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Sunday, March 9, 2003
Nature

How digger wasps ensure their survival
Nutan Shukla

PARASITIC in nature, digger wasps ammophila adriaansei are among the most industrious and efficient solitary insect predators. The females, in particular, do everything possible to ensure the survival of their offspring. However, they themselves do not live much longer after the eggs are laid. During the breeding season, her first job is to search for a suitable location in the ground or in dead wood where she can dig a hole to make a nest and then, having carefully covered the opening with small stones or pieces of earth, she goes off to seek prey. She makes provision of food for her young ones before the eggs are laid.

These wasps are very dangerous, specially to large spiders, including tarantulas (common name given to most large spiders, many of which have hair that produce rashes if touched. Some species are poisonous too) because they are their choicest food. However, the predatory wasp also preys upon caterpillars.

Tackling a caterpillar is quite an easy job but the spiders do not give up easily. Being predators themselves, they too have the killer instinct. In this case it is very natural that whenever these creatures come face-to-face, they grapple with each other. In a seemingly one-sided fight, the small but agile wasp is able to outmaneuver and overpower its victim, even the tarantulas, which are very large as compared to wasps. While the fight is on, the wasp avoids the spiderís formidable, poisonous fangs and stabs it with its sting, located at the tip of her abdomen.

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Unlike snake venom, the waspís nerve poison acts with great precision and selectivity on the central nervous system of the victim. It immobilises the spider by paralysing those muscles that are used for the purpose of locomotion, but it does not create any difficulty in breathing or in blood circulation. In this way the victim is not killed, it becomes a living food store for the waspís offspring. She drags the immobilised victim to its nest, removes the stones and hauls it in after her. It lays an egg on the victimís body, climbs out of the nest and replaces the stones to close the entrance. When the wasp larva comes out of the egg, it takes its first meal from the packed food that the mother has left for it.

Besides immobilising the prey, the other function of the waspís venom is that it has an anti-bacterial property that prevents the normal decomposition of the body. Consequently, the food remains fresh until it is consumed and the larva is ready to pupate.

Some species of digger wasp make several nests while on the move. In these cases, the female must, therefore, remember exactly where each nest is. So, before setting off from its nest hole it flies in two or three circles to memorise special landmarks, like a stone on the ground next to a tree or a fallen pine cone. Having remembered where each nests is, she also seems to know intuitively how much of the food supply is left, and she will keep on replenishing the nests until the larvae are fully grown and about to pupate. The female wasp has performed her duty and she does not survive much longer. After a few days, the pupae emerge as adult wasps and the life cycle goes on.

The fact that digger wasps remember the landmarks around their nests was established by G. P. Baerends, a Dutch scientist. To study the behaviour of these insects he performed very simple experiment in which he put fir cones round a female digger waspís nest hole, then moved them a short distance away to one side after she had left. Baerends observed that on her return, she went straight to the centre of the cone circle rather than to her nest, which she found later only after a thorough search.

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This feature was published on March 2, 2003
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