Length: fly in photo 3.9 mm
Three genera of Black Flies are found in Central America: Gigantodax, Tlalocomyia, and Simulium. The latter two are relatively less common, developing in mountain streams. Only Simulium is a pest of humans.
In Simulium species and other members of the subfamily the facets of the eyes are larger on the dorsal side than on the ventral, particularly in males. The fly shown here was eagerly tapping into the photographer's circulatory system.
Photo location: Naranjo, Costa Rica.
American Insects site