THE HEAD AND MOUTH ARMATURE. 43 



larger than in the adult, but each only exhibits a simple pair 

 of channels, pseudo-tracheae. When expanded the stomal disc 

 of the young larva has a striking resemblance to the oral 

 sucker of a trematode, and entirely surrounds the mouth. 



The Prestomal sclerite (Figs. 5 / and 9 2 pi}, is a ridge of chitin 

 between the maxillae; its posterior extremity in the newly-hatched larva forms 

 a convex pad covered with minute bristles, and at this stage the discal sclerites 

 (Fig 9, 2} are slender rods diverging from the pad which support the pseudo- 

 trachea; of the disc. 



In the adult larva (Fig. 5, b} these structures are more strongly chitinized, 

 and are no longer connected with the pseudo-trachea; ; they form a kind of 

 pseudo-labrum in front of the external larval mouth, but are not situated at 

 the true orifice of the alimentary canal. Even in the adult larva the remains 

 of the invaginated forehead are still distinctly recognisable in a median 

 section between the prestomum and the anterior extremity of the pharyngeal 

 apparatus, which corresponds with the median tooth, labrum, of the newly- 

 hatched larva. 



The Labium, or lower lip, remains external ; it is broad from side to side, 

 and narrow from before backwards (Fig. 5, //;). It exhibits a median and 

 two lateral lobes, and almost covers the mouth orifice when the larva is not 

 feeding ; it is capable of being retracted by a pair of powerful muscles. 



The Pharynx. Immediately within the true or internal mouth orifice is 

 the massive pharynx. Weismann describes it in the following terms : ' The 

 pharynx is a cylindrical bulb, somewhat pointed in front and behind ; behind 

 it is continuous with the oesophagus, and it might be regarded as the thickened 

 commencement of the oesophagus itself, were it not for strong morphological 

 grounds, which compel us to take a different view. It originates in the 

 embryo as an invagination of the forehead and mandibular segment' [2, 

 p. 107]. This view of Weismann's is undoubtedly correct ; but he advanced 

 only slender evidence in its favour, so that subsequent writers have regarded 

 the strong pharyngeal sclerites as chitinous thickenings of the alimentary tube. 



The pharyngeal skeleton of the larva corresponds in all respects with the 

 fulcrum of the imago. It consists of two vertical plates, connected above 

 by a cross-bar (Fig. 9, j) ; these are the cephalo-pharyngeal apophyses. 

 They enclose a cavity between them, which contains the intrinsic pharyngeal 

 muscles, and are united below by the hypopharynx, a sclerite developed in 

 the cuticular layer of the stomodaeum or foregut. There is no distinct epi- 

 pharyngeal sclerite in the larva. 



The invaginated forehead lies above the stomodaeal tube, and sends a 

 hollow process downwards on each side of it. These processes correspond 

 with the inflected edges of the clypeus in the imago. A sclerite is developed 

 in the hollow of each of the processes ; this is the lateral plate of the pharyn- 

 geal skeleton, which is therefore covered by hypoderm on both sides. 



The connection between the involution of the edges of the clypeus and the 

 imaginal discs is well seen in Chironomus (Hammond, in lit.}. 



The cephalo-pharyngeal apophyses of the eucephalic larvae of the Diptera 

 are, without doubt, the chitinized margins of the plates of the head capsule, 



