A tale of two terrestrial isopods

60% of all the terrestrial isopod (Oniscidea) observations on iNat belong two two European species that have been introduced around the world. They are the Common Pillbug and one of several species of Woodlice (Porcellio scaber, P. laevis, and P. dilatatus). Because they are so commonly encountered and posted by amateurs there's a lot of mis-IDs resulting from confusion between these groups. Fear not! They are easy to tell apart!

The butt (posterior) end of terrestrial isopods have 2 projections (the 'uropods') sprouting from either side of their triangular shaped last body part (the 'telson'). If you have a Pillbug (aside from being able to roll up) the uropods are very short and don't protrude past the telson. The black arrow here points to one of the two short uropods:

If you have a Woodlouse (aside from NOT being able to roll up), the uropods are pointy and stick out past the telson. The black arrow here points to the telson:

Like these two groups, by far most of the terrestrial isopods in the US are a handful of introduced European species that are actually really easy to learn (and wait for future posts here). Another commonly seen group are the Sea Slaters which are always found along the shore in the US (except one weird mountain one in Hawaii). Pretty much every other native terrestrial isopod in the US belong to amazing but rarely seen critters you have to hunt for. I hope you'll join me in trying to track down and share as many as these crazy creatures as possible!