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SUMMARY Contrary to the impression given by their extreme scarcity among extant species of flies, moveable processes on the abdomen are apparently of relatively simple developmental origin, and they have evolved multiple times in males of the small family Sepsidae. They are used to stimulate the female during copulation in two groups, where they are probably independently derived. Because female cuing of reproductive decisions on particular types of stimuli will tend to favor male abilities to elaborate such stimuli, sexual selection by female choice may sometimes result in sustained selection for certain types of innovations in males. The lack of moveable appendages in most dipterans may be due not to developmental constraints, but to lack of selective advantages.