44 THE LARVA OF THE BLOW- FLY. 



which are more or less inflections of their edges, and correspond closely with 

 the internal cephalic skeleton of more perfectly developed head-capsules, 

 such as that of the caterpillar (Fig. 9, <$), from which the dilator muscles of 

 the anterior portion of the alimentary tube arise. 



These must all therefore be regarded as morphologically similar structures, 

 which are represented in the imago of the Diptera by the so-called fulcrum, 

 only a very small part of which, the hypopharynx, arises in the wall of the 

 stomoda:!um.* 



The part of the dipterous mouth which has been identified with the hypo- 

 pharynx by all recent writers is a totally distinct structure, which I term the 

 ligula, and has no relation to the hypopharynx of Savigny, which is a plate 

 developed in the lower wall of the stomodteum. 



As the pharynx is therefore exceedingly complex, I shall speak of the whole 

 organ, the skeletal structures and the muscles, as the fulcrum ; that por- 

 tion of the skeleton which belongs to the head-capsule I shall term the 

 cephalo-pharyngeal skeleton, or, for brevity, the cephalo-pharynx ; that 

 portion developed in the wall of the stomoda^um, the pharynx. The term 

 ' fulcrum ' is now in general use ; but it is not absolutely correct, as it is applied 

 to a part of the mouth of the Hymenoptera which has a ditferent morpho- 

 logical value. 



Between the cephalo-pharynx and the great hooks of the blow-fly larva 

 there is a second sclerite, which is developed in relation with the orifice of 

 the salivary duct and ligula ; it articulates behind with the cephalo-pharynx, 

 and in front with the great hooks. This is the 'connecting piece' of Weismann. 

 I shall term it the hypostomal sclerite, owing to its position in the floor of 

 the mouth. 



Above the hypostomal sclerite, in each lateral wall of the mouth, there is a 

 short rod of chitin, which abuts upon the fulcrum behind ; 1 shall term it the 

 parastomal sclerite. 



The Cephalo-pharyngeal Sclerite may be described as consisting of two 

 vertical plates united by a thin dorsal arch. Each lateral pldte exhibits three 

 processes— an anterior inferior, a posterior inferior, and a posterior superior 

 process. The latter I shall term the cornu. 



In the newly-hatched larva these processes are very slender (Fig. 9, /), 

 but after the second moult they become exceedingly thick and strong. 



The cornua consist, in the adult, partly of dark and partly of transparent 

 chitin ; they exhibit thick upper edges, which lie in close proximity, so that 

 they enclose a cavity, which contains a blood sinus and the dilator muscles' 

 of the pharynx. I shall speak of this cavity as the pharyngeal sinus. 



The Hypostomal Scleritef is H-shaped in the adult larva ; its posterior 

 processes articulate with the anterior inferior processes of the cephalo-pharynx, 

 and its anterior processes support the great hooks. I have been unable to 

 find any trace of this sclerite before the second moult. 



* Macloskie, G., in a preliminary note on the head of larval Musca;, held 

 views which are very similar to my own. 'Psyche,' Vol. iv., p. 21S, Cam- 

 bridge, Mass., 1884. 



t 'Zungenbein' of Schroder v.d.Kolk. 'M^moire surl'Anatomieet Physiologic 

 du Gastrus Equi.' Verhand. d. kl. Nederl. Inst., Bd. ii., pp. 1-155, 13 pi., 1845. 



