120 MANUAL OF ANIMAL BIOLOGY 



in the posterior part of the abdomen, through the thorax, to the 

 head region. Throughout most of this vessel there are paired 

 openings, or ostia, through which blood enters from the surround- 

 ing blood cavity (pericardial sinus). The contraction of the 

 dorsal vessel drives the blood to the head region where it leaves the 

 dorsal vessel and flows into the large general hemocoel which 

 occupies so much of the body space in the thorax and abdomen. 

 In the hemocoel, the blood moves posteriorly and ventrally, bathes 

 the various tissues, including the alimentary canal — from which 

 it receives the absorbed food materials for general distribution — 

 and the Malpighian tubules — to which it transfers nitrogenous 

 wastes for excretion. To complete the cycle, the blood, after 

 having been in contact with many of the body tissues, gradually 

 reaches the dorsal pericardial sinus and then passes into the dorsal 

 vessel again through the ostia. 



3. Excretory System 



In our previous discussion of the nutritive system, note was made 

 of the numerous fine, coiled Malpighian tubules lying in the abdo- 

 men surrounding the intestine. Each Malpighian tubule opens 

 directly into the intestine at the point of attachment. The other 

 end, which lies free in the hemocoel, is closed. The walls of these 

 tubules are composed of cells which are specialized for absorbing 

 the nitrogenous wastes from the blood in the hemocoel, which con- 

 tinually bathes them. The wastes thus picked up from the blood 

 are secreted into the cavity of the tubule, from which they pass into 

 the intestinal cavity and are excreted with the feces. 



4. Reproductive System 



In all Insects the sexes are separate. Great divergence, however, 

 is to be found with regard to the structural differentiation, or 

 sexual dimorphism, between the two sexes. Thus in some species 

 of Insects the structural differences between male and female are 

 so great that they appear to be from widely separated groups. In 

 others, such as the Grasshopper, the sexual dimorphism is very 

 slight and is marked externally only by structural differences in 

 the external genitals. 



Male Reproductive Organs. The sperm are produced in a 

 pair of testes which lie above the intestine near the dorsal wall in 

 the third to fifth abdominal segments. The essential elements of 



