459 



22. — As regards the form of the microthorax, no stem can be de- 

 termined, but it is probable that there has been much more specializa- 

 tion in the Anisoptera than in the Zygoptera. 



23. — The degree of complexity of the prothorax as a whole is 

 difficult to determine. Many sexual modifications occur in the adults 

 which must be considered as secondary characters having little bear- 

 ing on phylogeny. The distinctness of the propleural suture, how- 

 ever, is of some value. In Zygoptera, this suture is most distinct in 

 the Coenagrionidae (Lestinae) and is moderately so in the Agrioni- 

 dae. In the Anisoptera it is most distinct in the Aeshnidae, but is 

 as a rule indistinct in other families. According to this character the 

 Zygoptera seem to be generalized ; the Anisoptera specialized. 



24. — In the mesothorax and metathorax the most important fea- 

 ture, aside from the wing structure, is to be found in the interpleural 

 suture. As already mentioned, this suture shows no sign of disap- 

 pearance in any of the nymphs of Zygoptera, and still remains undi- 

 minished in distinctness in the adults of the family Agrionidae. In 

 the Coenagrionidae, however, the interpleural suture becomes obsolete 

 in great measure. In both nymphs and adults of Anisoptera, it is 

 indistinct. The degree of its distinctness is therefore an excellent 

 character for determining the degree of specialization or generaliza- 

 tion of the species and consideration of this fact alone leads to the 

 conclusion that the Zygoptera are the more generalized. 



25. — The varying degrees of approximation of the mesepi sterna 

 and the metepimera indicate an entirely different line of development 

 from that shown in 24. The primitive condition is one in which the 

 two mesepisterna and metepimera are separated by considerable inter- 

 vals, as has been shown for the nymphs. The approximation of the 

 metepimera on the ventro-meson is a much later development and does 

 not appear until the adult stage. Nevertheless, nearly the same line of 

 specialization occurs as in the former case, the simplest conditions 

 being found in the Aeshnidae and the Agrionidae, the more complex 

 in the Gomphidae, Libellulidae, and Coenagrionidae. 



26. — The development of the mesothoracic spiracles indicates that 

 the Libellulidae, again, are the most specialized, with the Agrionidae 

 and Aeshnidae at the bottom of the series. The size of the spiracles 

 in Libellulidae and the degree of their approximation on the dorso- 

 meson warrant this assumption, the primitive types being small in size 

 and rather widely separated, as in Zygoptera and some Aeshnidae. 



27. — A line of specialization is found in the length of the thorax 

 caudad of the metacoxae. In this the Coenagrionidae and Agrionidae 

 are decidedly the more specialized. 



