Zoological Notes: The silent red threat of the black widow

THE BLACK widow spider is famous for its most mysterious trait: its bite is toxic enough to kill animals thousands of times its own size. Including, occasionally, a human being.

Widow spiders live in most of the temperate and tropical regions of the world, though they are not native to the United Kingdom. Female widows around the world have the same shape: a bulging globe of an abdomen appended to a much smaller front section, from which eight wiry legs protrude. The colour of the females varies from place to place. Florida has a red widow; Israel has a white version. But the most common look for a widow is a black body with red markings, including an hourglass-shaped mark on the belly.

The males run much smaller - sometimes one-fiftieth the weight of a female. The males differ from the females in colour, too. In one species of "black" widow, the males are tan and white. The name "widow" refers to the females' sometimes abrupt termination of the courtship. In some cases the male seems to actually volunteer himself to the fangs of the female. Cannibalism during mating may strike us as a breach of etiquette but it's common among spider's. It has always seemed unfair to me to single out the widow for condemnation on this point.

Widows are not especially aggressive toward large animals like you and me. It's hard for a person to get bitten without actually pinching the spider. Unfortunately, it's easy to pinch a spider by accident. The widow's deadly poison might have remained virtually unknown if it weren't for the outdoor toilet. This happens to be an ideal habitat for widows - dark, attractive to insects, and shaped to accommodate a large web. Medical records from Madagascar, Australia, and the United States relate hundreds of encounters between human and widow in the setting most likely to inspire us with Freudian terrors. In the United States, reports of widow bites decreased dramatically, as indoor plumbing became the standard.

What happens to a person bitten by a widow? The blood pressure plummets, then rises to dangerous heights. The pulse races. The body sweats. The belly goes stiff. A plethora of other symptoms have been reported - swelling, convulsions, nightmares, psychotic hallucinations. The one symptom always mentioned is pain. Some doctors have called widow envenomation the most painful condition known. The venom somehow turns receptors in the nervous system, sending cascades of agony through the entire body. Despite this horrific array of symptoms, death is an unlikely outcome. Even before scientists developed an antivenin, the death rate from widow bites was only about 4 per cent.

We human beings are not unusual in our sensitivity to the widow's toxin. Mice and rats, cats and dogs, even horses and camels can die from the venom of a single spider. The widow has the unusual distinction of being poisonous as well as venomous: an animal that eats the spider is likely to get sick. Such a fate can easily befall grazing animals. Sheep, incidentally, are virtually immune to the widow's toxin, probably because non-resistant sheep in widow country don't often live to reproduce.

The widow, which rarely exceeds two centimetres in length, eats almost any small creature it can catch in its irregular web - insects, other spiders, small lizards and snakes, even an occasional mouse. Why an animal with such a diet should possess the power to kill animals as large as the camel and the horse is a mystery. One theory holds that the bright red hourglass on the black body of a typical widow is a warning. It's a common occurrence in nature - a distinctive colour pattern advertises danger, and something about the animal delivers on the threat. Perhaps the black widow's venom is the ultimate payment on its silent red threat.

Gordon Grice is the author of `The Red Hourglass' (Penguin, pounds 14.99)