The native species Exorista mella and the introduced Exorista larvarum are very similar. These species have 4 postsutural dorsocentral bristles. Other species of Exorista in North America have 3. These species have no discal bristles on abdominal segments 3 and 4; some other species do.
Exorista larvarum is an important parasite of the Gypsy Moth (Lymantria dispar). E. mella larvae are unable to penetrate the caterpillar's skin.