Dear Straight Dope: I have heard from numerous sources (many of them seemingly credible) that the average human consumes an average of four spiders per year in his or her sleep. Is there any truth to this fact? And if so, why would the spiders go in our mouths? Michael O'Shea

SDStaff Doug replies:

Amazingly, I cannot find this one debunked in either the alt.folklore.urban or snopes urban legend archives. I know it’s been debunked in the AFU newsgroup — I’ve been in on the debunking personally, but none of that comes up in a search on the AFU archive. So it’s up to me to do what must be done.

One amazing thing about urban legends is that they never appear the same way twice. This statistic variously appears as 10 per year, 57 per year, 19 per year, and all sorts of other numbers. No one can ever tell you where the statistic came from originally, either.

Realistically, the average number of spiders swallowed at night per person per lifetime is probably less than one. After all, most people breathe while they sleep (at least I do) and spiders, like virtually all arthropods, flee from breath. After all, there are lots of vertebrates that EAT arthropods, and if you’re an arthropod and something is breathing on you, it’s not a good idea to stick around. Simple enough.

For a spider to get into your mouth while you’re sleeping, (a) you must have your mouth OPEN, which is certainly not universal, so there’s a big chunk of people who can never swallow anything; (b) there has to be a wandering spider in your immediate vicinity, also something which — for most people in the civilized world, at least — is a fairly rare occurrence; (c) the spider has to either jump or fall into your mouth from a long distance, because they won’t go near your mouth otherwise (they’re not suicidal), and the odds are pretty astronomical of a spider randomly dropping into your mouth from the ceiling.

Put it all together, and it would be a miracle for a spider to end up in anyone’s mouth while they’re sleeping, except for one rare circumstance — when a spider egg sac hatches indoors. At that point, you can have hundreds of microscopic spiders, a millimeter long or less, leaping into the air in a short time span (under an hour total) and trying to ride the air currents to freedom. This is as known as “ballooning”; you may remember it from Charlotte’s Web. If you’re in a house where a bunch of microscopic spiders are ballooning around, you MIGHT accidentally inhale about a dozen one night, IF the air were circulating sufficiently for them to get airborne and stay there long enough to drift in front of your face. The odds of such a thing are obviously quite small, but it surely happens to someone somewhere from time to time, and that will boost the average — but not enough that I would believe that the AVERAGE person inhales the contents of one egg sac in their lifetime.

The majority of people probably never swallow ANY spiders in their sleep, so the statistic will be composed of a fair number of people (still a tiny minority) that swallow one or two by accident, plus a vanishingly tiny handful of people who swallow a large number, due to a freak occurrence as above. Not enough to add up to much, especially when you consider that 99% of the spiders swallowed are almost too small to see, and you wouldn’t feel them even if you swallowed them while awake. I know I won’t lose any sleep over it.

SDStaff Doug, Straight Dope Message Board

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