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Sunday
, April 28, 2002
Nature

Animals with chemical weapons!
Nutan Shukla

USE of chemicals as a means of defence rather than offence, is commonly found in the animal kingdom and and species that are highly colourful are usually those that are possessing chemical weaponry. Animals that produce unpleasant smelling or tasting secretions are generally colourful(warninglycoloured) to advertise their distastefulness. Specially, among insects many species synthesise their own chemical compounds whereas others get them from plants or animals they feed upon.

Certain grasshoppers exude repellent substances from their abdomen, and millipedes actually produce hydrogen cyanide from pores along their body. Wasps rely on black-and-yellow stripes to warn of poisonous stings, and many brightly coloured caterpillars advertise their ability to cause irritation and pain if the hairs on their bodies are touched.

Larvae of the sawfly feed on the leaves of pine trees. When threatened, a larva will emit a drop of fluid from its mouth and daub it over its attacker. This fluid smells like pine resin and chemical analysis has shown that it is indeed pine resin. Resin is produced by pine trees for the very purpose of deterring insects from feeding on them. Most insects cannot cope with chemical, but a few have become adapted to do so, and the sawfly larva is one of them. As the insect feeds on pine leaves, the resin is stored in a pair of pouches in the gut. When it is daubed on to an insect attacker, the resin acts as a powerful deterrent. Hence, in this species, a chemical compound produced by a plant to ward off insects is taken up and used as part of one insectís own defences.

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Even more bizarre is the case of one of the species of flightless grasshoppers. It Producesa frothy fluid from the front paid of thoracie (part of the body between neck and abdomen) respiratory openings (spiracles), the froth itself being generated by mixing the respiratory air with the liquid. The secretion thus produced contains several different chemical compounds including 2, 5- dichlorophenol. This chemical is a constituent of a herbicide commonly used in the area of Florida where the grasshoppers occur. It seems, that the insect has adopted a man-made herbicide and added it to its armoury of chemical defencesystem, the next best thing is to pretend that you possess it. Harmless animals take on the appearance of poisons ones and confer on themselves immunity from being attacked. The larvae of monarch butterflies in North America feed on milkweed plants and, in doing so, gain protective poisons that are separated and stored by caterpillar. The caterpillar pupates and, after meta- morphosis, the butterfly emerges complete with a chemical defence, for the milkweed poisonous are retained.

The viceroy caterpillar looks completely different from that of the monarch and feeds from fairly innocuous plants. But when the viceroy butterfly emerges, it looks exactly like a monarch. The only difference is that it does not have the poisons. The viceroy mimics the monarch in order to confer immunity to attack; and it works. In another monarch butterflies have been presented to scrub jays. The birds might take one butterfly, but the reaction is so violent that refuse to touch monarch again. Presented with the harmless viceroys, the birds behave in the same way. They will not touch them.

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