180 MR. R. J. TILLYARD ON THE RECTAL BREATHING-APPARATUS 



instars of AnucT. One of these was raised to the seventh instar. Later, a 

 large number of Anax larvse were found in stages which clearly corresponded 

 to this seventh instar of ^schna, besides a few in the sixth instar. Many of 

 these are still alive and at present are in the eighth or ninth instars (August). 

 As the formation of the du[)lex foliate gill-type is practically completed by 

 the seventh instar, these larvse supplied me, m toto, with a complete account 

 of the gradual formation of the gill-system. 



We shall divide our account of the ontogeny of the gills into three sections, 

 as follows : — 



(1) The state of the gill-basket at the time of hatching, and the changes 



that take place immediately afterwards. 



(2) A description of the gill-basket and its tracheal supply during the 



second instar *. 



(3) The gradual development of the duplex system of gills. 



(1) The state of the gill-basket at the time of hatching, and the changes 

 that take place immediately afterwards. (Text-figs. 17-19.) 



The young Anax larva emerges from the egg very rapidly, covered with 

 two cuticles. The first instar, or pronymph stage, lasts only a very short 

 period, ranging from a few seconds to a little over a minute. Then the 

 outer cuticle is rapidly cast off, and the larva emerges fully equipped for its 

 life-struggle, with its powerful mask formed ready for capturing its prey. 

 This is the second instar. 



At the moment of emergence from the pronymphal skin, the pulsating 

 organ of the head is still beating, though it subsides shortly afterwards. The 

 tracheae of all the posterior region of the body contain no air and are quite 

 invisible, except the large dorsal trunks^ which can be faintly seen. The 

 dorsal tracheae, in the region anterior to the midgut, are seen to be filled 

 with air, which appears like a black continuous rod travelling steadily back- 

 wards along the tracheae. At the moment when the larva gets free from 

 the pronymphal skin, the air has reached to the level of the midgut. Thence 

 it travels fairly quickly backwards, filling up and outlining in black each 

 tracheal branch which it meets with in its course. I am inclined to connect 

 this regular filling of the tracheae with air, from some point in the anterior 

 part of the body, with the pulsations of the head organ, which can be seen 

 to stop about the time that the tracheal system becomes filled with air. The 

 point, however, needs further elucidation. 



At first, there is no sign of gills in the rectum. But as the air travels 

 backwards to the rectum, six longitudinal gills begin to be outlined in very 

 regular and beautiful fashion. These gills are thrown into folds or undu- 

 lations in which numerous tracheal loops occur. The air enters each loop in 



* Here, as elsewhere, I consider the so-called " pronymph " stage to be definitely the 

 /i)-st instar, so that the larva commences its free existence in its second itiata?'. 



