102 THE CIRCULATORY SYSTEM [CH. 



The Aorta. 



The dorsal aorta (d.aort.) is a very delicate, straight, non- 

 contractile tube, which carries the blood stream forward through 

 the thorax to the head. It is not divided into chambers. Its 

 chief peculiarity, in Dragonfly larvae, is the presence of a pair 

 of small but distinct osteoles (ol) on its dorsal wall, in the middle of 

 the thorax. 



In the imago the dorsal vessel is excessively attenuated and 

 elongated, owing to the corresponding form of the abdomen. 

 No detailed studies have been made of it, but its structure seems 

 to be essentially similar to that of the larva. 



The Ventral Vessel. 



The ventral vessel or ventral sinus of the imago is not really 

 a closed tube, but a kind of groove overlying the ventral nerve-cord. 

 A membrane placed on either side of the nerve-cord serves to 

 separate it from the haemocoele. During the respiratory move- 

 ments of the long slender abdomen, each contraction, by narrowing 

 the abdomen transversely, forces the edges of these membranes 

 up into a set of curved ridges, one pair in each segment. The 

 blood is thus constrained to flow between these ridges, from before 

 backwards. Thus an almost completely closed circulation is set 

 up in the imago. 



(ii) The Haemocoele. 



As in all insects, the general body-cavity is not the same as 

 the embryonic coelome. The latter becomes reduced to the narrow 

 pericardial space, and the lumina of the developing reproductive 

 organs. A secondary enlargement of the blood spaces, on the 

 other hand, forms the general body-cavity. This is termed the 

 haemocoele. Owing to the position of the various organs in the 

 body, the haemocoele forms a kind of imperfect sinus along either 

 side of the middle line, chiefly ventrally. It extends into the 

 legs and antennae. It is filled with blood plasma, in which a weak 

 and irregular circulation of corpuscles, from before backwards, is 

 easilv noticeable. 



