Gigantodax is a genus of 68 species of black flies distributed along the Andes from Mexico to Tierra del Fuego in Argentina.

[4] In 1997, researchers from the Universidad Nacional del Comahue in Rio Negro, Argentina collected larvae of G. marginalis and other fly species in Lanín National Park, Neuquén Province.

After a close examination included chromosome mapping, they reported: Females are not anthropophilic and can be found as well as in mountain creeks from sea level to 4,700 m of altitude (Wygodzinsky & Coscaron 1989).

[1] These South American researchers also analyzed G. marginalis, G. fulvescens, and G. chilensis and reported that Cnesia, another Prosimuliini genus, is sympatric with southern populations of Gigantodax and that both genera breed on both sides of the Andean range in Argentina and Chile in subantarctic Patagonia.

These genus was re-examined in 2007 by entomologists from the Universidade Federal do Rio de Janeiro in Brazil who explained that in their paper:...the first phylogenetic hypothesis for the 13 Southern Hemisphere genera of Simuliidae is proposed, through a cladistic approach.