stone centipede

If you turn over any given rock in Michigan, you are likely to find one of these:

This is a “stone centipede”, order Lithobiomorpha, so called because that’s what they live under[1]. They run like water, flowing around obstacles and into holes in a way that’s very much like the way a stream of water flows, and are kind of hard to catch. On top of the speed, they are slippery, and their dozens of legs are very good at forcing them through crevices, or out from between your fingers. Even their antennae have a disturbingly fluid nature, flowing over surfaces and contorting in a way that is more like what you would expect from tentacles than from antennae.

That first picture was low-magnification because the whole centipede is much too long to fit into a single high-magnification image. Here’s a few showing it in more detail:

In the picture of the head end, you can see two bulges on the sides of the head. These are the “poison claws” that they use to grab their prey. Here’s a (somewhat blurry) picture showing the claws opened and looking very ominous:

Getting back to the body, here is the midsection:

While stone centipedes do have lots of legs, they don’t have a hundred legs. This one would have had 30 legs, except that it is missing one of the rear ones. There is one pair of legs per body segment, even though some of the legs look like they are coming from the junctions between segments. This is because the body segments are wildly variable in size, with some of them being practically nonexistent. I’m not sure why that is, I would have expected all the body segments to be about the same size.

The last few sets of legs point almost straight back, which doesn’t seem like it would be all that helpful for walking. Maybe they are used to push forward? Mostly, it just seemed to drag them around.

While stone centipedes are reputed to be able to “bite”, once again I can’t confirm that. I was picking this one up and pushing it around, and it was doubtless pretty annoyed with me, but it didn’t let me have it with the poison claws. I understand that some of the big tropical centipedes have pretty painful bites, but as I’m sure everyone has gathered by now, Upper Michigan is hardly the tropics[2].

Anyway, stone centipedes are pretty aggressive predators, preying on the other things that live under rocks like woodlice, worms, springtails, grubs, and who knows what else. They are easily the fastest-moving things you are likely to find under a rock, and their thin body shape makes it easy for them to squeeze quickly into crevices.

They are pretty stong, too. I had this one in my photographing dish with a glass slide over it, and it was strong enough to muscle the slide aside and crawl out. Then it was off like a shot, and I had to scramble to catch it again (this was when it had a really good opportunity to bite me, but didn’t take it). I ended up having to hold the cover slide down with my finger to keep it from getting away again.

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[1] I was looking at the “categories” list on the sidebar, and noticed that while I had multiple specimens for the other major classes of arthropods I’m likely to find around here (arachnids, crustaceans, and insects), for the myriapods there was only this one, lonely millipede. So, Sam and I went out flipping over rocks to find some more myriapods to fill out the list a bit.

[2] The term “subarctic” comes to mind[3], although I guess it is officially classified as a “humid continental” climate. I know a faculty member at the University who was visiting the Soviet Union (back when there was a Soviet Union), and mentioned where we were located. The Soviet professor he was talking to was convinced that our university must be the US Government’s equivalent of Siberia for exiling politically-disgraced scientists. I dunno. Maybe it is.

[3] The Keeweenaw Peninsula was just about the last place in Michigan to come out from under the ice at the end of the last ice age: the glaciers from the “Marquette Readvance” didn’t melt off until about 9,900 years ago, several thousand years after the lower peninsula of the state had melted off completely. Even then, a lot of the U. P. was periodically inundated for the next several thousand years until the lake levels stabilized. That sounds like a long time for wildlife to recolonize the area, until one considers that wingless arthropods like centipedes had to walk about a thousand miles to get up here since the ice melted. A mile every 10 years or so doesn’t sound that hard to us, but to critters less than an inch long that can’t fly, it’s a pretty fast clip. The centipedes move fairly briskly, so they probably got here comparatively quickly (maybe 5000 years ago), but things like woodlice must have had a hard slog of it (unless they hitch-hiked on birds or something). Some of the slower creatures, like earthworms, didn’t make it up here at all until they were carried up by human activities (earthworm eggs in mud stuck to tires, or abandoned fishing bait, for example). So, even though a lot of the things that live in the soil in, say, Ohio could probably live just fine up here, they just haven’t gotten this far yet.