CHAPTER 3 



The Circulatory System and Associated 

 Tissues 



In the insect body there is only one tissue fluid, the blood or haemo- 

 lymph, occupying a single cavity, the haemocoel, and separated from 

 the tissue cells only by the delicate but continuous basement mem- 

 branes. With few exceptions, there is only one blood-vessel, which 

 runs along the midline of the back. The posterior segment of this 

 pulsating vessel, the 'heart', is provided with a series of valved 

 openings or ostia through which the blood can enter; the anterior 

 segment, the 'aorta', is a uniform contractile tube. After passing 

 through the brain above the oesophagus, the aorta ends more or less 

 abruptly; and from this point the blood simply percolates slowly 

 backwards through the tissues. But in many insects it is still subject 

 to some direction : the aorta may discharge into definite vessels carry- 

 ing the blood in different directions ; the antennae and limbs are often 

 divided by longitudinal membranes, the blood entering the limb on 

 one side and leaving on the other; and the abdominal cavity in some 

 insects is divided by two horizontal membranes, the dorsal and 

 ventral diaphragms, into three sinuses: the pericardial sinus con- 

 taining the heart, the perineural sinus containing the nerve cord, and 

 the visceral sinus between. The perineural sinus tends to conduct the 

 blood to the posterior end of the abdomen before allowing it to re- 

 turn to the heart; but it is perforated and incomplete laterally, so that 

 some of the blood circulates transversely across the abdomen below 

 it. The dorsal diaphragm, when present, is also fenestrated, and thus 

 allows the blood to enter the pericardial sinus throughout its length 

 (Fig. 6). 



The mechanism of the circulation 



In insects with a rigid cuticle, the blood in the body cavity, when the 



