314 MR. J. DAVIDyON ON THE MOUTH-PARTS AND 



The clypeal sclerites and maxillary scleriiesare separated by the maxillary 

 folds, mx.f. These represent the mandibular folds o£ Muir and Kershaw. 

 As in the case of the clypeal folds, the in-turned edges of the maxillary and 

 clypeal sclerites are held firmly together by connective tissue. In sections 

 the connective tissue tears away and the sclerites become separated. The 

 maxillary and clypeal folds extend towards the proximal end of the fore- 

 head^ where the sclerites merge together to form its lateral walls. 



The internal, lateral walls of the hypopharynx (figs. 11-14) are composed 

 of stout chitin and become continuous with the wall of the pharvnx, forminu" 

 part of the stout chitinous supports which surround its anterior end. The 

 pharynx is held firmly in position by these supports, vide fig. 14. 



B. The Endoskeleton of the Head. 



The endoskeleton of the head consists of strong, hollow, chitinous bars, 

 formed by invaginations of the integument of the head. These bars, which 

 form the arms of the tentorium, give support to the fore-head, and also 

 afford attachment for several muscles. Figs. 2 and 3 show the cliitinous 

 endoskeleton as seen in potash preparations of the head. 



The arms of the tentorium expand at their extremities and form a broad, 

 hollow, transverse plate, t.j)., which is situated in the posterior region of the 

 head, beneath the supra-cesophageal ganglion, and forms the central support 

 of the tentorium. The antero-dorsal arms, the antero-ventral arms, and the 

 ventral rods *, are attached or continuous with the transverse plate. The 

 chitin of this latter structure is thinner towards the middle, but becomes 

 much stronger towards its ends, from whence the tentorial arms pass. 



The antero-dorsal arms, a.d., are formed by two invaginations at the 

 posterior end of the clypeal region of the head. They extend in a posterior 

 direction to the transverse plate, fig. '6, a.d. 



The antero-ventral arms, a.v., pass forward ventrally, diverging from one 

 another, towards the infra-posterior angles of the fore-head. Each then bends 

 upwards, along the lateral wall, and ends at the proximal end of the maxillary 

 fold. From this region a stout triangular bar of chitin passes into the fore- 

 head from each side, which form two levers, md.l., for the dorsal or anterior 

 pair of stylets (mandibles). The position of these levers corresponds to the 

 position of the ends of the maxillary folds, and each lever appears to be a 

 thickening of the fold in this region, fig. 16, md.l., being part of the 

 cl3'peal sclerite. 



The ventral chitinous rods, v.r., fig. 2, and also in figs. 18-21, originate at 

 the posterior ends of the buccal folds, and passing beneath the large pump- 



* These terms are used for convenience in description, and are not necessarily homologous 

 with the arms of the tentorium as described by Comstock (1902) in insects generally. 



