The common isopod Armadillidium nasatum, shown in the photo above, is ordinarily grayish-brown in color. Yesterday, taking advantage of the warmer weather, I went out in the backyard to see how the creatures under the rocks were coming along. Among the assorted slugs, earthworms, millipedes and isopods, there were 2 yellow isopods under 2 separate rocks.
I photographed them and took one of the specimens. They were easy to identify as A. nasatum from the prominent projection on their heads (arrow), a characteristic of the species. When disturbed, they roll up into a ball (not every terrestrial isopod species can do that).
I also e-mailed photographs of the yellow isopods to my usual isopod contacts, Joan Jass in Milwaukee and Ferenc Vilisics in Budapest. They have since informed me that such yellowish color morphs are occasionally seen in A. nasatum populations.