Volvation

Volvation (from Latin volvere "roll", and the suffix -(a)tion; sometimes called enrollment), is a defensive behavior in certain animals, in which the animal rolls its own body into a ball, presenting only the hardest parts of its integument (the animal's "armor"), or its spines to predators.

Caecosphaeroma burgundum: two of three pillbugs have curled themselves into "pills"; found in the Nancy spéléodrome (an abandoned underground aqueduct in Villers-lès-Nancy, France).

Among armadillos, only species in the genus Tolypeutes (South American three-banded armadillos) are able to roll into a defensive ball; the nine-banded armadillo and other species have too many plates.[1]

Volvation is used by earthworms during periods of extreme heat or drought.

Among pill millipeds, volvation is both a protection against external threats and against dehydration.[2]

Pillbugs curl themselves into "pills" not only for defense, but also to conserve moisture while resting or sleeping, because they must keep their pseudotrachaea ("gills") wet. Volvation is particularly well evolved in subterranean isopods, but only Caecosphaeroma burgundum is able to roll up into a hermetic sphere without any outward projections, and thus "approaches perfection in volvation".[3]