Pillbug – Armadillidium

2008 February 16

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.

[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.

8 Responses
  1. Cecilia Perez permalink
    August 13, 2008

    Actualmente esta ocasionando grandes daños en la producción de tuberculo-semilla de papa a partir de vitroplantas en los invernaderos del Campo EXperimental Las Cuibas – Estado lara

  2. August 14, 2008

    Here, I’ll try a translation of the previous comment, even though it’s been a long time since high school spanish. Please let me know if I got it wrong:

    “At the moment, this is causing a great deal of damage in the production of seed potatoes in the Las Cuibas Experimental Fields greenhouses in Lara State (Venezuela).”

  3. Retha permalink
    May 21, 2009

    Gee, Thanks

    I have so many of these in my back yare. They are invading the plants that I try to trans plant. I cant figure out if they are eating the leaves. How do you rid them. Are they beneficial. Your pictures are perfect and the information if far more that what I expected.

  4. May 22, 2009

    Retha: They mainly eat rotting organic matter and fungus in moist areas, and as far as I can tell they don’t do much to living plants. They basically have the same sort of effects as earthworms, speeding decomposition but not doing any direct damage. They sometimes will eat stored root crops like potatos, like Cecilia Perez said above, but they’re only a problem there because root crops contain moisture which keeps the pillbugs from drying out, where other crops (like grains) are stored dry. From what I’ve seen they mainly peel off the skin of potatos and then gradually eat the material underneath as it rots. I’ve never had them cause any problems in the garden, so I don’t think there is any real need to control them there.

  5. November 13, 2009

    My daughter is doing a science fair experiment with these little guys. We’ll be blogging photos and videos in a few weeks.

  6. November 17, 2009

    That will be interesting to see, be sure to let us know when it’s ready.

  7. tasha permalink
    May 5, 2010

    My question is how do i get rid of potatobugs they are livnging in my home. i have them all over my house living room,bathroom,bedrooms, some times i find them on my bed.. It’s so nasty!!! How do i get rid of them???

  8. May 6, 2010


    Generally you get them when there is moisture in the house, for example in damp basements. The best way to get rid of them is to make sure your house is bone-dry. Depending on where you live, a dehumidifier might help.

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