326 MR. J. DAVIDSON ON THE MOUTH-PARTS AND 



During the insertion of the stylets, the proximal part of the proboscis is 

 withdrawn into the body by the protractor muscles (7?^.pr.). This enables 

 the stylets to be forced deeper into the tissues of the plant ; at the same time 

 they are strengthened by being supported outside the plant in the proboscis- 

 groove. I have indicated above, that the proximal end of the proboscis is 

 very thin and membranous, which allows it to be readily retracted. 



The mandibles remain free throughout, although they are closely apposed 

 to the compound maxillary stylet, and strengthen this structure during the 

 piercing of the plant-tissues. 



When the stylets have been forced into the desired part of the plant- 

 tissues, suction commences. 



Owing to the very minute lumen composing the suction-canal, s.c, it is 

 very probable that the plant-juices ascend up this canal, largely by means of 

 capillarity, and there is the further factor of the pressure of the sap itself. 



As the divaricator muscles of the pharynx contract, they pull out the 

 flexible, dorsal wall of the pharynx, thus greatly enlarging the lumen of that 

 structure. This causes a sucking action, and as the plant-juices make their 

 way from the suction-canal up the pharyngeal duct, they are drawn from the 

 latter duct into the pharynx proper. 



The divaricator muscles acting on the posterior half of the pharynx are not 

 so strong as those acting on the anterior half. I have shown that the stont, 

 ventral wall of the pharynx between the pharynx protuberances is deeply 

 concave, so that when the divaricator muscles of this part are relaxed the 

 dorsal wall becomes adprcssed to the ventral wall and practically occludes 

 the entrance from the pharyngeal duct. As the other divaricator muscles 

 are relaxed the dorsal wall tends to regain its original position by virtue of 

 its own elasticity, and as a result, the lumen being considerably reduced in 

 size, the juices are forced backwards into the oesophagus, and thus into the 

 stomach. At the entrance of the oesophagus into the stomach, there is 

 formed a well-defined oesophageal valve, which prevents regurgitation of the 

 food into the pharynx *. 



The working of the salivary pump is produced by the large pump-muscle, 

 p.m. When this muscle contracts, the proximal wall of the pump-cylinder 

 is pulled out (fig. 23), thus enlarging the lumen of the pump-chamber. By 

 the contraction of the small ventral muscles, fig. 25, v.m., the entrance from 

 the median salivary duct into the pump-chamber is opened and the saliva 

 passes into the pump-chamber. 



It will be noticed in fig. 23 that the thick proximal rim of the pump- 

 cylinder, which is slightly inturned, is also pulled out, and the strong walls 

 of that structure are thus forced outwards, which is rendered possible by the 

 elasticity of the thick chitin. It is for this reason that the pump-muscle is 



* Davidsou, 3., Quart. Journ. Micr. Sci. vol. Iviii. (1913) p. 680. 



