Sunday, December 8, 2002		Nature

Of parasites & their hosts
Nutan Shukla

THERE are various kinds of stimuli to which animals must be able to react in order to survive the changes in their environment. The responses an animal makes are generally designed to protect it from harm, to allow it to reproduce and to maintain it in favourable conditions. The sum of these activities is called the behaviour of the animal.

Many invertebrates respond to heat stimuli. This is of particular importance in the orientating responses of animals that are parasitic on warm-blooded animals. The warmth is only a ‘token’ stimulus of the host’s presence as the parasite is not seeking warmth but food. Many blood-sucking ectoparasites do not remain on the host once they have sucked enough blood to satiate them. They only respond to the stimulus of the host’s warmth when hungry.

The temperature range to which certain parasites respond is often quite specific. The stories of lice and fleas deserting the body of a host before it dies are an example of this. The parasites are attracted to the host by its normal body temperature, but if this temperature rises due to a fever the parasites no longer receive the stimulus to which they are adapted and leave the host.

Chemical stimuli are used by many parasites to locate their host. A fish leech senses the presence of a fish by the vibrations caused by the fish as it swims. But before the leech can attach itself to the host a secondary stimulus, a chemical secretion given off by the fish, must also be present.

The solitary wasp Idechth is parasitic in its larval stage on the larva of the flour moth. The wasp lays its eggs in the eggs of the moth. A danger of parasitism is that if too many parasites are present in one host the host may die and so will the parasites. To ensure that this does not happen, the female wasp will not lay eggs in the eggs that have the characteristic odour left by another wasp. If the eggs are washed free of this odour the wasp will begin to lay in the eggs but will detect the presence of the other eggs with her ovipositor and stop laying. Idechtha can locate a culture of the flour moth from a distance of 800 metres.

Certain moths are renowned for the ability of their male to locate a female even when a female sex pheromone (a chemical) is released at as much as four kilometres from the male.

Sea-anemones and hydra belong to a class of animals known as coelenterates. They have a tube-like body with a mouth at one end and surrounded by a circle of tentacles by means of which they catch their food. The other end of the body is used to attach them to the substratum and for locomotion. The tentacles are very sensitive to touch, if one touches them the sea-anemone responds by rapidly withdrawing them. Tentacles have specialised cells called nematocysts which are thought to be able to respond to stimuli of certain chemical substances and touch without control from the animal’s nervous system. These cells shoot out barbs that paralyse the prey and long threads that wraps round it. As soon as the prey touches a tentacle other tentacles move over and grasp it, and it is then moved to the mouth and swallowed. Once this pattern of responses has started, removal of the food and even a change of water will not necessarily cause the animal to stop completing the pattern. This is an example of an external stimulus triggering off a pattern of responses which the animal then executes in the absence of the original stimulus. A hungry hydra will respond to a tactile stimulus only, but a satiated one requires a tactile and chemical stimulus to trigger off the response.

An animal will sometimes be seen to make an alarm reaction to a stimulus which, although not potentially harmful to it, is strange. If it persisted in making such responses to all strange stimuli whenever they were perceived the animal would not perform efficiently. In order to eliminate such useless responses, some animals exhibit a type of behaviour known as habituation, by which the animal becomes adapted to a strange stimulus and no longer responds to it.

The various types of behaviour described can only give a brief illustration of the very varied methods by which invertebrate animals, possessing a very simple nervous system, respond to stimuli from their environment. These simple forms of behaviour play an important part in ensuring the animal’s survival in an environment that is potentially and may actually be hostile.