Abstract
This chapter deals with the family Pirenidae, a monophyletic group of parasitoids attacking either Cecidomyiidae (Diptera) or various families of coccids (Hemiptera). They are currently grouped into five subfamilies previously classified in Pteromalidae and Eriaporidae. Generally, they have small (1–2 mm, rarely more), weakly sclerotized and superficially sculptured bodies, and antennae with a reduced number of flagellomeres. An identification key to the five subfamilies is provided.