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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. (the northern widow, Latrodectus variolus; the black widow, L. mactans; and the "western" widow, L. hesperus) 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 webs are typically constructed in quiet, undisturbed locations that are usually, but not always, close to the ground. The female spends her entire life in the web and retreats into the tunnel when disturbed. She places her eggs in white, spherical sacs within the web. The abdomen of a young black widow is patterned with red, white and yellow.

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, the bite site becomes quite painful. Symptoms from the bite of a black widow 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. But if symptoms do become severe, diazepam may be administered for muscle pain and cramps. A bite is usually more serious for small children and the elderly.

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 1¼ inch. The males are slightly smaller than the females. There are many species of brown recluse spiders in the U.S. 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 for web-building. Unlike black widows, brown recluse spiders hunt for prey some distance from their webs. They usually come into contact with humans when they take temporary refuge in clothing or bedding. 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, particularly in regions where the brown recluse is prevalent.

Bites are rare and are usually the result of unused rooms suddenly being put to use, or accidental contact resulting from pressing the spider between the body and either 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. Young children, the elderly, and the infirm are most likely to be affected severely. 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 waits in its funnel, and when it feels vibrations rushes out to grab its prey. This spider mates in the summer and early fall, and the female lays eggs in the fall in silken sacs that are placed behind or beside the web. Eggs hatch in the spring and the spiderlings develop for a year before they are sexually mature. 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. Bites most commonly occur from July to September when males are wandering in search of females. Often bites occur when the spider is squeezed between clothing and a person's body. 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 eventually scabs over. The lesion may take months to heal.

For more information about spiders, please visit the University of Florida's School IPM Web site (http://schoolipm.ifas.ufl.edu). Click on Technical Information - Pests, then select "IPM for Spiders in Schools."

Edited by: Angela Brammer, University of Florida