Sunday, December 22, 2002		Nature

Crustaceans on the move
Nutan Shukla

ALL types of barnacles, crustaceans that have evolved a protective shell and live attached to rocks or ships etc., have one thing in common that they cannot move from one place to another until they attach themselves to any moving animate or inanimate object. To colonies new places their fertilised eggs hatch has free-floating larvae, drifting in the ocean and feeding on tiny plants, building up fat reserves. After about three weeks or so these tiny creatures stop eating, transform into site-seeking larvae and search for a place to settle.

Once a suitable site is found these free-floating animals start congregating by means of an ingenious mechanism. Larvae of barnacles have antennules that feel around on hard surfaces for a good place to settle, in order to metamorphose into adult barnacles. Since all species like company of their own kind, they react positively to the presence of other barnacles of their own species, or to the traces of cement left on a rock by other barnacles. These traces are good enough signs for the new larvae to settle down because if any spot suits one barnacle it is good enough for the others too.

There are exceptions to this; these larvae usually avoid a smooth surface like glass, even if other members of their species are fixed to it. If a rough surface is available nearby they will prefer it. Thus the distribution mechanism seems to be a combination of response to the texture of a surface and a more specialised response to the clues left by other barnacles. The total effect is to keep barnacle colonies only in those areas where conditions for development are good.

Many winged insects are distributed over wide areas by a behaviour pattern that they exhibit just after they have metamorphosed. At once they are impelled by some inner mechanism to fly upwards, or in some particular direction which may be determined by the angle of the light at the moment of departure. In some insects, especially aphids and locusts, this migration is a highly developed dispersal mechanism and enormous numbers of newly emerged insects swarm the air. Both of these insects fly upwards and get caught in rising air currents, which sweep them away, high into the sky.

Pilots of planes flying at 1,000 to 5,000 feet recount how masses of insects float there on a warm day. But in the evening the air cools, and the sky becomes clear again. The insects fall slowly to more normal levels, and there green leaves attract the aphids. They land, and if the leaves are a suitable breeding ground they stay there, living out the rest of their lives in a much less exciting way. The drives which make different types of insects fly upwards are varied. Aphids are attracted by the sun’s ultra-violet light, and the Scolytid bark beetle is attracted by the brightness of the sky. But whatever the drive, there is a brief period after emergence when it overcomes all other drives, including sex and hunger drives. In the case of aphids, the drive is soon replaced by the drive to find green foliage, but the bark beetle responds to height in a different way. On the way up it swallows air, and the size of the air-bubble in its stomach seems to signal a change in response — it turns from the light, and heads towards pine trunks, which are its breeding grounds.