164 THE CIRCULATORY SYSTEM [CH. 



Wandering fat-cells are often to be seen in the blood of 

 Dragonflies. The large cells known as oenocytes also appear to 

 be present, at any rate in the abdomen of the larva ; but I know 

 of no definite observations concerning them. 



One of the principal functions of the blood is undoubtedly to 

 receive from the mid-gut the products of digestion, and to carry 

 them to the various organs, or to the fat-body for storage, until 

 such time as they may be needed. The blood also dissolves the 

 carbonic acid gas formed in the body of the insect. In the larva, 

 it almost certainly plays some direct part in the process of 

 respiration, by carrying this gas to points where it may be 

 exchanged for oxygen from the water, by the ordinary processes 

 of diffusion. 



The Circulation. 



In the young larva, directly after hatching or ecdysis, the whole 

 course of the blood can be followed with ease. The blood enters 

 in two streams on either side of the hind-heart, the current passing 

 quite suddenly from sluggish to rapid as the blood approaches 

 the ostia. The pulsations of the hind-heart are fairly regular, 

 and usually from 60-70 per minute. Each pulsation drives a 

 considerable quantity of blood forward into the next chamber. 

 Between pulsations, the corpuscles remain almost stationary in the 

 chambers of the heart. On reaching the aorta the corpuscles are 

 forced forwards in jerks, so that an irregular stream of them is 

 seen to enter the haemocoele in the upper part of the head. 

 This stream at once spreads out, passing at first slantingly down- 

 wards beneath the brain. The main stream then circles forwards 

 and upwards over the brain, and finally divides into two separate 

 streams passing slantingly downwards and backwards towards the 

 neck, through which the blood passes in two closely contiguous 

 streams, separated by the nerve-cord. Some of the corpuscles 

 pass into the bases of the antennae, where a circulation can be 

 seen in the scape and pedicel. 



In the thorax, a circulation along either side of the nerve- 

 cord includes the basal joints of the legs (coxa and trochanter). 

 A weaker current also passes backwards more dorsally. In the 



