Are Pill Bugs Insects? Why Roly-Polies Are Crustaceans

What kind of animal is a pill bug?

Pill bugs, also called roly-polies, belong to the arthropods, the huge group of animals with jointed legs and a hard outer skeleton. Arthropods include insects, arachnids, myriapods and crustaceans, and pill bugs sit firmly among the crustaceans. Here is where the common pill bug fits:

Woodlice are the land-living branch of the isopods, a group of roughly 10,000 species that mostly live in the sea and in fresh water. Some sea-living isopods are giants: the deep-sea giant isopod of the Atlantic can grow to more than 30 cm (12 in) long. About 3,700 species of woodlice have been described, and experts think there may be 5,000 to 7,000 in total.

Pill bug vs insect: the key differences

The quickest way to tell a pill bug from an insect is to count its legs. Insects always have six legs as adults, while a pill bug has 14. There are many other differences:

None of these features alone proves a creature is a crustacean, but together they leave no doubt. The second pair of antennae is a classic crustacean trait: in woodlice the first pair has shrunk to tiny stubs, while the second pair does the feeling and smelling.

How do pill bugs breathe out of water?

Insects breathe through a network of air tubes called tracheae, which open to the air through small holes along the body. Pill bugs inherited a different system from their sea-living ancestors. Under the abdomen are five pairs of flat appendages called pleopods. In marine isopods these work as gills; in pill bugs some of them contain white, spongy patches of branched air tubes, often called pleopodal lungs.

These lungs work only when they are damp, which is why pill bugs hide under stones and logs by day and come out at night. Woodlice also lose water through their shells much faster than insects, whose waxy outer layer keeps moisture in. Another crustacean feature is their blood, which carries oxygen with hemocyanin, a copper-based pigment, and much of their nitrogen waste leaves the body as ammonia gas through the shell rather than as solid waste.

A brood pouch like a sea creature’s

Female pill bugs carry their eggs in a fluid-filled brood pouch called a marsupium, made of overlapping plates on the underside of the thorax. This is a trademark of the peracarids, the group of crustaceans that includes isopods, sand hoppers and opossum shrimps. The young develop in the pouch and hatch as pale miniature adults called mancae, with six pairs of legs instead of seven.

In effect, the mother carries a small private pond. Instead of laying eggs that would dry out, she keeps them bathed in fluid until the young are ready to face the land. Most insects, by contrast, lay eggs and leave, and many of their young look nothing like the adults.

How woodlice moved onto land

Woodlice descend from marine isopods that crept up onto the shore. Living examples of this transition still exist: sea slaters of the genus Ligia scuttle over rocks at the edge of the tide and rarely stray far from the splash of the waves. Over millions of years, other woodlice moved farther inland, into forests, grasslands and even deserts.

The oldest woodlouse fossils, preserved in amber from Spain, France and Myanmar, are about 100 million years old, from the middle of the Cretaceous period. Because woodlice were already varied and widespread by then, scientists think they first came ashore much earlier, perhaps more than 300 million years ago. Today the most extreme survivor is the desert woodlouse Hemilepistus reaumuri of North Africa and the Middle East, which lives in family groups in deep burrows that keep the air humid.

Pill bugs, sow bugs and other look-alikes

Not every woodlouse is a pill bug. Sow bugs, such as the common rough woodlouse, are flatter, have two little tail points at the rear and cannot roll up; only the pill bugs and a few relatives can make a ball, as explained in why do pill bugs roll into a ball?

Another look-alike is the pill millipede, which also rolls into a ball but has 17 or 19 pairs of legs and is a myriapod. Millipedes, centipedes, spiders and woodlice are all arthropods that people often call bugs or insects; if you want to sort out the eight-legged ones, see spider vs insect. For ordinary use, the word bug is fine, but to a biologist a roly-poly is a crustacean that lives on land.