FEEDING AND BREATHING 35 



steady flow passes into each chamber of the heart from 

 either side of the shallow blood-space in which it lies. 



The front end of the heart passes into a narrow median 

 tube, the aorta or main artery of the body (Fig. 12, a). 

 Thus there is beneath the dorsal wall of an insect, a con- 

 tinuous longitudinal blood-vessel, of which the hinder 

 region is heart and the front portion aorta. The relative 

 lengths of these two portions differ in different insects ; 

 from what has already been stated it may be realised that 

 while in a cockroach the aorta begins at the front end of the 

 thorax, in a bee it begins towards the front end of the 

 abdomen. Through the aorta therefore the blood is pro- 

 pelled forsvards into the head. Now, in back-boned animals 

 it is well known that the blood passes from the aorta into a 

 well- developed system of branching arteries and then 

 circulates through a network of minute vessels, the capil- 

 laries (through whose excessively thin walls exchange of 

 substances can go on with the fluid lymph that bathes the 

 tissues), and is returned to the heart by a set of veins. But 

 in insects there is no such *' closed " circulatory system. 

 Here the blood streams out from the open front end of the 

 aorta, bathes the brain, passes backwards through the 

 thorax surrounding the large muscles that move the legs 

 and wings, and flows into the abdomen, whose cavity has 

 already been defined as a great blood-space surrounding 

 the digestive tube. In many insects there is in the abdomen 

 a definite ventral blood-space, in which the nerve cord lies ; 

 this space is roofed by a delicate diaphragm whose con- 

 tractions drive the blood upwards into the main cavity of 

 the body. This cavity having the ventral diaphragm just 

 mentioned as its floor, is roofed by the thin membrane 

 already described as the floor of the pericardial space. 

 Into this latter the blood streams from the large main 

 cavity below, passing in some insects — cockroaches, for 

 example — through minute holes or pores in the membrane, 

 in others — such as beetles and bees — around the segmentally 

 scalloped edges of the membrane. From the pericardial 

 space, as already mentioned, the blood flows back into the 



