Baltic amber fossils reveal early evolution of sexual dimorphism in stalk-eyed flies (Diptera: Diopsidae)

A cladistic analysis of 16 species of extant and amber fossil stalk-eyed flies of the family Diopsidae places the fossil †Prosphyracephala succini (Loew) as the sister group of all other Diopsinae, the subfamily in which eye stalks occur. The study is based on a scoring including five old and 23 new finds of †P. succini from Baltic amber, and for the first time allows a morphometric analysis of eye span and various body size parameters in this species. The data indicate that sexual dimorphism of the eye stalks already existed in †Prosphyracephala, suggesting that this feature evolved early in the Diopsinae. Contrary to recent views that the ancestral condition of diopsine eye stalks was monomorphic, the new results suggest that sexual selection was involved in the evolution of eye stalks from the very beginning of the lineage.