Illinois Natural History Survey - University of Illinois

Brown Recluse Spider

Susan Post


“Just received word the University of Illinois is planning some pest management monitoring here at the Forbes Natural History Building for brown recluse spiders. They will be placing a few monitoring stations here in the building this next couple of business days (and may knock on your door for access).”

E-mail message from Cathy Bialeschki (INHS), Friday November 13, 2009

Cathy’s e-mail message piqued my curiosity, especially when a few days later a black rectangular spider trap was placed under my desk. What do these spiders look like?  Would they really be in my office?  Should I be concerned?

The brown recluse spider, Loxosceles reclusa, is found throughout the south-central and midwestern United States. These spiders are rare outside of their range and are widely over reported. They may be transported to a non-native area in boxes or furnishings, but infestations seldom become established.

Only one-half inch or less in length, this spider is easily overlooked, especially in dark corners. They may be brown, gray, or deep yellow in color and have long, thin legs that lack conspicuous spines. Their cephalothoraxes (unlike insects, spiders only have two body parts—the cephalothorax and the abdomen) have the eyes, fangs, and legs attached. The dorsal markings resemble violins with the neck of the violin pointing to the rear of the spider. This mark, while less obvious in young spiders, has led to the common names of fiddleback spider, brown fiddler, or violin spider.

The eye pattern of recluse spiders is the definitive diagnostic feature, but most people will need a hand lens to see it! They have six eyes arranged in pairs; most other spiders have eight eyes. Even with this many eyes they have limited vision and rely mainly on touch.

During the day brown recluse spiders usually seek refuge in dim, secluded areas. These refuges are lined with dark, irregular webbing. Outdoors (their natural habitat) the webs may be built under rocks, logs, woodpiles, and debris. Rotting tree bark is a favorite habitat. Yet these spiders have adapted to living indoors with us, hiding in cracks and corners of our homes. Indoors they seem to favor cardboard and are able to persist many months without food and water. Females seldom venture far from their retreat, while males and juveniles tend to wander, ending up in shoes, clothing, or bedding where they may become trapped against someone’s skin.

Unlike most web weavers, they leave their webs at night to hunt, seeking insect prey, either dead or alive.

From May through July the female will deposit 40–50 eggs in an off-white, silken sac. These sacs are 2/3 of an inch in diameter. Spiderlings (which resemble tiny versions of a fully grown adult) emerge from the sac in about a month and will molt up to eight times before becoming adults. It takes about a year for the spiders to mature. These spiders have an annual life cycle, but may live up to three years in captivity. A female will produce up to five egg sacs during her lifetime.

Most spiders have poison glands, which they use to paralyze their prey (any creature that happens to wander into their web or near the spider). The brown recluse is one of only four spiders in the United States whose venom poses a danger to humans. The black widow, hobo, and yellow sac spiders are the others.

As the name suggests, the brown recluse is not terribly aggressive. Bites, while rare, usually occur when a person inadvertently trespasses on a spider’s turf. Most bites happen in response to body pressure, when a spider is inadvertently trapped against bare skin. These spiders have small fangs and cannot bite through clothing. The spider’s venom is cytotoxic, meaning it kills cell tissue. The initial bite is usually painless. Several hours later the bite site may become red and swollen. Most bites remain localized and will heal within three weeks.

So far the spider trap under my desk remains empty.  But there is no need for me to worry. These tiny arachnids are more inclined to avoid me, as much as I am them. Whether we encounter each other or not, we seem to have achieved a very natural and peaceful coexistence.

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