Grades K–2 · Organisms
How can a pill bug roll into a ball?
A pill bug has a hard outer coat made of joined pieces. It can walk on fourteen little legs. When it curls up, those legs disappear inside a small ball.
In the diagramJoined plates let the body curlWalkingCurled upSeven pairs of legsLegs tucked insideThe hard back wraps around the soft underside
The hard pieces overlap rather than forming one stiff box. Their joins allow the body to bend. Muscles pull the body inward, bringing the head and rear closer together.
The hard back becomes the outside of the ball. The soft underside and legs tuck inside. Curling can help protect the animal and reduce water loss. Not every small creature called a woodlouse can make such a tight ball.
About this explanation
Starting knowledge: Understand bending and inside versus outside.
Sources
- Natural History Museum of Los Angeles County — Wheel Through the Museum: Roly-poly roll call
- Smigel and Gibbs, Journal of Insect Science — Conglobation in the pill bug, Armadillidium vulgare, as a water conservation mechanism