1. An account is given of the morphology, development, and changes following emergence of the ptilinum of the blow-fly.
2. The ptilinum in Calliphora erythrocephala develops from the integument of the anterior part of the head. Differentiation from the surrounding integument begins first as a local thickening of the epidermis. The structure of the thickened area is similar to that of an epidermal imaginal bud. At the end of the third day after pupation at 25° C. this epidermal thickening grows inwards and, as its surface increases, its walls become gradually thinner. Eudiments of the ptilinooesophageal muscle are obvious even immediately after eversion of the head; other muscles do not appear until the fourth day after pupation.
3. In the newly emerged fly the ptilinum is continuous with the integument of the frons, genae, and vertex. Two unpaired and one pair of retractor muscles are inserted on the ptilinum; six other muscles appear to be accessory in connexion with pulsation of the head during emergence.
4. By rhythmic expansion and contraction of the ptilinum the puparium is ruptured and emergence of the fly assisted. After final retraction of the ptilinum into the head-cavity, and hardening of the surrounding head integument, no more use is made of the organ.
5. During the adult life of the fly the ptilinal integument is reduced to a layer of spinules. Before two days have elapsed after eclosion practically all the ptilinal and accessory muscles have disappeared. Their disappearance does not appear to be due to phagocytic activity. From histological evidence the cause and mechanism of muscle degeneration cannot be ascertained.
6. A possible method of origin of the ptilinum from a membranous area above the antennae is suggested. Eristalis , without ptilinum, is compared with Calliphora. From the evidence brought forward it is suggested that the Aschiza and Schizophora represent two independent lines of development.
Anatomy, Physiology, Morphology, and Development of the Blow-fly (Calliphora erythrocephala)