---
title: "Common Pill Bug (Armadillidium vulgare) | AnimalTracks.wiki"
description: "A land-living crustacean that rolls into a ball, and in which bacterial DNA has become a new sex chromosome."
image: "https://animaltracks.wiki/images/animals/common-pill-bug-1.webp"
published: "2026-10-11"
updated: "2026-10-11"
---
# Common Pill Bug (*Armadillidium vulgare*)
> **Status**: Checked against 4 sources, listed under Sources below.
> **Tagline**: A land-living crustacean that rolls into a ball, and in which bacterial DNA has become a new sex chromosome.
> **Soundtrack Vibe**: Britpop & 90s Madchester // Oasis, "Roll With It"
> **Soundtrack Links**: [Spotify](https://open.spotify.com/track/1Xip3AM9yC2aeUIuhKftP5)
> **Vibe Match**: Press hard on a pill bug or shake the ground near it, and it curls its body into a sphere-like ball. Roll With It rides one chugging, rolling guitar riff from the first bar to the last.
## Biological Superpowers & Stats
- **Taxonomy**: *Armadillidium vulgare*
- **Danger Level**: Low
- **Conservation Status**: Not Evaluated ([IUCN Red List](https://www.iucnredlist.org/))
- **Superpower**: Curls into a ball when squeezed or shaken
- **Habitat**: Under stones and logs and in garden soil; native to the Mediterranean edge, now on nearly all landmasses
- **Diet**: Decaying leaves, with some roots, fruit and seeds
- **Tags**: crustacean, invertebrate, armored, woodland, urban, nocturnal
## Natural History & Behavior
## Armour that curls into a ball
Press hard on a common pill bug (*Armadillidium vulgare*) or shake the ground near it, and it curls its rear body joints in toward its front ones into a sphere-like shape. The trick is called conglobation. Animal Diversity Web says it may be a shield for the softer belly and legs, or a way to cut water loss. The genus was once named *Armadillo*, after the armored mammal that does something similar. The defence has limits: birds such as the common starling can beat it, and tropical ants of the genus *Leptogenys* have mandibles long enough to pry rolled-up pill bugs apart.
## Not an insect
The nickname says "bug", but a pill bug is an isopod, a crustacean that lives on land. A typical young adult is 10 millimetres long and 5 wide, and Animal Diversity Web lists a mass range of 0.060 to 0.116 grams. Its shell is mineralized: a study of the cuticles (outer shells) of two woodlice species, this one included, found crystalline magnesium calcite plus two non-crystalline forms, calcium carbonate and calcium phosphate.
Armour has a price. A pill bug must shed its cuticle to grow, and in the usual molt cycle, for about two days after a molt the new shell is soft, and the animal cannot move or eat and is open to predators and drying out. Females carry their eggs in a fluid-filled pouch called a marsupium for two to three months. Animal Diversity Web gives a range of 6 to 300 young, averaging 100.
## A life ruled by water
Much of a pill bug's behaviour is a fight against drying out. It moves slowly when the air is humid and faster when it is dry, and it is usually more active at night. Roughly 13 metres a day in summer is its pace, falling to 6.6 in winter. It cannot swim. At 20 to 30 degrees Celsius, pheromones (scent signals) draw pill bugs into clumps, which cuts the surface each one exposes to the heat.
The species comes from the Mediterranean's edge and has been carried to nearly all landmasses. It lives under stones and logs and in the soil of fields, gardens and hedgerows. Mostly it eats decaying leaf litter, and some data suggest almost ten percent of its diet is its own droppings, which recycles microbes and nutrients it missed the first time.
## Bacteria that rewired its sex
> **Borrowed chromosome**
> Researchers found a 3-megabase insert of a feminizing Wolbachia genome inside the pill bug's own nuclear genome, and their results support it now acting as a female sex-determining region.
Wolbachia are bacteria that live inside cells and pass from mother to young. In this pill bug, at least two strains turn genetic males into females. The researchers concluded that the chromosome carrying the insert is a new W sex chromosome, which would make a bacterium's DNA part of what decides whether a pill bug is female.
## Frequently Asked Questions
### Why does a pill bug roll into a ball?
Animal Diversity Web says it curls up when its body is pressed hard or the ground shakes nearby. Scientists suggest the shell shields the softer belly and legs, or that the ball helps the animal hold on to its body water. *(Source: [Animal Diversity Web]())*
### How big is a common pill bug?
Tiny. Its body is an oval about twice as long as it is wide, reaching around 10 millimetres by 5 in a young adult, and it weighs well under a gram: 0.060 to 0.116 grams, says Animal Diversity Web. *(Source: [Animal Diversity Web]())*
### How far does a pill bug walk in a day?
Not far. The same account says individuals travel about 13 metres a day in summer and 6.6 metres in winter, and that they are usually more active at night. *(Source: [Animal Diversity Web]())*
### What does Wolbachia do to a pill bug?
Wolbachia is a bacterium that lives inside cells and passes from mothers to young, and in this species it turns genetic males into females. In one study, a 3-megabase stretch of its DNA had moved into the pill bug's own genome, and the authors concluded that this stretch now works as a female sex-determining region. *(Source: [Leclercq et al. 2016]())*
## Sources
1. [Armadillidium vulgare. Animal Diversity Web.]()
2. [Becker, Ziegler & Epple (2005). The mineral phase in the cuticles of two species of Crustacea consists of magnesium calcite, amorphous calcium carbonate, and amorphous calcium phosphate. Dalton Transactions.]()
3. [Leclercq et al. (2016). Birth of a W sex chromosome by horizontal transfer of Wolbachia bacterial symbiont genome. Proceedings of the National Academy of Sciences.]()
4. [Cordaux et al. (2004). Evidence for a new feminizing Wolbachia strain in the isopod Armadillidium vulgare: evolutionary implications. Heredity.]()
*Published October 11, 2026. Last updated October 11, 2026.*
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