322 ME. J. DAVIDSON ON THE MOUTH-PARTS AND 



The maxillary levers. 

 The maocillary levers are two curved rods of chitin which articulate one at 

 the proximal end of each maxillary sclerite ; each passes into the fore-head, 

 beneath the maxillary stylets (figs. 2 & 3, m.v.l., and fig. 17), to the sv^'ollen 

 proximal ends of which they become attached, thus affording moveable 

 supporting levers for those structures. The levers project a little way 

 beyond the internal face of the stylets, as shown in figs. 2 and 18, and thus 

 afford attachment for some of the retractor muscle-bands, mx' .re. 



The mandibles. 



The anterior stylets are the mandibles. They lie dorsal to the maxillary 

 stylets, being disposed slightly further from the median line (fig. 3, md.). 

 Although slightly longer, they resemble the latter in general appearance, 

 but remain separate throughout. As they enter the buccal cavity they 

 approximate towards the median line, and extend above the maxillary stylets 

 along the internal walls of the hypopharynx (rzVZe figs. 4-14). 



Towards the extremity of this structure the internal faces of the mandibles 

 are somewhat concave, and fit closely against its lateral walls. Beyond the 

 junction of the maxillary stylets the mandibles lie by the side of the com- 

 pound stylet, the internal concave faces of the former closely fitting against 

 the convex outer faces of the latter. They then extend by the side of this 

 stylet along the proboscis-groove. 



The mandibular levers. 

 The mandil)les are supported by two stout arms of chitin (md.L, fig. 3), 

 which extend, one on each side of the fore-liead, from the proximal end of 

 the maxillary fold (fig. 16). They are much stronger than the maxillary 

 levers. Each passes to the base of the mandibles, to which they become 

 attached. 



J. Musculature of the Stylets. 



The mandibles and maxillary stylets can be protracted or retracted by 

 a system of protractor and retractor muscles. 



The retractor muscles of the mandibles are inserted on the internal face, at 

 the proximal end of those structures (fig. 3_, md.re., and figs. 16-21), and 

 extending backwards into the head, divide into two groups, one group 

 becoming inserted along the antero-dorsal arms of the tentorium, the other 

 on the transverse plate of chitin, at the base of these arms. 



The jyrotractor muscles, md.pr., are inserted along the base of the mandibular 

 lever, and pass forwards to become inserted along the pharyngeal struts. 



The retractor muscles of the maxillary stylets (fig. 2, and figs. 18—21, mx.re.) 

 consist of two groups of muscles. One group is inserted on the internal face 



