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Identification and biology of three dangerous spiders

Black widow spider

Adult females of the three most common species of black widows in the U.S. (Latrodectus spp.) are large (body size is ½ inch or larger). They are typically shiny black spiders with a red hourglass design on the underside of their abdomen. Because their webs are near the ground and the spiders hang upside down in the web, their distinctive marking is readily apparent. The adult male is not dangerous. The female black widow spider spins an irregular, tangled web with a tunnel in the center. The female spends her entire life in the web.

Black widows are shy, retiring creatures that bite reluctantly and then only in self-defense when threatened. However, when a female is defending her egg sac, she can become quite aggressive. A bite may not initially cause pain, but after a few minutes it becomes quite painful. Symptoms include headache, general body ache, nausea, shortness of breath, intense muscle pain, and rigidity of the abdomen and legs. If reactions are mild, no treatment is usually administered. If symptoms do become severe, diazepam may be administered for muscle pain and cramps.

Brown recluse spider

Brown recluse spiders (Loxosceles spp.) have long thin legs, an oval-shaped abdomen that is light tan to dark brown, and a very distinctive violin-shaped mark on the back. Their overall size is ¾ inch to 1 ¼ inch. They are found mostly in Midwestern and south-central states, the Southwest and Puerto Rico. These spiders are shy, preferring dark, undisturbed places on or near the ground. Brown recluse spiders hunt some distance from their webs. Items left undisturbed on the floor, such as supplies, toys, or clothing, are perfect daytime refuges for these spiders. Such objects should be shaken out thoroughly if they have been on the floor for any length of time.

Bites are rare, usually the result of unused rooms suddenly being put to use or accidental contact resulting from pressing the spider between the body and clothing or sheets. The bites are almost always very unpleasant, producing an ulcerous wound called a necrotic lesion that turns dark within a day and takes a long time to heal. Victims should seek medical attention but should only allow a doctor to excise the affected tissue in extreme cases.

Aggressive House Spider

The aggressive house spider (Tegenaria agrestis) is a fairly large (1¾ inches, including legs), fast-moving spider. Its legs are long and hairy, and its body is brown with darker markings on its oval abdomen. This spider builds a funnel-shaped web in moist, dark places. The aggressive house spider is found throughout the Pacific Northwest, Idaho and Utah and appears to be rapidly expanding its range.

Very few people are bitten by this spider, and even fewer develop severe symptoms. The bite of an aggressive house spider can produce symptoms similar to those produced by a brown recluse. The initial bite may not be painful, but within a few minutes a hard, sensitive area develops. Other symptoms include severe headache, nausea, weakness and joint pain. Later, the area blisters, oozes serum and scabs over. It may take months to heal.

Please visit the University of Florida's School IPM Web site (http://schoolipm.ifas.ufl.edu).

Edited by: Angela Brammer, University of Florida