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Masters
of disguise 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.
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 relationship between ants and aphids
is beneficial to both. Some species of aphid have special
hairs to hold droplets of honeydew until an ant can
collect it. Woolly alder aphids belong to this group.
Aphids who are tended by ants leave all their
security problems to ants and are therefore
less able to defend themselves than those that are not
tended thus. 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. |