THE THORAX 503 



natural elasticity of the parts to assert itself, as they slip back to the 

 position of rest. If, on the other hand, the points of the piercing stylets 

 are fixed in the skin, alternate protraction and retraction of the maxillae 

 will bore a hole. Once the external resistant layer has been pierced, 

 vis a tergo can come into play, and the whole organ is thrust in by the 

 muscles of the body till the layer of blood is reached. A forward thrust 

 can be readily imparted to the proboscis in the case of the bug, as there 

 is no membraneous neck. 



The thorax of Cimex is divided into three segments, of which the first 

 is much the largest and is the best differentiated. There are no wings, 

 but the mesothoracic pair are represented by small 

 elytra. The legs are well developed, but are not spe- 



. * , ' HI 3,16 LX I ' 



dally adapted to enable the bug to cling to its host as 

 in the case of lice, or for jumping as in the fleas. 



The prothorax is the most conspicuous segment of the body when 

 seen from the dorsal surface. It is about twice as broad as long, and 

 articulates closely with the head in front and the mesothorax behind. 

 The anterior borders are concave, and embrace the posterior end of the 

 head ; the lateral borders are strongly convex, and are produced forwards 

 into prominent angles ; the posterior border is straight on both surfaces. 

 All the sclerites are welded into a compact whole, the lateral borders, 

 where the tergite and sternite meet, being sharp. The dorsal surface is 

 arched, and bears many hairs, while the ventral surface is flatter, and 

 is interrupted on each side of the middle line by an oval space for the 

 articulation of the first leg. The two spaces are separated from one 

 another by a prominent spur which projects between them from the 

 front. 



The mesothorax and metathorax are small and rather irregularly 

 arranged. On the dorsal surface there is a small triangular plate, artic- 

 ulated in front to the prothorax, which represents the mesonotum ; its 

 posterior angle projects between the elytra. Posterior to it there is a 

 broad plate, resembling an abdominal tergite, and lying under cover of 

 the elytra at the sides, which forms the metanotum ; it is only thinly 

 chitinized. On the ventral surface in the middle line there is first a 

 smooth transverse plate, corresponding to the mesosternum, and behind 

 it, and between the coxae of the middle pair of legs, a more elongate 

 plate, at the posterior border of which the opening of the stink glands is 

 situated ; this latter plate is regarded by Landois as a process of the 

 mesothorax, but it appears to correspond more nearly to a metaster- 

 num. At the sides of the mesosternum there are two slight elevations, 



