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Myths about "Dangerous" Spiders

Myth: You can always tell a spider bite because a spider leaves 2 punctures.

Fact: There is a germ of truth in this idea, but only a very tiny germ. Spiders do have two venom-injecting fangs and typically bite with both at the same time. However, in any spider smaller than a tarantula, the entry points of the two fangs will be so close together that there is little if any visible separation. Also, the fangs are so slender and sharp that the actual entry points are all but invisible.

When you have a "bite" with two separated marks, it is either caused by a bloodsucking insect that has bitten twice (a common occurrence), or is a double skin eruption arising from one disease condition or arthropod bite, also a common occurrence.

Myth: Some spiders are poisonous and others are not.

Fact: There are two problems with this idea: a technicality, and a set of false assumptions. First, the technicality. "Poisonous" and "venomous" are two different things. Offhand, I can't think of any spider that is poisonous (harmful to eat, breathe or touch). Mushrooms are sometimes poisonous, but spiders are not; they are venomous (their toxins are proteins and work by being injected, not by being eaten).

Second, we have a set of false assumptions about spider venoms. Almost all spiders are venomous; only two small families lack venom glands. The purpose of spider venom is to subdue the spider's prey, almost always insects. In brief, it's an insecticide. Spider venom does not exist to harm creatures, like humans, which are too large for spiders to eat, and in nearly all cases has little if any effect on humans.

A minority of spider species have venom that can cause localized pain in humans, like the venom of bees or wasps. (Bees and wasps are far more dangerous than spiders, however; wasps cause many deaths annually). Of around 50,000 spider species known, only about 25 (1/20 of 1%) have venom capable of causing illness in humans, to a greater or lesser extent. In any given locality you can expect to find from zero to (at most) three such species. These species are called "medically significant" spiders.

"Is it poisonous" or "is it venomous?" is not a meaningful question.

Text © 2003, Burke Museum of Natural History & Culture, University of Washington, Box 353010, Seattle, WA 98195, USA Phone: 206-543-5590 Photos © as credited

Queries to Spider Myths author, Rod Crawford This page last updated 22 August, 2011 This site best viewed at 800 x 600 using IE 5.0 or above.