Population stresses during the larval development of A. quadrimaculatus produce characteristic effects on the resulting pupae and adults. Thus, pupae from underpopulated colonies are more uniform, heavier and more viable than pupae from overpopulated colonies, while the adults usually have a higher biting rate and are consistently more constant in their reactions to a blood meal than the adults from overpopulated colonies. The biting rate of adults from overcrowded colonies is related to the rate of pupation, and pupal and adult mortality is increased as the larval developmental period is lengthened. The ratio of males to females within a given adult population is a factor in determining the biting rate of the females, the biting rate increasing as the ratio of males to females is increased.