pill bug

noun In both senses also called slater, wood louse; also called regionally doodlebug.

1. A small terrestrial isopod crustacean of the genus Armadillidium closely related to the sow bug and having a convex, segmented, flexible body enabling it to curl into a ball when disturbed. In this sense also called roly-poly.
2. A sow bug.

Usage Note: Many people, especially science teachers, like to make a distinction between pill bugs and sow bugs. According to this usage, a pill bug, or roly-poly, is a terrestrial isopod crustacean whose segmented body gives it the ability to curl up into a ball when disturbed. A sow bug is any of several similar looking creatures that lack this ability. In popular usage, however, the names pill bug and sow bug, along with a number of other popular names, are used to refer to any of these tiny segmented crustaceans that are often mistaken for insects.