SCHISTOCERCA AMERICANA DRURY 151 



rectum is again much bigger. The anus opens between the 

 podical plates, dorsally to the ovipositor. Morphologically ter- 

 minal in position in all insects, the anus belongs to the twelfth 

 somite. A pair of salivary glands are situated one on either side 

 of the crop. The salivary ducts open on the ligula of the lower 

 lip into the mouth cavity. 



Excretory system. The numerous malpighian tubes which 

 open into the alimentary tract between the mid- and hindgut 

 function as excretory organs. 



Circulatory system. The heart is situated under the body 

 wall in the mid-dorsal line. It has the shape of a long tube and 

 consists of a series of ventricles typically one for every somite, 

 inclosed in a pericardial sinus. Each ventricle is separated from 

 the preceding one by a pair of valves and communicates with 

 the pericardial sinus by a pair of ostia. Thus there are a pair of 

 ostia and a pair of valves for every somite. Anteriorly the heart 

 forms a short aorta which opens into the body cavity in the head. 

 There are no blood-vessels and the blood, which contains amoe- 

 bocytes, circulates freely in the body cavity before it returns 

 to the heart. Metamerically arranged triangular muscles serve 

 to enlarge the pericardial sinus and to draw blood into it. The 

 blood current in the heart is always from the rear end forward. 



Respiratory system. The American locust, together with 

 all other air-breathing insects, possesses a complicated system of 

 tracheal tubes communicating with the outside by a series of 

 paired openings. These openings or spiracles (stigmata) are 

 twenty in number. One pair belongs to the mesothorax, one 

 pair to the metathorax, and the rest to the first eight abdominal 

 somites. In the thorax they are situated in the pleura, dorsal 

 to the places of attachment of the legs; in the abdomen on the 

 terga not far from their lateral edge, one on each side. The 

 spiracles lead into short tubes, opening into two longitudinal 

 trunks. Each trunk gives off numerous branches which pene- 

 trate between the cells of all organs. The smallest branches 

 end blindly in a terminal cell. The two trunks communicate 



