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



finished feeding, and these muscles contract, the fore-head is pulled back 

 into the normal deflexed position. 



There are two other bands of muscles attached to the ends of the transverse 

 plate of chitin (text-fig. 2, m.e.p.). They extend through the head, almost 

 parallel, in a plane continuous with that of the large pump-muscle, and 

 are inserted on the wall of the head. These are the elevator muscles oj 

 the transverse plate. From their position the function undoubtedly is to 

 counteract the pull on this structure, of the large pump-muscle and the 

 retractor muscles of the stylets. 



I have shown that the salivary pump is held firmly in position by chitinous 

 supports. When the large pump-muscle contracts, thus operating on the 

 proximal wall of the pump- cylinder, there must be also a considerable pull 

 on the transverse plate, and if this structure were allowed to be pulled in the 

 direction of the pump, the muscle would be deprived of a considerable 

 amount of its force exerted on the pump. This is overcome by the action of 

 the elevator muscles of the transverse plate. When these muscles contract 

 they exert a pull along the same plane as the pump-muscle, but in the 

 opposite direction. 



L. How Suction is accomplished. 



I have indicated when describing the anatomy of the mouth-parts how 

 the plant-juices are conveyed into the pharynx, and how the saliva is 

 conveyed into the plant-tissues. 



I shall now o-ive a concise account of the way it seems to me the mouth- 

 parts work during the operation of feeding. 



Owino- to the minute structure concerned it has not been possible to work 

 the parts experimentally, or to observe them working in the living insect, so 

 that the following conclusions have been arrived at by means of a careful 

 study of the anatomy, and especially the distribution of the muscles. 



The proboscis is raised from the body, and a suitable part of the plant-host 

 is selected in which to insert the stylets. The distal article of the proboscis 

 is freely moveable, and exerts a tactile function by means of the tactile hairs 

 on its extremity. 



The fore-head is also raised from its deflexed position by the contraction 

 of the two elevator muscles of the fore-head (m.d.p.). The stylets lie 

 along the longitudinal groove on the anterior face of the labium, to which 

 the labrum is closely adpressed. 



The fore-head and proboscis are now in the position for the working of 



the stylets. 



By means of the large protractor muscles (fig. 23, mx.pr.), the compound 

 maxillary stylet is forced into the tissues of the plant. This stylet, as has 

 been described above, is held firmly in position by the deep grooves on the 

 internal face of the epipharynx and labrum and the grooves on the internal 

 walls of the maxillary sclerites. Proximally, before they come together, the 



