434 RECTAL GILLS IN LARVAE OF ANISOPTERID DRAGONFLIES, 



would absorb and hold far more oxygen. The question of the 

 chemical activity of gill-pigments(4) has been seriously enter- 

 tained by a number of authors, but ought to be finally disposed 

 of by the evidence given in the case of Anisopterid larvae, where 

 closely allied genera may have either pure white (unpigmented), 

 or deep purple or black (heavily pigmented) gills, and both carry 

 on respiration equally well. 



The passage of the oxygen along the tracheal tubes, — Several 

 authors have sought for elaborate explanations of the method by 

 which the oxygen is carried along the tracheal tubes to the 

 various parts of the body. It was thought possible that the 

 larger tracheae themselves might expand or contract in cross- 

 section, and so drive the gas forwards. This has been shown to 

 be impossible to any appreciable extent, except in the case of 

 vessels with specially distensible walls, such as the air-sacs of 

 certain insects. A very curious alternative explanation has 

 been offered by Sadones(6). He showed that, by reason of the 

 discontinuity of the spiral threads, the tracheae can be pulled 

 out longitudinally, and then let back again, like a concertina. 

 From this, he seems to have concluded that such a concertina- 

 like action actually takes place, and is the means whereby the 

 gas was moved along the tracheae. This conclusion, he admits, 

 was arrived at only after cudgelling his brains for a long time 

 ("nous nous frappions la tete depuis longtemps")! Now it seems 

 clear that such a supposition is not only unnecessary, but is un- 

 supported by any morphological evidence. For the tracheae are 

 devoid of muscle and elastic tissue, and any extension they are 

 capable of is purely passive, and not active. If one watches the 

 respiration of a transparent Anisopterid larva under a low power, 

 it will be seen that the dorsal tracheae move up and down in the 

 body-cavity in connection with, and not independently of, the 

 pulsations of the rectum. There does not appear to be any 

 change in volume, but merely a change of position. Further, if 

 the tracheae forced the gas onwards in any way, one would 

 naturally expect a considerable increase in pressure to take place 

 within the tubes, as is the case in strongly-flying imagines. But, 

 in the Dragonfly larvae, the large tracheal trunks do not collapse 



