Adaptive variation in offspring size in the terrestrial isopod, Armadillidium vulgare.

Variation in the birth size of offspring of the terrestrial isopod, Armadillidium vulgare, was observed in laboratory experiments and in field populations. In the laboratory, larger offspring were produced when the mother's food supply was reduced. In field populations, larger offspring were produced during the summer, a period of reduced food availability. Smaller offspring are produced in the spring, when food is readily available. Females may be making larger young to increase survival during the more severe conditions of the summer breeding period.