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Adults of the rain-pool dwelling midge Chironomus pulcher are sexually dimorphic with females larger than males. This difference is achieved by females remaining in the growing larval stage for longer. When food is scarce, as under crowded conditions, the average size of both sexes is reduced, but the difference between them is maintained. However, more females perish, presumably because they are unable to achieve a minimum female size. Under conditions of crowding, there is thus a switch in favour of males. Since crowding due to a diminishing habitat is a frequent event in rain-pools, males seem fitter than females in C. pulcher. This paradox is discussed.