Touch

Touch is a stimulus widely used in the animal world but it is less apparent in plants. There are, however, two well known examples of plants which are sensitive to touch. The Venus fly-trap is stimulated to shut its leaves when an insect touches the sensitive trigger hairs on its surface. When the Mimosa pudica plant's leaves are touched they rapidly fold up.

Since touch involves an animal being in physical contact with another animal, it is a stimulus which is often used between members of the same species but it is not likely to be used between members of different species. Two exceptions to this are shown by insectivorous animals and jellyfish, both of which find their food by touch.

The noses of some animals, such as those of the shrew and the mole, have very sensitive hairs which are able to detect a worm only 0,1mm long. The jellyfish's tentacles react to touch by activating special cells which not only produce a poison to stop the prey moving, but also surround the prey with 'threads' which effectively tie it up.

Many social animals use touch as a method of communication. Young bees use their antennae to stimulate the worker bees to produce food. The young bee touches the mandibles of the older bee with its antennae. When the older bee is ready to stop giving food it lowers its own antennae so that they touch the antennae of the younger bee. Since it is dark inside the hive, the bees use touch as their main method of communication.

Monkeys spend many hours searching through each other's fur for ticks and lice. Apart from the obvious advantage of this action, touching each other also has a role in keeping the group together and in identifying individuals.

Touch can be very important when certain animals mate. The male wolf spider has to enter the burrow of the female in order to mate with her. In the dark burrow, visual stimuli would be ineffective. The male wolf spider uses his front legs to touch the female in such a way as to inform her of his intentions. If he does not do this correctly he is in danger of becoming her next meal.

The ability to sense vibrations is particularly strong in spiders which spin webs. These spiders have long, fine hairs on their legs which pick up the vibrations caused by insects flying into their webs. Following these vibrations helps the spiders to locate their prey. Birds are able to detect small vibrations in the twigs and branches on which they are sitting. This is useful because it helps them to escape from predators when it is dark.

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© Paul Billiet, Shirley Burchill, Alan Damon and Deborah James 2009