When a blood-sucking insect 'bites' a sensitized host it must, if it is to survive, depart before the host is alerted by the irritation that accompanies or precedes the 'immediate' reaction. There is a safety period between initial salivary injection by the insect and onset of irritation in the host during which the insect's meal must be completed; immunity from attack by the host must, therefore, depend partly on the speed at which the insect can tap the blood supply and complete its meal, and partly on an adequate delay in the onset of irritation in the host. In mosquitoes, and other insects that depend on a blood meal for egg production, only those that complete the meal within the safety period can lay a full complement of eggs; the others will either be killed or injured by the host before egg development begins, or they will be disturbed before completion of the meal and so lay fewer eggs. Thus, fast feeders and those producing a delay in the onset of irritation will tend to lay more eggs, and these two properties will be maintained by natural selection, with the onset of irritation in the host acting as the main selection force. Experiments were made comparing a population of wild mosquitoes (Aedes africanus) with that of a colonized species (A. aegypti), which had been protected from retaliatory action by the host during the previous 3 years. They showed that while the period between salivary injection and the onset of irritation was the same in both species (ca. 3 min), feeding was usually fast in the wild species, which had presumably been subjected to rigorous selection against slow feeding, but variable and often slow in the captive species, which had been reared for generations in the absence of this pressure. The results also indicated a slight compensatory delay in the onset of irritation following slow feeding in the wild mosquitoes; presumably a difference in antigenic properties of the saliva affects the speed at which the insect can take up blood, and provides what appears to be a remarkable alternative pathway to successful feeding, and so to maximum egg output and thus to survival.