head-on which gives the aphids a better chance of taking evasive action. When attacked, an aphid kicks itself free, or atleast tries to do so, or walks away. Some aphids drop to the ground, which is hazardous as death from starvation is almost inevitable.