Abstract
The prestomal teeth in mouthparts of the face fly ( Musca autumnalis ) are long (ca. 60 μm), with jagged terminal points clearly projecting beyond the pseudotrachea when the labellae are in the scraping position. In contrast, the teeth of the house fly ( Musca domestica ) are more reduced (ca. 20 μm) and barely extend beyond the labellae when they are in the scraping position. These morphological differences and the behavior of the face fly in contacting the eye conjunctival tissues during feeding account for the observed damage caused by this fly to the eyes of cattle. This damage is not observed for the house fly.