House Pseudoscorpion

What timing! I’d just gotten the camera mounted to take nice, high-magnification photos of tiny things, and then S. tells me there is a pseudoscorpion in the bathtub, just *begging* to be photographed. So, here it is:

Then, we flipped it over to have a look at the underside:

And then, finally, zoomed in on one claw:

Considering that this little guy was only about 3 mm long, I think the photos came out pretty sharp. We figured that this should be good enough to get a full, positive ID, so I found an identification key to the pseudoscorpions of Michigan in the MTU library[1] . . . and had my illusions pretty much dashed. It turns out that you distinguish pseudoscorpion species from each other based on very fine details of the claws, and where exactly the sensory hairs (setae) are located. Unfortunately, even though the claw detail photo is pretty good, it isn’t good enough to count setae. According to the author of the ID key, even an expert might take as much as a week to prepare, dissect, and examine a pseudoscorpion to determine the species. There are supposed to be at least 29 species in Michigan, so the next ones we find might get taken in to the lab for a more thorough examination.

While a really positive ID isn’t practical from these photos, the fact that it was in the house (and, specifically, in the tub) means it was almost certainly Chelifer cancroides, also known as the “house pseudoscorpion” or “book scorpion”. This is a “cosmopolitan” species, meaning that we have carried it around the world and it pretty much lives everywhere that humans live. They are actually very common, but are rarely seen except when they, say, venture into the bathtub to get water. They spend the rest of their time eating other, even smaller arthropods (mites, clothes moth larvae, and the like), and generally get dismissed as being harmless and of no economic impact. They are reported to live for 2-3 years, which is pretty good for such tiny creatures.

Even though they look superficially like scorpions, they are in a separate order (Pseudoscorpiones), and are about as closely related to spiders as they are to scorpions. Instead of poisoned tails, they have venom glands in their claws that they use to kill prey. Of course, at less than about 3 mm long even for the largest species, they are completely incapable of breaking the skin of a human, so their venom is irrelevant to us. Pseudoscorpions frequently get around by hitching a ride on other insects, like the one in this rather startling picture.

[1] Sigurd Nelson Jr., (1975), “A Systematic Study of Michigan Pseudoscorpionida (Arachnida)”, The American Midland Naturalist, Vol. 93, No. 2, April 1975, pp. 257-301