(1) Flight in adults of the rain-pool dwelling midge, Chironomus imicola Kieffer, is sexually dimorphic and can be related to selective pressures of the habitat and to the disparate roles of flight in males and females. (2) For females, the temporary nature and isolation of the habitat necessitate a mandatory dispersal that involves flying, often for many kilometres and for over an hour, with a full load of eggs. Characteristics of sustained flight are relatively long and broad wings that are slow-beating and have a large amplitude of beat. (3) Males, by contrast, have relatively short, narrow, fast-beating wings with a short stroke, and their flight lasts for less than three quarters of an hour. These are characteristics of aerobatic flight appropriate to the acquisition of mates in the face of male competition in a mating swarm. (4) Sexual differences hinge on a disparity in size, males being smaller, on average, than females. (5) These observations do not support the traditional views that large size is advantageous to males in acquiring mates and that males emerge earlier, on average, than do females in order to increase the time available for mating. Small size in males is likely to be advantageous to insects mating on the wing and requiring aerobatic ability. As the larval growth period is reduced for smaller males, protandry may be a device primarily for achieving small size rather than for increasing the time available to males for mating.
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