70 GNATS OR MOSQUITOfeS — CHAPTER V , 



head, the mechanism being specially well developed in the 

 female. The peculiar air-containing sac already described 

 is also developed at this time. It is obvious euough that 

 the former mechanism is capable of being actively dilated 

 by muscular action, and may therefore assist in suction, but 

 it is difficult to understand how any one can have fallen into 

 the error of ascribing such a function to the latter organ. 



The circulatory system consists of a long dorsal vessel, 

 which is broad and actively contractile in the abdomen and 

 contracts into an " aorta " in the head and thorax. From 

 its sides membranes, the alee cordis, which serve to suspend 

 it, run out between the extensor muscles and the stomach, 

 to attach themselves to the tracheal trunks. Each ala 

 consists of a dorsal and ventral lamina, and the space between 

 them has been called the pericardium. It contains the 

 pericardial cells and communicates freely with the body 

 cavity by the spaces between the alae. There is no distinct 

 constriction of the heart into chambers, and the paired ostia 

 or slits, which put it in communication with the "peri- 

 cardium," open backwards in the first segment, and forwards 

 and inwards in segments three to seven. 



In the space between the alec cordis are also the peri- 

 cardial cells, which are brown in colour and arranged in 

 ovoid masses, of which there are four pairs in each abdominal 

 segment, two of which are in its anterior and two in its 

 posterior portion. The protoplasm of these cells is extraor- 

 dinarily spongy and contains numerous granules which stain 

 deeply with borax carmine. The nuclei vary in number 

 from 3 or 4 to 10 in each mass, but the boundaries between 

 the cells cannot be made out. The glandular character 

 of these cells has been shown by Kowalevsky (" Biolog. 

 Centralblatt," ix., 1889), their function being probably some- 

 what analogous to that of the lymphatic and other ductless 

 glands of the higher animals. I reproduce more fully the 

 histological characteristics of these masses, as, alike from 

 their position close to the walls of the stomach, from which 

 they are separated only by the ventral layer of the ala cordis, 

 and from their general appearance they might easily be 

 confused with the parasitic "coccidia" of malaria recently 



