CRUSTACEA > MALACOSTRACA > ISOPODA > Oniscidea > Crinocheta > Armadillidiidae

Armadillidium depressum superficially resembles the Common Pill Woodlouse, Armadillidium vulgare, but has some distinguishing features that clearly set the two species apart.

Both species are robust and much the same size, although Armadillidium depressum averages a millimetre or two larger as mature individuals can reach 20mm. They also have a very similar uniform slate-grey appearance, except that whilst Armadillidium depressum sometimes has a longitudinal pattern of contrasting yellow flecks (especially in immatures), it doesn't have any of the mottled patterning or coloured variations that can occasionally be seen in Armadillidium vulgare.

However, the main difference comes when you look more closely at the actual shape of the body, and see that in Armadillidium depressum the pleonites have shallow, gently splayed lateral margins giving the species a slightly flattened appearance, particularly noticeable when viewed from behind. The telson and uropods have a marginally different shape but, to me, the general appearance of the pleonites is the telling feature, as they have a distinctive flattened top line (as can be seen in the final photo below) as opposed to the continuous shallow curve that occurs in Armadillidium vulgare.   

The species also has a distinctly raised scutellum - the projecting structure in front of the eyes and between the antennae - that is broad, much wider than tall, and more of a third of the width of the head (best seen in the fourth photo), whereas in Armadillidium vulgare it is much flatter and less pronounced. This feature, which can only be properly appreciated from a low-level, front-on view, is also useful when distinguishing the species from the less common Armadillidium nasatum that has a much narrower snout-like projection.  

A further characteristic, is that due to its slightly depressed form, Armadillidium depressum cannot roll into a fully enclosed and protected ball in the same manner as Armadillidium vulgare, or indeed the rarer and much smaller Armadillidium pictum because, even when tightly closed, there's always a small gap where you can see the antennae (third photo). But, on the other hand, it has a much better grip than other pill woodlice, which allows it to clamp on to substrate including vertical walls. In fact, there are many observations about the species clamping down for protection rather than enrolling, so I guess that when this species is found in a ball it's probably just resting.        

As its common name suggests, Armadillidium depressum is very much a southern species with a distinct south-western bias, albeit with scattered sightings from further afield. It's strongly associated with synanthropic habitats where it is most frequently observed under, and on, hard surfaces, such as loose or slightly raised stones and rocks, wall capping or under bridges. The individual featured below was found in our back garden under a large stone planter.