ORNAMENTS IN THE DIPTERA

Occasionally, flies bear sexually dimorphic structures (ornaments) tha t are used, or are presumed to be used, in courtships or in aggressive interactions with sexual rivals. These are reviewed, beginning with p rojections from the head, continuing through elaborations of the legs and finishing with gigantism of the genitalia. Several functions for o rnaments are considered, including advertisement of genetic properties , subversion of female mate choice and ''runaway' sexual selection. Ne ither the type of ornament nor the degree of elaboration necessarily i ndicates which of the above processes is responsible for a particular ornament. Resource distribution and the resulting possibilities for re source defense and mate choice explain the occurrence of ornaments in some species. The phyletic distribution of ornaments may reflect forag ing behaviors and the type of substrates upon which courtships occur.