MOVEMENT 



67 



Fig. 24B, — Longitudinal Section through Head of Plant-bug For 

 explanation, see Fig. 24A. 



the beak. The liquid food obtained by the piercing action 

 of the jaws is sucked in by movement of the walls of the 

 pharynx, the region of the food-canal into which the 

 mouth opens and from which the gullet leads out. There 

 is a group of divaricator muscles {d) passing from the 

 inside of the facial head region to the front wall of the 

 pharynx ; the contraction of these increases the capacity 

 of the organ. They are antagonised by lateral pharyngeal 

 muscles which arise from the head skeleton on either hand 

 and are inserted into thickly chitinised paired regions of 

 the pharyngeal wall. When these are pulled apart by the 

 contraction of the lateral muscles, the dorsal and ventral 

 walls of the pharynx are drawn closely together, and the 

 cavity is almost obliterated. 



For the digestion of an insect's food, movement of the 

 walls of the digestive canal are necessary, and the action 

 of the muscular layer of the proventriculus in constricting 

 its aperture has been discussed in the previous chapter 



