MECHANISM OF SUCTION IN SCIIIZONEURA LANIGERA. 327 



SO powerful, as the for.ce required to squeeze oat the walls ol: the cylinder 

 must be very considerable. The resulting pull on the transverse plate o£ the 

 tentorium must be very great, and to counteract this there are the two 

 elevator muscles of that structure, m.f.p. These muscles exert a pull in the 

 same plane as the pump-muscle^ but in the opposite direction. 



If the only function of the pump-muscle were to pull out the flexible 

 ])roximal wall such great force would not be needed. Further, it is very 

 improbable that the return of this wall would give sufficient force to propel 

 the saliva from the pump-chamber, along the minute efferent salivary canal, 

 and then down the ejector canal formed between the maxillary stylets. 



When the saliva has collected in the pump-chamber the small ventral 

 muscles (v.m.) are relaxed and the opening from the median salivary duct 

 (afferent canal) is closed. The large pump-muscle is then relaxed and the 

 walls of the pump-chamber return to their normal position. The force of 

 the contracting walls of the cylinder is such that the saliva is forced into the 

 efferent canal, down into the ejector canal. The flexible proximal wall of 

 the cylinder gradually returns to its normal position and the cavity of the 

 chamber is considerably reduced in size^ the contents being forced down the 

 salivary ejector canal into the tissues of the plant at the extremity of the 

 compound maxillary stylet as explained above. When the chamber is 

 emptied, the process is repeated as before. 



As regards the function of the saliva I am not able at present to say any- 

 thing definite. Meek (1903) remarks that the saliva of Hemiptera is alkaline, 

 and has the power of changing starch into sugar. He refers to the work of 

 Plateau (1874), 'Recherches sur les Phenonomene de la Digestion chez les 

 Insectes,' who holds that primarily the saliva serves as a digestive fluid. 



It may be, considering the extreme minuteness of the suction canal, that 

 the ascent of the cell-sap along this canal is largely due to capillarity, and 

 that the addition of the saliva causes the surface-tension of the sap to be 

 lowered, thus facilitating its ascent up the suction-canal. 



When suction is finished the pump-muscles and the protractor muscles of 

 the stylets are relaxed, the stylets being withdrawn from the tissues of the 

 plant by the contraction of the retractor muscles md.re., mx.re. The 

 integument at the posterior end of the buccal cavity is thin and flexible, thus 

 allowing the stylets to be withdrawn for some distance into the head, into a 

 kind of integumental sheath. The pull exerted on the tentorial arms by the 

 retractor muscles is largely counteracted by the two elevator muscles of the 

 transverse plate. 



When the stylets are withdrawn from the plant the proboscis muscles are 

 relaxed and that structure returns to its normal position of repose. Similarly 

 the depressor muscles of the head pull the fore-head into its normal deflexed 

 position. 



