The depth and habits of pupation, shown by the three main species of sheep blowflies (Lucilia sericata, Chrysomyia chloropyga, and C. albiceps) are discussed in relation to parasitism by the Pteromalid, Mormoniella vitripennis.

Experimental series showed that the majority of Lucilia larvae pupate in situations which are inaccessible to the parasite, a result which agrees with field observations; whereas the two species of Chrysomyia have a large proportion of their pupal populations exposed freely to attack by the parasite. The degree of protection thus sought by the larvae appears to be directly proportional to the suitability of the puparia of the species as hosts for the parasite.

Because of this difference in pupation habits, a large proportion of the Lucilia population which survives the competition for food and the predatory attacks of C. albiceps on the carrion will produce adult flies; whereas the more exposed C. albiceps and C. chloropyga populations will be further reduced and in a proportionate manner by parasitism. This explanation is put forward as one of the main reasons for the comparative success shown by field populations of Lucilia and the unexpectedly small field populations observed in the case of C. albiceps.

It is pointed out that this phenomenon is a disadvantage where natural control of the primary flies is concerned since C. albiceps is an important controlling factor in larval populations of Lucilia.