If you grew up thinking these cute little bugs were some kind of beetle, you're not alone. Most of us think that roly-polies — also known as pill bugs, potato bugs and wood shrimp — must be a type of insect since they're always found under rocks and logs with other creepy crawlies.
However, these unique little animals are like fish out of water, in a way. Roly-polies are crustaceans that crawled out of the sea onto land millennia ago.
How do we know this? Well for one thing, they still have gills!
"Gills work great in the water. They’re basically exposed mucous membranes that absorb oxygen out of the water and into the blood that feeds the rest of the body. But on land, gills are a liability. If the pill bug dries out, its gills won’t function properly and the pill bug can suffocate. That’s why you usually only find them in damp areas, like under a dead log... [O]ver evolutionary time, the pill bug’s gills have adapted to life on dry land. Folds in the surface of their first two pairs of gills eventually turned into hollow branched structures, almost like tiny lungs."
So the next time you come across a roly-poly curled up in a tight ball, know that you're looking at the dry-land descendant of a once-aquatic species.
Check out this fascinating video from KQED that highlights the unique features of roly-polies: