Sunday, July 21, 2002		Nature

Housefly tastes food through its feet
Nutan Shukla

ANIMALS with poor or non-existent vision rely heavily on their senses of smell and taste. Using their olfactory powers (olfaction is the sense of smell), they learn to recognise their territories, can track their food and avoid predators. Sometimes, too, mating is contingent on the male being able to smell when the female is ready and receptive for him; the cat on heat, for example, releases, a strong sex stimulant.

Some animals have extraordinary powers of smell. The salmon can find its way back across miles of ocean to the fresh water river where it was reared. It is believed that once back at the mouth of the river, the salmon finds its way back to the spawning ground by remembering the smells of various parts of its outward journey.

Smell and taste are very much bound with each other, and when we talk of ‘tasting’ a substance, particularly something with a lot of flavour, we very often mean that we are ‘smelling’ it. We have only to have a heavy cold, or to hold our nose while eating, and we will find the food tasteless. The reason is simple; the tongue can only distinguish between four main classes of flavour — sweet, sour, bitter and salt. The other flavours that we taste are in fact selected by our noses.

Although most of us can tell between good and bad smells, a sense of smell is not as important for humans as sight or hearing. The aroma of good cooking or a subtle perfume will certainly arouse us, but we will not, like the dog, be able to track down our food simply by sniffing the ground or air. In fact man, together with the apes and most marine mammals, is somewhat exceptional among the higher animals in having a relatively poor sense of smell.

Even the lowest forms of animals — the single-celled protozoans — can react to changes in their environment, and they will either be attracted towards a pleasant stimulus, like food, or try to escape a noxious one. Thus, squirting a drop of dilute acid near the protozoan Parameciim makes it swim furiously away. Every substance is basically chemical and, therefore, capable of reacting with other chemical compounds. The ability to detect such a change in the environment depends on a process called "chemoreception’: the most elaborate forms of chemoreception are the sensory cells of taste and smell.

Insects have probably developed the most sophisticated ability to smell and taste. Very often communication between one insect and another is effected by means of substances that are liberated by one insect and picked up by another. Thus bees and ants depend on smell and taste to recognise members of the hive or nest, and any intruder is promptly dealt with. The sense of taste is very often on the tip of the proboscis; the housefly is able to sense the presence of sugars or salt through special receptors on its feet.

Sometimes antennae are used for taste, as in the wasp; more often, they are used to distinguish smells. A cockroach can therefore smell its food with its long, pointed antennae and will follow the trail to a piece of cheese. A few insects, like the cabbage white butterfly, smell with their ‘palps’ which are small projections on their mouths. Whether they are for taste or smell, the sense organs have very similar structures and they are, in fact, derived from the same cells, the ectodermal cells. They are connected to nerve fibres which run to the insect’s brain, and its behaviour can be completely governed by what it smells. For example, the male silkmoth is endowed with large feathery antennae, and these are so sensitive that they can pick up the scent of a female silkmoth that may be several miles away.

It has been calculated that only a few molecules of the scent need strike the antennae for them to be stimulated. This phenomenal ability to smell the female is one way in which reproduction is ensured. The adult silkmoth does not feed and at most, only lives for about nine days after emerging from the cocoon. In that short span of time, the silkmoth must mate and lay its eggs. To attract the male, the female thus secretes a volatile substance into the air from special glands on her abdomen. This substance — called a pheromone because it is a secretion of one animal that can change the behaviour of another animal — is the substance the males are so sensitive to, and once they have picked up the scent, their sole object in life is to find the source and mate with the female.

Cockroaches live for much longer than the adult silkmoth and they go through several reproductive cycles. The female only mates at certain times when she releases a pheromone and only then is she attractive to the male. If a filter paper touched by a receptive female is put into a cage containing male cockroaches, they immediately go berserk and clamber over each other to get to source of the pheromone.

The secretion of the pheromone is under the control of hormones and these hormones are released from glands associated with the brain of the insect. There is thus a tie-up between the season, the year, the animal’s reproductive state, and the animals’s behaviour — and as we have seen smell plays an important part.

The gypsy moth produces a pheromone known as gypsol and this has now been synthesised by chemists. In the USA, plagues of gypsy moths are controlled by putting out containers of synthetic gypsol. The male moths are attracted to the containers, where they are trapped and can be destroyed.

Female mammals on heat, also produce substances to attract the males. It is believed that women too release a pheromone-like substance when they are at the most fertile periods of the menstrual cycle. If they do, it is unlikely that men have sufficiently good noses to know.