VIII] THE CIRCULATORY SYSTEM 163 



(iii) The Blood. 



The blood of Dragonflies is a lymph-like fluid, in which corpuscles 

 of various kinds live and float. The fluid or plasma is not absolutely 

 colourless, but is usually of a delicate yellowish or greenish tint. 

 In the larvae of some Gomphinae it is a very bright green. 



The blood corpuscles appear to be of only two kinds, 

 miocytes and amoebocytes. The former are the more numerous. 

 In a transparent larva they can easily be seen travelling up the 

 dorsal vessel, or passing backwards round the bases of the legs, 

 and along the edges of the abdominal segments. They are of 

 an elongate oat-shape. In the newly-hatched larva of Anax 

 papuensis they vary in length from 13 to 15 /A. The probable 

 limits of size, in larvae of different genera, may be placed at from 

 10/x to 16 p. In the very young larva, the number of corpuscles 

 present in a single chamber of the heart at any given moment 

 is usually only from six to ten ; while, in the whole animal, there 

 do not appear to be more than two or three hundred of them. 

 There can be little doubt that, after each ecdysis, their number is 

 largely added to by the manufacture of new corpuscles in the 

 osteoles and phagocyte-organs. Both miocytes and amoebocytes 

 are often spoken of as phagocytes, since they are supposed to play 

 a part closely similar to that of the leucocytes, or white corpuscles, 

 of the blood of vertebrates. 



The amoebocytes differ from the miocytes chiefly in their 

 irregular amoeboid form. Voinov [187] states that it is the 

 amoebocytes alone which undertake the function of phagocytes. 

 But he also denies the presence of the phagocyte-organs described 

 by Zawarsin. It seems possible that both the oat-shaped corpuscles 

 and the amoebocytes are only variations of a single form of 

 corpuscle. Both have been seen to undergo binary division. 

 Under certain circumstances the shape of the elongated corpuscle 

 is known to alter. The distinction between them may therefore 

 be only an arbitrary one. 



There are no corpuscles in the blood of insects corresponding 

 with the red corpuscles of vertebrates. The presence of a compli- 

 cated tracheal system renders oxygen-carrying cells unnecessary. 



112 



