Mating experiments with several strains of C. p. pipiens and C. p. fatigans at different temperatures showed that fatigans was more active at all temperatures, but was less inhibited than pipiens at the highest temperature. With decreasing intensity of illumination, fatigans began swarming sooner than did pipiens, but ceased shortly after complete darkness was attained, whereas pipiens continued to swarm in the dark.
It is concluded that fatigans does not exhibit a specific temperature adaptation for mating, but that the higher insemination rates at all temperatures were the result of an inherent sexual aggressiveness. However, such differential activity could be a means of limiting the establishment of pipiens in warmer areas, where it would have to compete with the more active fatigans. Furthermore, these differences in response to temperature and changes in illumination could be a mechanism for keeping the populations segregated in some areas where they coexist.