The Most Poisonous Spiders In North America

Only two varieties of North American spiders are poisonous: the black widow and the brown recluse. Each causes dozens of illnesses and deaths each year. This article outlines their habits, and shows you how to recognize and avoid them.

In their lengthy history, insects and their kin have evolved every conceivable trick to enhance their survivability, along with a few that have to be seen to believed. Bombardier beetles, for example, gas their enemies; some wasps lay their eggs in living tissue; click beetles can flex their chitin, allowing them to pop several inches into the air with an audible click; fireflies use chemical light to attract mates; and whip scorpions use acetic acid (vinegar to you and me) to hunt their prey. In addition to all these strategies, many species use toxins to immobilize prey and/or to protect themselves. Some of these toxins are among the most potent in nature, and can kill a human in hours or days. Aside from those individuals with rare allergies, North Americans are fortunate in that we face few native bugs that are, individually, dangerous to us. Two of the most potent are spiders, the black widow and the brown recluse, and it behooves us to be able to identify them upon contact, and to understand the threat they may pose to us.

The Black Widow

The name "black widow" is almost universally recognized and feared. The name for these distinctive arachnids is apt, for only the females bite; the males are small and harmless. In fact, it was long thought that the males serve primarily as meals for the females once mating is completed, though recent evidence suggests that this is not always the case.

Interestingly, the black widow belongs to a genus of spiders, Lacrodectus, that lives only in grape-growing regions of the world. More than thirty species of black widow are known worldwide, with the North American black widow being one of the most easily identified species. The female Lacrodectus mactans is characterized by a glossy, black, globular body, marked by a red spot on the abdomen, often in the shape of an hourglass. They are most common in California, although they occur in most areas of North America (Alaskans and most Canadians don't have to worry, though). Legs and all, this spider is less than an inch in length. Although all spiders are venomous to some extent, black widows -- which are as much at home inside human structures as in the wild -- inject an especially potent neurotoxin along with their bite. This type of poison blocks nerve impulses to the victim's muscles, resulting in cramps, muscle spasms, rigidity, and, in extreme cases, paralysis. In ordinary circumstances, the black widow uses its venom to paralyze insects, and then proceeds to suck them dry while they're still alive.

In humans, common reactions include the tightening of the abdominal muscles and diaphragm, often affecting breathing. The bite may also be accompanied by excruciating muscular pain, and sometimes by vomiting. These symptoms are temporary, and rarely result in death; according to the evidence, most of those who die from black widow bites are those who expect to, so if you suffer a black widow bite, don't panic. Prior to the development of black widow antivenin in 1943, only 32 of the 578 cases reported in the prior 200 years resulted in death. Many of the recorded cases occurred in the woods or during the use of an outdoor privy, both of which are rare these days in California.

As for our fine furry friends, it turns out that only cats and horses are particularly susceptible to black widow venom. A single bite can kill either of these species (not to mention even larger critters like camels). Dogs, sheep, and rabbits seem mostly immune to the toxin's effect.

The Brown Recluse

The brown recluse is a drab little bug, much less well known than its distant cousin the black widow. The Laxoceles clan, including the North American representatives, L. reclusa and L. rufescens, are ordinary looking, medium-sized brown spiders less than an inch long. They like living in human structures, and often set their webs in corners or in heating ducts. They also enjoy bunking with humans, and have a nasty habit of crawling into clothing as well. Most victims are bitten while dressing or sleeping.

In its own way, the brown recluse's venom is much nastier than that of the Lacrodectus. Recluse venom is a necrotic toxin: that is, instead of affecting a victim's nervous system, it acts directly on the skin and musculature to kill the tissues immediately surrounding the bite. The dead tissues heal very slowly, if at all. The effects include a stinging sensation in the vicinity of the bite, coupled with vomiting, restlessness, and malaise. If the venom fails to penetrate the skin, the bite is not life-threatening; however, if it does, the victim may suffer more severe symptoms, including kidney failure (which may turn the urine black) and toxic shock reactions. However, death in unlikely; since 1896, fewer than ten of 130+ brown recluse cases recorded in the United States have resulted in death. In severe cases, necrosis may lead to gangrene, which can require amputation.

Interacting with Poisonous Spiders

The best way to avoid black widows and brown recluses is to stay out of their habitat; however, this isn't always possible, especially when their habitat and yours overlap, of when they come to you. If you see a spider you know is poisonous, or one you're unsure about (since recluses can be hard to identify), avoid it if you can. Otherwise, try to remove it by trapping it in a container and placing it outside. Of course, for most people the instinctual reaction would be to kill the offending arachnid; it's your choice. If spiders are a recurring problem in your home, contact a pest control agency and have them treat the problem. It isn't particularly expensive to do so, and they usually have a great deal of experience with spiders.

As you can see from the figures cited above, spiders are rarely dangerous to humans, but they can hurt or even kill you. Hopefully this article will help you understand our endemic poisonous spiders, as well as the threat they pose -- or the lack thereof. Now you only have to worry about the other million or two species of bugs in your neighborhood"¦

© Demand Media 2011