(84) 

 524 0. 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 (ra. s.), and the posterior border is bounded 

 by a lingual-like process (I.) that is bilobed at its anterior 

 extremity. 



Cephalo-pharyngeal sclerites (PI. 30, 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 

 larvae 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 (L p.). 

 The lateral pharyngeal sclerites or plates recall the shape of 

 the fulcrum of the adult fly. Each is wider posteriorly than 

 anteriorly, and the posterior end is deeply incised ; at the 

 base of this incision the nerves and tracheae 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. 31, figs. 17 and 18) in certain respects is 



