BY R. J. TILT, YARD 



f.l3 



I judge to be either of the fourtli or fifth instar. The differen- 

 tiation of the hiemocoele is much more clearly shown in this type, 

 the primary or afferent blood-canal 

 being much smaller, more regu- 

 larly formed, and more strongly 

 walled than the secondary; the 

 latter indeed appears to be merely 

 the large remainder of the hajmo- 

 coele left over after the trachete and 

 the primary blood-canal have been 

 closed off. The beginnings of the 

 alveolar mesh work can be clearly 

 seen within it. It is much to be 

 regretted that I could not obtain 

 a series of this very rare, archaic, 

 larval type, so as to follow out the 

 changes in the blood-canals more 

 fully. 



Structure of the Gills in the Si.rth 

 Instar. (Text-figs.40, 41). 

 At this stage, the gill is defin- 

 itely tiattened for about two-thirds 

 of its length, though still very 

 narrow (Text-fig. 40). The blood- 

 canals are distinguishable in the 

 whole gill, particularly where they 

 run through the pigment-zone in 

 Austroagrion cyane. Transverse 

 sections now begin to show the de- 

 finite differentiation into rhachis 

 and blade, as may be seen from 

 Text-fig. 41, which is taken from 

 a little beyond half-way along the 



Text-fig. 40.* 

 At this stULje, 



the 



* Distal expanded portion of a lateral gill of Austroayrio7i cyane Selys, 

 sixth instar, side-view; hd, dorsal blood-canal; hr, ventral blood-canal; p^, 

 pigmented zone; t, main trachea; t', branch-trachea, which has become the 

 ■second main tiachca; ( x 90). 



