Why Do Pill Bugs Roll Into a Ball? Conglobation Explained

How does a pill bug roll up?

A common pill bug is built for rolling. Its back is covered by a series of curved armor plates that overlap like the bands of an armadillo. When it is alarmed, muscles pull the head down and the tail end forward, and the plates slide over each other until the body forms a closed shell.

The fit is remarkably precise. The head tucks against the triangular tail plate, the side edges of the plates meet, and the legs fold inside, out of reach. The common pill bug closes into an almost perfect sphere with no gap, which is one way experts tell it from some close relatives: two other European pill woodlice, Armadillidium nasatum and Armadillidium depressum, leave a small gap when they roll up.

Rolling usually happens in an instant, triggered by touch, a sudden jolt or strong vibrations in the ground.

Protection from predators

The main reason for rolling up is defense. A pill bug’s upper surface is covered by a thick shell hardened with calcium carbonate, while its underside, legs and breathing organs are soft. As a ball, the animal shows an attacker only its armor.

This works well against small hunters that must grip their prey, such as ants, many ground beetles and small spiders. A rolled pill bug is also hard to hold, and it may simply roll away and drop into a crack in the soil.

The defense is not perfect. Shrews, toads, frogs, hedgehogs and many birds can swallow a rolled pill bug whole. The woodlouse spider (Dysdera crocata) specializes in woodlice and has unusually long fangs that can pierce the armor. Pill bugs also have a second trick: when badly disturbed, they may freeze and play dead, which can make a predator that hunts by movement lose interest.

Saving water

Rolling up has a second job. Woodlice lose water through their shells and breathing organs much faster than insects do, and drying out is the biggest danger they face. A ball exposes far less surface to the air than an open body, and it closes off the damp underside where the breathing organs sit.

In one set of laboratory experiments, pill bugs that were allowed to roll up in hot, dry air lost less water than ones prevented from rolling. In the wild, pill bugs mostly avoid drying out by hiding in damp places by day and huddling together in groups, but rolling up gives them an emergency option when they are caught in the open.

Which woodlice can roll into a ball?

Most woodlice cannot roll up at all. Complete rolling is found in a few families, notably the pill woodlice of the family Armadillidiidae, found mainly in Europe and the Mediterranean, and the Armadillidae of warmer regions. Some other woodlice can curl partly.

Sow bugs and other non-rolling woodlice have their own defenses. They run fast, cling tightly to bark and stones so they are hard to pick up, or release sticky, distasteful secretions from glands in the shell. Understanding why pill bugs are crustaceans, not insects also helps explain why they rely on armor rather than speed.

How and when do pill bugs unroll?

A rolled pill bug stays closed while it still senses danger. When everything is quiet, it slowly relaxes, opens a little and tests its surroundings with its antennae. Then the legs appear, the body straightens and the pill bug walks away, often heading quickly for darkness and shelter.

If a pill bug ends up on its back after unrolling, it waves its legs and twists until it can grip something and turn over. Pill bugs that have just molted are soft and less able to protect themselves, so they tend to stay hidden until their new shell hardens.

Other animals that roll into a ball

Rolling into an armored ball has evolved many times. The pill millipede (Glomeris marginata) of Europe looks so much like a pill bug that people often confuse them, but it is a millipede with 17 or 19 pairs of legs. This is a textbook case of convergent evolution, where unrelated animals arrive at the same solution.

Among mammals, pangolins and hedgehogs curl up to protect their soft bellies, and of all the armadillos only the two three-banded species can roll into a complete ball; see can armadillos roll into a ball? Even some trilobites, extinct sea arthropods that lived hundreds of millions of years ago, have been found as fossils rolled up, apparently for the same reason a pill bug curls today.