Seasonal variations in brood size have been described in samples of Asellus aquaticus (L.) Crust. Isop.; the samples were caught monthly in a freshwater pool and kept in the laboratory, at constant tem perature and exposed to natural light. It was shown that:
(1) the average number of larvae per deposition varies from about 35 (in summer) to about 50 (in winter);
(2) when the brood size reaches maximum or minimum values, the mating frequency reaches minimum or maximum values respectively, reestablishing equilibrium.
(3) high temperatures and long photoperiods increasing mating frequency neutralize the reduction of brood size, while low temperatures and short photoperiods decreasing mating frequency neutralize the increase in brood size.