CHAPTER VIII 



THE CIRCULATORY SYSTEM 



The Circulatory or Blood-vascular System of the Dragonfly 

 has been very little investigated, chiefly owing to the excessive 

 delicacy and fineness of the structures forming it. In the larva, 

 the broader and shorter abdomen, and the more abundant blood 

 supply, make the study of the blood-system more easily carried 

 out. We owe to Zawarsin [203] a very complete account of the 

 heart in the larva of Aeschna. 



It will be found convenient to treat the Circulatory System in 

 three parts, (1) the closed system, (2) the open system or haemocoele, 

 (3) the blood itself. 



(i) The Closed System. 



The Closed System in the larva is entirely dorsal, and consists 

 only of the Dorsal Vessel. In the imago there is, in addition, 

 a partly closed Ventral Vessel. The Dorsal Vessel is a slender 

 tube lying dorsally above the alimentary canal. It begins in 

 the ninth abdominal segment, where it is widest, and runs forward 

 to the second segment, where it narrows sharply to a very fine tube. 

 This latter penetrates through the thorax into the head, opening 

 into the haemocoele above the brain. The broader portion of the 

 Dorsal Vessel is called the Heart, the narrower portion the Aorta. 



The Heart (figs. 53, 71). 



The Heart proper differs from the Aorta, not only in its greater 

 width, but also in possessing definitely contractile walls. It 

 consists of eight chambers, corresponding to the eight abdominal 

 segments (viz. numbers 9 to 2) in which it lies. These chambers 

 will be spoken of as the first, second, etc., beginning from the 



