316 ME. J. DAVIDSON ON THE MOUTH-PAETS AND 



this region that the stout pharyngeal struts and the thick, internal walls of 

 the hypopharynx meet, thus forming a firm support for the pharynx. 



The pharynx proper (z. e., the part which exerts the sucking force) begins 

 here, and as is seen in the transverse sections, figs. 12-22, the dorsal wall is 

 composed of thin flexible chitin, which is acted upon by large divaricator 

 muscles, d.m. Anterior to the pharynx protuberances the continuation of the 

 pharynx dips down, and its walls are entirely composed of very stout, rigid 

 chitin, figs. 7-11. The thick chitinous upper wall is formed by the epipharynx, 

 e.p., and the lower wall by the hi/pophari/iuv. This portion does not exert 

 any sucking action, and is simply a duct, the pliaryngeal duct, ph.d., which 

 conveys the plant-juices into the pharynx proper. This duct, as will be 

 shown later, leads directly into the suction-canal formed by the approxi- 

 mation of the maxillary stylets. The pharynx proper extends from the 

 position of the pharynx protuberances to the transverse chitinous plate 

 of the endoskeleton. It is concave in section, the thin dorsal wall fitting 

 against the stout ventral wall, and possesses a fairly wide lumen. 



The pharytiw protuberances are two hollow, dome-shaped prominences, 

 formed by the meeting of stout chitinous arms in this region, as shown in 

 figs. 12 and 13. They afford a strong support for the anterior end of the . 

 pharynx. The dorsal wall of the pharynx, which is thin and flexible, fits 

 snugly between the protuberances into the deep bay formed by the thick 

 ventral wall. In this region the dorsal wall is acted upon by strong 

 divaricator muscles, d.m., which are attached to a median^ vertical tendon, 

 m.t. It will thus be seen that when these muscles are relaxed, the upper 

 wall of the pharynx fits closely against the ventral wall between the pro- 

 tuberances and practically occludes the opening into the pharynx, thus 

 acting as a kind of entrance-valve. It seems to me, therefore, that the 

 function of the protuberances is simply to strengthen the anterior end of 

 the pharynx, and to render this valvular action more complete. 



The pliaryngeal duct, which really forms part of the pharynx, can be 

 discussed separately, because its function differs from that of the pharynx 

 proper, in that it exerts no sucking-force, but is simply a conducting-canal 

 through which the plant-juices are conveyed from the suction-canal. 



The pharyngeal ditct, -^gs. 10 and 11, ph.d., dips down and leads to the 

 extremity of the hypopharynx lip *, Its roof is formed by a specially thick 

 plate of chitin, which also forms the floor of the clypeo-labrum, cl.l. This 

 structure is the epipharynx, ep., figs. 3 and 7-11. 



* The mouth, or oral opening, which is really the entrance from the buccal cavity into 

 the pharynx, may be considered as being here. Owing to the special mode of feeding in 

 Hemiptera however, the food-juices, a.s will be shown later, really enter at the extremity 

 of the compound maxillary stylet, and pass upwards through a special canal into the 

 pharyngeal duct, so that the actual mouth is situated at the extremity of this stylet. 



