56 The Form of Insects 



central line of the pericardium below the heart 

 (I, 2, 30, a). 



Circulation. — It is now possible to trace the course 

 of the blood as it circulates. In living, transparent 

 insects the flow of the blood from behind forward 

 through the heart and aorta can be watched through 

 the skin. The blood passes through the opening of 

 the aorta into the body cavity, where it bathes and 

 nourishes the various organs, receives food-substance 

 from the food-canal, gives up its waste matters to 

 the kidney-tubes and air-tubes, receiving pure oxygen 

 from the latter. Then it passes upward through the 

 perforated membrane into the pericardium, whence 

 by the series of paired slits it finds its way back into 

 the cavity of the heart. From the body-cavity the 

 blood flows into the various appendages — legs and 

 wings. In the wings it circulates through the nervures, 

 where, in some cases, it surrounds air-tubes. In the 

 hind wings of the male cockroach the blood has been 

 observed to flow outwards along the nervures near 

 the two margins and to return by the central 

 nervures (29). Some water-bugs have a special pulsat- 

 ing organ at each knee-joint, which assists the flow of 

 blood through the legs (31). In dragonflies and locusts 

 there is a pulsating diaphragm stretched across within 

 the lower wall of the body, forming a special blood 

 space around the nerve-cord (30, b). 



Body-cavity. — It has been stated that the blood 

 in insects flows through the body-cavity, which, as 

 we shall see when we consider its development, is 

 very diflferent in its origin from the body-cavity of 

 back-boned animals. It may really be regarded as 

 a collection of blood-spaces (60). Within the body- 

 cavity of the cockroach, surrounding the food-canal 

 and other organs, is to be noticed a large irregular 

 white mass — the fat-body. 



