Sunday, September 1, 2002		Nature

Female spider judges prey from vibrations in the web
Nutan Shukla

THE sense of touch is capable of giving accurate information, particularly of a comparative kind. In humans, all parts of the body surface are not equally sensitive to touch. The fingertips, the lips and the palms of hands, which we use constantly in touching, are among the most sensitive parts. Animals also use this faculty in different ways to their advantage.

A remarkable example of touch in insects is the ant-lion, an insect common in the southern deserts of the USA. It lies at the bottom of a pit of sand and waits for ants to land on the edge of the pit. A few grains of sand falling on the ant-lion triggers off its sensitive nervous system, and it begins to shower the ant with grains of sand. In its state of confusion, it becomes easy prey for the ant-lion.

The activity of the queen bee is also triggered by touch. She wanders over the face of the honeycomb, weighed down by her enormous egg-filled abdomen, feeling the cells of the honeycomb to decide what type of egg to lay in each. If the cell is small, a reflex of her nervous system releases a valve inside her reproductive organs and allows a few sperms to pass through to fertilise the next egg. Shortly afterwards, she deposits the egg in the small cell, where it will eventually become a worker bee. If the cell is large, the reflex does not operate and an unfertilised egg is deposited which can develop only into a drone.

Many spider species extend their sense of touch by building webs so that they can detect instantly any insect which lands on the web. In the spider, the sense of touch is specially well served by sense organs on her legs, and she is able to sit at the edge of the web and sense what is happening over a wide area.

The female spiders which build webs are sensitive to the size of the movement in the web. If the vibration is too small, the spider will not respond. On the other hand, if a large insect, a beetle, for example, gets caught in the web, the spider will cower in a corner of the web while a considerable part of it is destroyed by the beetle’s struggles.

In human beings, the sense of touch is sufficiently well developed to make it easy to tell a smooth pane of glass from one that is etched to a depth of only 1/2500 inch. This ability has been put to professional use by ‘cloth-feelers’ whose job is to feel cloth to tell its quality and type. Many of these men can tell the exact type of a cloth merely by rubbing it with a stick. Others can distinguish a particular type of cloth even if the only contact they have with it is a momentary tap with a fingernail.

Unlike that of the primitive paramecium, but like that of most higher animals, the human sense of touch is linked with and mediated by a complex nervous system. Cells within skin are capable of reacting to touch, pain and pressure. These are linked with fine nerve-endings which transmit messages back to the brain. The activity of the brain itself influences what is felt.

For humans, of course, touch is not the most important sense. It plays a large part in the minutiae of our lives, but has relatively little survival value.

One stage of life in which touch plays a predominant part, however, is in the very young child. A baby gets most of his information about his restricted world from the things he touches. His contact with his mother is even more important. Unless he is nursed and fondled during these early months and years, his development will be impaired.

Experiments with animals have shed light on this problem. A third of the rats in one experiment were left in their cages without being handled at all. Two other groups were placed by hand at intervals into special boxes, where one group was given mildly painful electric shocks regularly while the others were merely left alone once inside. The interesting result was that the two groups of rats that had been handled, both those that had been given shocks and those that had not, were friendly and ‘tame’ when they grew older. It was the third group, the unhandled, unstimulated rats, that showed a different behaviour. As they grew up they covered timidly in the corners of their cages, showing all the signs of fear and anxiety. When they grew older, they underwent brain surgery which turned them into extremely vicious rats. The other rats, after similar surgery, remained comparatively tame. It appeared that the presence of stimulation rather than its quality was the vital factor.

This experiment, with emphasis on the role of the sense of touch in the development of sociability, leads on to the consideration of the role of the mother in raising normal children.