ABSTRACT

Flies with structures protruding from the head, termed eye stalks or antlers, have long fascinated biologists. In this chapter we consider the possibility that such flies either share developmental predispositions to head modification or face similar selective pressures to augment head width. By randomizing the occurrence of eye stalks and antlers among families of acalyptrate flies over a phylogeny, we demonstrate that eye stalks and antlers have evolved especially often within one superfamily and that eye stalks and antlers exhibit correlated evolution. Because extreme sexual dimorphism of antlers and eye stalks occurs in four families, while correlated evolution of eye stalks in females occurs in others, we suggest that the degree of sexual dimorphism, but not the occurrence of head projections, has been influenced by historical genetic factors. Comparison of the ecology and mating systems of flies from the genus Phytalmia, all species of which possess antlers, with flies from the family Diopsidae, all species of which possess eye stalks, indicates selective similarities. All headprojection flies studied to date exhibit some form of resource–defense mating system. Phytalmia spp. defend rare oviposition sites, whereas dimorphic diopsids defend protected sites where females aggregate at night. In both groups, males assess the body size of competitors from the head projection, and males of similar size engage in extended physical contests for control of a resource. Females of at least two diopsid species also use eye–stalk length to select mates and can influence sperm precedence by ejecting male spermatophores.