524 C. GORDON HEWITT. 
channels to the mouth. Distally many of the channels unite ; 
the resulting channels all converge and run into the mouth. 
The anterior border of the oral aperture is occupied by the 
mandibular sclerite (m. s.), and the posterior border is bounded 
by a lingual-like process (/.) that is bilobed at its anterior 
extremity. 
Cephalo-pharyngeal sclerites (Pl. 80, fig. 4).—The 
sclerites associated with the cephalo-pharyngeal region are 
rather similar to those of the second larval instar; they are, 
however, of a more solid and of a thicker character. Between 
the oral lobes is seen the median uncinate mandibular 
sclerite (m.s.). The homology of this sclerite is obscure. 
Lowne regarded it as being the labrum; some authors con- 
sider that it represents the fused mandibles. As we know 
at present so little of the comparative embryology of these 
larve it will be best to retain the name by which it is 
generally known. The basal extremity of the mandibular 
sclerite is broad, and at each side a dentate sclerite (d.s.) is 
articulated by means of a notch in the side of the mandibular 
sclerite, the function of which has been shown already in 
describing the muscles. The mandibular sclerite articulates 
posteriorly with the hypostomal sclerite (h.s.). This consists 
of two irregularly-shaped lateral portions united by a ventral 
bar of chitin; it is anterior to this bar of chitin that the 
salivary duct opens into the front of the pharynx. The 
sides of the hypostomal sclerite articulate with two processes 
on the anterior edge of the lateral pharyngeal sclerites (J. p.). 
The lateral pharyngeal sclerites or plates recall the shape of 
the fulcrum of the adult fly. Hach is wider posteriorly than 
anteriorly, and the posterior end is deeply incised; at the 
base of this incision the nerves and tracheze which supply the 
interior of the pharynx enter. The lateral sclerites vary in 
thickness, as will be seen in the figures of the sections of the 
pharynx. They are united dorsally at the anterior end by a 
dorsal sclerite (d. p. s.), and ventrally they are continuous with 
the floor of the pharynx. 
The pharynx (PI. 3], figs. 17 and 18) in cabal respects is 
