Stalk-eyed flies of the family Diopsidae exhibit a unique form of hypercephaly, which has evolved under both natural and sexual selection. Male hypercephaly is used by female diopsids as an indicator of male quality. By choosing to mate with males expressing the mostexaggerated hypercephaly, females can benefit both from the enhanced fertility of these males and the transmission of other heritable advantages to their offspring. Stalk-eyed flies are close relatives of the model organism,Drosophila melanogaster.We have shown that similar genetic and cellular mechanisms regulate the initial development of the head capsule in fruitflies and diopsids. The great diversity of stalk-eyed fly species, exhibiting varying degrees of hypercephaly and sexual dimorphism, constitutes a major advantage for comparative studies of their development and evolution.