Subject: big fly
Location: catoctin mountains, maryland
September 9, 2013 10:42 am
I saw this fly outside on the table and ran to snap a picture. Is it a horse fly? It is about an inch long and entirely black.
Yes this is a Horse Fly, and as you noted, it is black, so it makes sense that it is commonly called a Black Horse Fly, Tabanus atratus. Your individual is a blood sucking female Black Horse Fly as evidenced by the space between her eyes. The males of the species, like all Horse Flies, have much larger, more close-set eyes. One can only speculate that such sexual dimorphism is due to mating and sexual reproduction. We guess that male Horse Flies use their large eyes with 360 degree vision to spot females.