Flip over a rock or a pile of damp leaves in almost any Colorado yard this month and something small and gray will curl into a ball. Kids call them roly-polies. Colorado State University calls them pillbugs, and its fact sheet about them carries an unusual title: ‘Colorado Crustacean of Interest.’ That title answers the question. Roly-polies are not insects.
The short answer: they are crustaceans
CSU Extension’s fact sheet on millipedes, centipedes and sowbugs describes sowbugs and pillbugs as small, gray crustaceans usually found outdoors under rocks or other cover. CSU’s pillbug fact sheet places the common pillbug in the order Isopoda, and University of Kentucky entomologists describe isopods as crustaceans capable of living on land.
The quickest test is to count legs. An insect has six. A pillbug has seven pairs, 14 in all, each about the same size and shape. Its back is a row of plates that CSU compares to the look of an armored vehicle, usually blue-gray with some yellow spotting. Adults are rarely more than three-quarters of an inch long, according to the Kentucky fact sheet.
Why you always find them somewhere damp
Because they are crustaceans, pillbugs take in oxygen with gills, and CSU’s sheet says those gills need continuous wetting. The same sheet notes that the small appendages at the tail end can wick up water, so a pillbug can drink from both ends. Kentucky’s entomologists add that rolling into a ball is not only a defense. It also helps the animal hold on to water.
That need for moisture explains the rest of their habits. CSU describes pillbugs as scavengers that feed mostly on moist, decaying plant matter, usually at night, and spend the day under cover. Kentucky lists leaf litter, mulch and the undersides of stones as their usual outdoor homes, which makes October’s leaf piles a good place to look.
Five true things to tell a kid
- They are immigrants. CSU says the pillbug is an introduced animal, originally native to Europe, that can now be found statewide in Colorado.
- Mothers carry their eggs in a pouch. CSU says a female carries about two dozen eggs at a time in a pouch called a marsupium, often two broods a year.
- They molt in halves. The front half of the old shell comes off first, and the back half follows two or three days later.
- They live longer than you would guess. CSU says pillbugs can survive two or three years.
- Not every roly-poly rolls. Pillbugs curl into a ball. Their look-alike cousins, sowbugs, are flatter and cannot curl up tightly.
A 15-minute backyard investigation
These steps are our suggestions, built on the facts above. They work for a preschooler with a grown-up or an older kid with a notebook.
- Look in the dampest spots first: under a rock, a flowerpot or a log, in mulch, or at the bottom of a leaf pile. Lift slowly and put everything back the way you found it.
- Count the legs with a magnifying glass or a phone camera on zoom. Seven on each side means crustacean, not insect.
- Sort your finds. Touch one gently. A pillbug rolls into a tight ball. A sowbug hurries away.
- Run a choice test. Line a shoebox lid with paper towel, dampen one half and leave the other half dry, then set a few roly-polies in the middle. Ask kids to predict which side the animals will pick and why, then check after five minutes. The gills are the clue.
- Draw one, then let them all go in the same damp spot within the hour, and wash hands.
For nervous parents: CSU Extension says sowbugs and pillbugs do not bite and are harmless to humans.
If they turn up in the basement
CSU Extension says sowbugs and pillbugs may move into homes at some times of the year, particularly after extended wet spring weather. They do not last long indoors, and the fact sheet says control is generally unnecessary. Kentucky’s entomologists say the animals usually die inside for lack of moisture, and that clearing leaves, mulch, rotting wood, stones and flowerpots away from entrances and the foundation may greatly reduce the number that wander in. The practical version is to sweep them up and put them back outside.
A picture book that starts the conversation
One read-aloud built on these facts is ‘Curio Power and the Roly-Polies,’ a picture book by Linda Ellis, illustrated by Jean Ruth, about two sisters who shrink down into their garden and help a crew of roly-polies look for a new home. The book’s site lists it as a 32-page picture book for ages 3 to 8, priced at $24.99. Its Meet the Roly-Polies page has a short list of roly-poly facts in kid language (crustaceans, gills, 14 legs, the famous roll and a diet of decomposed leaves) that lines up with what the university fact sheets say. It adds one we did not find in those sheets: that a lost leg grows back in about a week. The story itself is described on the About the Book page, and our Local Business Spotlight on Curio Power has where to buy it.
Disclosure: Curio Power and the Roly-Polies is a Holland Health Marketing client.
Take it further around the Front Range
- For a guided look at invertebrates, see our listing for Butterfly Pavilion field trips in Westminster.
- For more places to turn over logs with a naturalist, try the CSU Environmental Learning Center in Fort Collins, Majestic View Nature Center in Arvada or Bear Creek Lake Park in Lakewood.
- Wondering what to do with the leaves once the hunt is over? Watch CSU Extension’s PlantTalk video on mulching leaves into your lawn.
- Homeschool families counting this as science time can check the state’s rules in what Colorado requires of homeschool families.
Sources
- Colorado State University, ‘Colorado Crustacean of Interest: Pillbug’ fact sheet, listed on CSU Extension’s Western Colorado Insects page on pillbugs and sowbugs
- Colorado State University Extension, Fact Sheet 5.552, Millipedes, Centipedes and Sowbugs
- University of Kentucky Department of Entomology, ENTFACT-439, Sowbugs and Pillbugs
- curiopowerbook.com, Meet the Roly-Polies and About the Book pages
More like this: Learn · Family + Learning · Homeschool nature resources · Wild Bear Nature Center group programs
