32 CAUDAL GILLS OF ZYdOPTERID LARV^, 



(the Anisoptera) possesses larvse which breathe chiefly — one 

 may say, ahnost entirely — by means of delicate gills situated in 

 the rectum. The second sub-order (the Zygoptera) is remarkable 

 in possessing larvpp in which the caudal processes are ver}' con- 

 spicuously developed. These processes vary much in size and 

 shape, but in general they serve as one of the principal means of 

 respiration, though by no means the only one. These organs are 

 now generally known as the caudal gills, though it is by no means 

 certain that they function as such in all cases, since in some 

 genera they appear to have undergone reduction from disuse. 

 During 1913-14, I completed my study of the morphology of the 

 rectal gills of Anisoptera, in a paper which was sent to the 

 Linnean Society of London in November, 1914, but which, 

 through the unforeseen delays caused by the war, has only 

 recently appeared in print. "^ A sequel to this, dealing with the 

 physiology of the same organs, appeared in the Proceedings last 

 year (Vol. xl., Part 3, pp.42'2-437, Plate xlvii.). With the com- 

 pletion of these two papers, I passed on to the study of the 

 Zygoptera. The state of aftairs in connection with this sub-order 

 offers a remarkable contrast with that existing for the Anisoptera. 

 In the latter, the unique beauty and high complexity of design 

 of the rectal tracheal gills have been sufficient to attract the 

 attention of workers from many fields of Biology, and the study 

 of these organs stands well advanced. But, in the Zygoptera, 

 the condition is one of comparative neglect, so that at present 

 no general study of the morphology of the gills has been 

 attempted, nor is there as yet any clear idea as to how respira- 

 tion is carried on. As the case stands at present, it would 

 appear that at least ^five parts of the body of a Zygopterid larva 

 may function as organs of respiration, viz., (l)the general in- 

 tegument, (2) the spiracles (on certain occasions only), (3) the 

 rectum, albeit lacking in the highly specialised gills of the An- 

 isoptera, (4) paired lateral abdominal processes or gills (in 

 certain CalojHeryyida' only), and (5) the so-called "caudal gills." 



* "On the Rectal Breathing- Apparatus of Anisopterid Larvae." Journ. 

 Linn. Soe. London, ZooL, xxxiii., No.2'2*^, lOlfl, pp. 127-196, Plates xviii.- 

 xxii, 



