Common Pill Bug (Armadillidium vulgare)

It’s funny how sometimes the most common animals are the ones I know the least about. I know a fair bit about tenrecs, cheetahs and lyrebirds, but know next to nothing about pill-bugs. In fact, I’ve barely even thought about them, asides from when I played with them as a kid. But this blog is a place for people to learn about animals (me included), so let’s talk about pill-bugs!

Pill-bugs are crustaceans, like crabs and lobsters, and belong to the genus Armadillidium, which currently has 178 known species. That’s an awful lot of species to cover, so I’m going to focus on one of the most common, and the most studied, species: Armadillidium vulgare, otherwise known as the common pill-bug. Common pill-bugs originated in Europe, near the Mediterranean, but have since been introduced around the world, with particularly dense populations in the United States.

Pill-bugs require moist soil to survive, as they do not possess a waxy cuticle to prevent themselves from drying out. As well, pill-bugs breathe from psuedotrachea, a type of modified lung that must be moist to function. Humidity levels of around 50-60% or more are ideal for pill-bugs. They are most common in temperate or Mediterranean-like climates, and are often found under debris, such as logs, stones, or human waste.

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I think pill-bugs are kind of cute, what do you think? Image credit: Franco Folini via Wikipedia

Common pill-bugs are oval shaped, and have armour-like plates along their bodies. They are not overly large, reaching sizes of 18 millimetres. Most pill-bugs are darkly coloured, with some having spots along their backs that can be yellow, brown or red. Common pill-bugs have seven pairs of legs, and a single pair of antennae.

Pill-bugs have to constantly worry about drying out, so much of their behaviour is centred around conserving moisture. When humidity is high, pill-bugs move slowly, and when humidity drops, they start to move more quickly, to search out moister areas. They move almost twice as much during the summer than in winter, and tend to be more active during the night, when moisture loss is reduced. When temperatures get to be 20 to 30 degrees celsius, pill-bugs release pheromones that cause them to bunch together, which reduces the surface area of individuals in the group, meaning less moisture is lost.

Many of you probably know that pill-bugs curl into balls, but do you know why? There are two likely reasons, the first being that all the soft, squishy parts of pill-bugs are located under their shells, so by curling into a ball pill-bugs can protect their vulnerable parts. The second is that rolling up into a ball helps prevent moisture loss, and as we’ve seen, moisture conservation is of paramount importance to these little guys.

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A pill-bug unrolling from a defensive curl. Image credit: Franco Folini via Wikipedia 

Pill-bugs fall prey to a number of animals, including birds and arthropods. They are not entirely defenceless — in addition to their armour and their rolling-in-a-ball technique, pill-bugs can release nasty secretions when threatened. Unfortunately, pill-bugs’ defences only really work against small invertebrates, and birds will readily snatch these guys up. Also, a clever genus of ants have super long mandibles that can pry open a pill-bug when it’s curled in a ball.

Pill-bugs don’t have a particularly yummy diet — they feed on leaf litter and other decaying organic matter. They are, however, quite adaptable, able to survive for several months without food, and when food is scarce, readily switch to other sources of nutrients. Pill-bugs will eat the roots of plants, seeds, fruit, other pill bugs, and even their own poop.

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I had a lot of fun drawing this fella! 

The mating season in pill-bugs depends on the climate, with mating usually occurring in warmer times. Thus in regions with mild winters, mating can occur year round, while in other areas mating usually starts in the spring. Both male and female pill-bugs mate with multiple partners, and females can store sperm for up to a year. Pill-bugs moult regularly on a 29 day cycle, during which time the bugs cannot move or eat, and are extremely vulnerable. A special moulting cycle occurs in females, called the parturial moult. Mating can only occur during a female’s parturial moult, so males have to be prepared! Female pill-bugs retain their eggs in a pouch until they hatch, for about two to three months. The little pill-bugs stay in the pouch for a few days, and after moulting a few times, are independent from their mothers.

Hopefully by now you realize just how fascinating pill-bugs are, and have a greater appreciation for them. I certainly do! These are some awesome little insects (and they’re pretty cute!)