A Tough Pill to Swallow

What am I looking at?

This is a composite image of a single woodlouse (Armadillidium vulgare) in a defensive position (1), transitioning out of a defensive position (2), and in a relaxed position (3).

Biology in the background

This woodlouse, commonly known as a pill bug, gets its less formal name from the way it can roll itself into a tight sphere when it feels threatened, taking on the shape of a pill. Its back and sides are covered in layers of chitinous plates that serve as armor for these tiny arthropods. When they’re disturbed or agitated, they pull in their vulnerable appendages and roll into a ball, leaving only their armored back and sides exposed.

Woodlice are crustaceans – more specifically, isopods – which makes them relatives of shrimp, rather than of insects, centipedes, or millipedes. But speaking of the latter creatures, pill millipedes came up with a very similar defensive strategy. In fact, they often share their habitat with pill woodlice and can easily be mistaken for them.

Pill woodlice originated in the European and Mediterranean region, but they now exist in most temperate areas around the globe due to their introduction to new habitats by human travelers. They pose no threat to ecosystems; in fact, they can be beneficial, taking part in the process of nutrient cycling – converting detritus, mostly dead plant matter, into forms that other organisms can utilize. And, yes, the derivation of the name of their genus is supposed to evoke an armadillo.

This species of pill bug can grow up to 18 millimeters long, or roughly the width of a human thumbnail.

Technique

This image was created using macrophotography.