147 



The Larva. 



The great increase in our knowledge of the larva ^ of the 

 Odonata is a striking characteristic of our period. As shown 

 above, many of the authors already cited, following the embrvo- 

 logical method, have found in the larvai the starting-})oints for 

 their special investigations and have in cons(>quence added 

 information on these early stages. 



GiLSON and Sadones (i8g6), and Sadones alone in a longer 

 paper (1896), described the anatomical and histological features 

 of the ahmentary canal of the larva of a Libellula in greater 

 detail than had been given previously. They supported the 

 view that the gizzard with its teeth has a triturating and sub- 

 dividing action on the bolus, although not on the food particles 

 themselves. They described for the first time two epithelial 

 plates or disks in the pre-rectal ampulla and a blood-cavit}' in 

 the gill-plates of the rectum, and called attention to the situa- 

 tion of the terminal loops of the tracheoles in the subcuticular 

 la3'er of cells of these gill-plates. They suggested that the 

 absorption of oxygen from the water which is drawn into the 

 rectum through the anus is accomplished by the vital activities 

 of this subcuticular layer, which in turn continually delivers the 

 ox^'gen as a gaseous secretion to the tracheoles ; and that the 

 ox\'gen is caused to pass along the tracheœ of the rectal gills 

 and of the body by the alternate increase and decrease of the 

 tracheal lumina. The increase was thought to be due to elonga- 

 tion of the trachccC by a smoothing out of their cuticular linings, 

 the decrease by the reverse process. The elongation and shorten- 

 ing of the tracheae were referred in turn to the oscillations of 

 cœlomic and intrarectal pressures which must accompany the 

 respiratory movements so familiar to those who have watched 

 living Anisopterous larvœ. On the other side of the respiratory 

 account, th(^ elimination of carbon dio.xide was suggested to be 

 the rôle of the blood-cavities in the gill-plates, and of the epi- 



1 In my " Intiuduction to the Study of Odonata" [Trans. Amer. Eut. 

 Soc, XX., p. 195 and elsewhere), I have used, in common with other writers, 

 the term nymph to designate " that stage of Odonate existence between 

 the egg and the transformation into the imago," but I do not see now why 

 the more general term larva should not be employed instead. 



