Immature Pillbug

2022 June 26

Here’s an immature pillbug that I caught in my pitfall trap on April 13, 2022. It was very small, about the size of an uncooked grain of rice.

It looks like an Armadillidium, with a rounded tail instead of a tail with projections like most other woodlouse species have.

The main difference between an immature pillbug, and a mature one, is the mature adults are generally much darker and considerably bigger. See for example this one that I photographed back in may of 2012:

Anyway, pillbugs are able to roll completely into a ball, which is aided by the fact that they are practically hollow underneath. This gives them a place to put their legs when they roll up.

On the underside, there are two white patches (one of which is circled in the next picture):

I believe these are the gills [1]. Pillbugs are crustaceans, like shrimp and lobsters, and crustaceans apparently never evolved lungs as such. Instead, they keep their lungs moistened and aerated, and I think the white color is due to them being essentially covered by foam. Their gills are really sensitive to drying out, which is why you mainly see pillbugs in moist areas and not running around in dry places.

This little feller is most likely the Common Pillbug, Armadillidium vulgare, since that is the species that has been most thoroughly spread around the world. However, there is another species that has also been transplanted to North America: the Nosy Pillbug, Armadillidium nasatum. The Nosy Pillbug has something of a projection on the nose, which I expect is probably the source of the name. Otherwise, the two species are very similar to each other and it probably doesn’t matter to anyone other than them which one they might be.

[1] I used to think those white patches were the “marsupium” pouches where the females incubated their eggs, but I have since found out that they carry the marsupium further forward. One of these days I will probably find a female pillbug with a full marsupium.

2 Responses
  1. Ben permalink
    July 1, 2022

    Good post Tim! When I was in the garden I found a good sized specimen, and I could see the white patches you mentioned. Luckily it cooperated and didn’t roll into a ball. I believe they call them woodlice here.

  2. July 16, 2022

    Great photos as always. I didn’t realize their gills were that far back in the chassis.

    Here in San Diego, almost all of ours die off by July because everything is do dry.

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