Abstract
Agria [= Pseudosarcophaga] affinis (Fall.) and Aphaereta pallipes (Say) make an excellent host-parasitoid couple for a determination of effects of host diet on a parasitoid. The anatomy, life history, and behaviour of these insects have been described (Salkeld, 1959; Coppel et al., 1959). Moreover, A. affinis can be reared axenically on chemically defined diets and it is readily attacked in the laboratory by A. pallipes, a parasitoid of the onion maggot, Hylemya antiqua (Meig.). This parasitoid deposits eggs into the body cavity of its host and these increase in size: the duration of the egg stage depends on the age of the host larva when parasitized (Salkeld, 1959).