Animals called roly polies are a kind of land-going crustacean called terrestrial isopods. They are characterized by having seven pairs of legs. They are also called woodlice, pill bugs, sow bugs, slaters, potato bugs and a whole bunch of other things, depending on where you live. Only some species can roll into a ball--those are the ones usually called roly polies.
They roll into a ball in order to protect themselves from predators. The ones that can't roll into a ball have a flatter body shape and longer legs so that they can run away much faster. Some species protect themselves by clinging tightly to the wood they are on, like a limpet or starfish does in the tide pools.
In nature, woodlice are detritivores, which means that they eat fallen dead plant leaves, fungi, bacteria, and fecal matter in the soil. They are nature's garbage-collectors and play an important role in returning nutrients to the soil for plants to use to grow. In the lab, we feed them cut up carrots and potatoes. They will also eat paper, fish food, cat food and almost anything, including poop.
The main predators on woodlice are lizards, frogs, birds, small insect-eating mammals such as shrews (these are uncommon in Southern California, though), and spiders. They don't seem to suffer from heavy predation, however, due to their behavior (stay hidden) and awkward shape for predators to bite. People who keep terrariums use them as food for tropical frogs as well as to help keep the environment clean and sanitary.
Yes! But you need a magnifying glass or low-power microscope. The females have a smooth belly, while males have a pair of pointy appendages making a V-shape on their belly. You can also know that you are looking at a female if she is pregnant.
After a female mates, she grows flap-like plates over her belly and lays her eggs into this marsupial brood pouch. The eggs hatch and the babies grow until they are big enough to survive on their own inside this protected environment while the mom supplies them with nutrients and water. The flaps separate for the babies to crawl out. They are almost clear and only 1.5 mm when born--bigger than a poppy seed but smaller than a sesame seed. They grow to adulthood (around 6 mm) in a couple of months and can live up to three years.
Terrestrial isopods are distributed globally except for anywhere that is freezing cold for too long during the year. There are thousands of species world wide. Some species have become common around the world in human-modified environments. The kind you are most likely to see is called the common woodlouse, Armadillidium vulgare. It is native to the Mediterranean region in Europe but has been found in Southern California since the 1920's.
Places that you are likely to find them are dark and moist, like in your compost pile, under rocks, wood and mulch in irrigated landscaping or gardens. They will come into your house, especially if it is cold outside, usually in basements or garages. If it is too hot or cold they will burrow deep into the soil where the temperature is more moderate.
A day in the life of a roly poly is pretty simple. During daylight hours, they mostly shelter. That means they stay still under a piece of wood or another safe, dark and moist place. At night they will forage for food. If they are at a good food source they will stay there; if not they will explore the environment until they find food, which they can smell. They seem to like switching between foods rather than eating the same thing all the time. This may help them get the right nutrients they need in their diet. They also can smell each other and aggregate together (form groups) in good sheltering sites. In spring and summer, males may be more active searching for mates.
They are not dangerous to humans or really anything else. They can't bite anything larger than their tiny mouths and don't have any venom or poison. In some regions they can be crop pests and they may eat some of your garden plants, but normally they eat fallen leaves, not living plants. They are theoretically consumable, but they would tast bad, since they release their body wastes as ammonia. Plus, there is a lot of shell and very little else.
For woodlice identification, the most important characteristics are the antenna, the shape of the head, the shape of the tail end. Knowing what environment it was in (such as garden versus beach), whether it rolls into a ball or not, and whether it runs or clings when disturbed can help identify the species out of the handful that are common in Southern California. Visit the Terrestrial Isopods of North America Guide at iNaturalist to learn more about what species are found here and to identify the ones you see. Post your pictures there, too and contribute to our knowledge of how roly polies are distributed in the habitat.
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