130 HORTENSE BUTLER. 



in the early stages (PI. IV, Figs. 3 and 4) two rudimentary setae 

 appear on the lateral lobe, just before the movable hook. 



The Gomphinse and ^schninae are very similar in both nymphal 

 and adult labial characters, and the affinities of Tachopteryx are 

 clearly with these groups. The Libellulidte have almost no points 

 of resemblance to the other families, and appear very distinct Cor- 

 dulegaster is in many respects intermediate, the adult labium show- 

 ing strong gomphine affinities, the nymphal labium being strikingly 

 mask-like and Libelluline in form ; but this latter may be the result 

 of a parallelism and due to its manner of life. 



Summary and Conclusion. 



In conclusion it will be well to note the principal points which 

 have been brought out in this discussion, and to give in a short space 

 the essence of the conclusions reached. 



Beginning with the discussion of the mechanism of the labium in 

 the nymphal stage, it was observed that all movements which the 

 labium executes are provided for by four pairs of muscles, which 

 obtain the necessary leverage by pulling over two hinges. Two 

 pairs of these muscles, the abductors and the adductors, are situated 

 in the mentum, while the flexors and the extensors occupy the 

 submentum. 



In ontogenetic study observations were made upon the embryonic 

 and early nymphal stages of Lestes, Anax, and Basiceschna. The 

 remarkable similarity of the embryonic stages of Anax and Lestes 

 was noticed, contrasting markedly with the divergence of their 

 development during nymphal life. In these embryonic stages the 

 retardation of development of the laminae (galea and lacinia) in 

 ('<)mparison with that of the palpus is clearly evident. 



Finally, in a comparison of the adult and nymphal labia by 

 groups, similarities and differences being carefully noted, there was 

 brought out the fact of the great and independent specialization of 

 the nymphal labia. They develop distinctive characters which are 

 wholly absent in the adults of the same species. 



