Face fly, Musca autumnalis DeGeer, puparia have brittle and calcified cuticle containing 63% ash and 16% Ca; whereas cuticle of sclerotized puparia of house flies, M. domestica L., posses 16 and 1%, ash and Ca, respectively, and are thinner and more elastic than that of face flies. The adaptive value of this unique process of cuticle modification is not known. Although parasitic Pteromalid wasps that develop in face fly pupae can’t chew their way out of the puparium, the killing of the parasite can’t offer any selection advantage to the parasitized host. Evidence is presented to support the idea that puparial calcification in face flies developed to prevent oviposition by parasitoids.