4l8 THE INVERTEBRATA 



the rostrum is bent beneath the body, and when the insect feeds it 

 is extended forward and the stylets projected to penetrate the host 

 tissues (Fig. 309). In some plant-feeding species the stylets are 

 immensely long and very slender and it is difficult to explain the 

 mechanism by which they are forced into the tissues as far as the 

 vascular bundles, but the mechanical insertion of the stylets is greatly 

 assisted by a solvent action of the saliva which appears to loosen the 

 plant cells from one another and to allow the stylets to pass between. In 



Fig. 309. Mouth parts of the Hemiptera. A, Sagittal section through head 

 of Graphosoma italicum. After Weber. B and C, Diagrams of mouth parts 

 and adjacent region of the head. C is a transverse section across B at the 



point X X. After Imms. Ibm. labium; Ibr. labrum; 7nd. mandible; 



mx. maxilla; ph. pharynx; /)/j.^. muscles of pharyngeal pump ; sty. stylets. 



Aphis rumicis the phloem cells of the plant are eventually pierced and 

 their contents sucked out. The pumping action is performed by the 

 muscles of the pharynx. 



The order is divided into the Heteroptera (Fig. 310), in which save 

 for their membranous apex the front pair of wings are harder than 

 the hind, and the Homoptera with the front pair uniformly harder. 

 The Heteroptera contain such families as the Capsidae living on plant 



