The Backyard Arthropod Project

And, in the “Things found under rocks” category, we have this specimen from back in May[1]. It is a “pillbug” (also known as “potato bugs”, “sow bugs”, “woodlice”, “roly-polys”, and probably dozens of other names).

From the underside, we can see that the bulk of the body is mostly an illusion: the volume under the shell is mostly empty space, to the point where this critter is almost hollow. This probably makes it a lot easier to roll up into a ball.

I expect that this is a species of the genus Armadillidium, which are land-living isopods (a type of crustacean) with habits similar to the other isopods, Porcellio scaber, that I posted almost a year ago. The difference is that the pillbugs can actually roll up into a tight little ball, while the Porcellio scaber can’t. At least around here, the Porcellio scaber are much more common, so it is likely that the ability to roll up into a little armored ball isn’t as much of an advantage as one might think.

While isopods aren’t actually related to myriapods like millipedes and centipedes (they are more closely related to crabs and shrimp), there are two orders of millipedes that look rather strikingly similar. These “pill millipedes” can be told apart from Armadillidium by looking at their last few body segments: the pill millipedes have a body ending in one big segment, while the Armadillidium bodies end in a bunch of small segments, as you can see here:

The final segment is a part of a crustacean called the “telson”, in aquatic crustaceans like lobsters this is the segment that is shaped like a fish’s tail and is used for swimming. Unfortunately it isn’t very clear here, but a trapezoid-shaped telson is an ID feature for the pillbug species Armadillidium vulgare, which is the most common pillbug species, and is probably what this one is.

I tried zooming in on the head, to see if there were any interesting details.

The mouthparts are not very pronounced, and look like they are mainly adapted for scraping organic muck off of stones and the like. This is not surprising, seeing as how they eat organic debris under rocks, logs, sticks, dead leaves, and pretty much else that holds moisture and protects them from drying out. I don’t see any eyes, and in fact I’m not so sure that they even have eyes. Given that they live in dark places, eyes are probably not a big priority for them.

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[1] You may have noticed that the pictures from around last spring are pretty variable in quality, mainly because I was still experimenting with lighting and camera settings. I’m afraid this one wasn’t one of the better sets of images.

Other articles related to this topic:

• Woodlice
• Soil Centipede
• Female Crab Spider
• Freshwater Amphipod - Gammarus
• Elongated Running Crab Spider