Why Do Spiders Bite Humans?

Spiders Aren't Built to Bite Humans

Black widow spider.
The widow spider is one of the few spiders capable of harming humans. Getty Images/PhotoLibrary/John Cancalosi

Spider bites are actually rare. Spiders really don't bite humans very often. Most people are quick to blame a spider for any unusual bump or mark on their skin, but in the vast majority of cases, the cause of your skin irritation is not a spider bite. This belief is so pervasive that doctors often misdiagnose (and mistreat) skin disorders as spider bites.

Spiders Aren't Built to Bite Large Mammals

First of all, spiders are not built to do battle with large mammals like humans. Spiders are designed to capture and kill other invertebrates. With few exceptions (most notably, that of widow spiders), spider venom is not lethal enough to do much damage to human tissues. Chris Buddle, an Associate Professor of Insect Ecology at McGill University, notes that "of the almost 40,000 spider species, globally, there are less than a dozen or so that can cause serious health problems to the average, healthy human." And even those with venom potent enough to threaten harm to a human are ill equipped to bite us. Spider fangs simply aren't made for puncturing human skin. That's not to say spiders can't bite humans, but it's not an easy thing for them to do. Ask any arachnologist how often they suffer bites while handling live spiders. They'll tell you that they don't get bitten, period.

Spiders Choose Flight Over Fight

One of the main ways that spiders detect threats is by sensing vibrations in their environment, much like they detect the presence of wayward insects in their webs. People make a lot of noise, and spiders are well aware that we are coming their way. And if a spider knows you are coming, it's going to choose flight over fight whenever possible.

When Spiders Do Bite

Now, occasionally, spiders do bite people. When does this happen? Usually, when someone unknowingly sticks his hand into a spider's habitat, and the spider is forced to defend itself. And here's a disturbing little tidbit of spider bite trivia for you, courtesy of entomologist Dr. Gilbert Waldbauer in The Handy Bug Answer Book:

The majority of [black widow spider] bites are inflicted on men or boys sitting in an outdoor privy, or pit toilet. Black widows sometimes spin their web just beneath the hole in the seat, often a good place to catch flies. If the unfortunate person's penis dangles in the web, the female spider rushes to attack; presumably in defense of her egg sacs, which are attached to the web.

So if this mark on my skin isn't a spider bite, what is it?

What you thought was a spider bite could be any number of things. There are plenty of arthropods that do bite humans: fleas, ticks, mites, bedbugs, mosquitoes, biting midges, and many more. Skin disorders can also be caused by exposure to things in your environment, including chemicals and plants (like poison ivy). There are dozens of medical conditions that can cause a skin irritation that looks like a bite, from vascular disorders to diseases of the lymphatic system. Bacterial or viral infections are often misdiagnosed as arthropod bites. And you might be surprised to learn that one of the most common causes of  "spider bites" is actually MRSA (methicillin-resistant Staphylococcus aureus).

 

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