456 



seems to be greater specialization in the shape of the labial palpi or 

 lateral arms in the Coenagrionidae, notably the Lestinae, than in any- 

 other group. The simpler types are found in the Aeshnidae, Gomphi- 

 dae, and Agrionidae, and a highly specialized form again in the Libel- 

 lulidae. 



7. — The condition of the maxillae and the mandibles in species 

 existing prior to the present time can only be surmised, since there 

 are no embryological or paleontological data on the subject. These 

 appendages are so nearly alike in shape in the two suborders that no 

 comparison can be profitably made. 



8. — The primitive prothorax, according to both paleontological 

 and embryological evidence, was a simple ring of the same size as 

 the mesothorax and metathorax. Specialized conditions are found in 

 the Anisoptera where, owing to the size of the head and the growth 

 of the compound eyes, the cephalic part of the pronotum is depressed. 

 The condition of the prothorax is probably primitive in Zygoptera. 

 The sclerites are not as distinct in the Anisoptera as in the Zygoptera, 

 indicating that obsolescense of the sutures has begun in this suborder. 



9. — The next feature of note is found in the interpleural suture. 

 Stages of disappearance occur in all Anisoptera, the suture being 

 completely lost in the Libellulidae and perfectly distinct in all nymphs 

 of Zygoptera. The cause of this modification is unknown, but it is 

 probably due to the excessive development of the wing muscles with- 

 in the thorax. In respect to this feature, then, the primitive forms 

 are found in the Zygoptera; the specialized, in the Anisoptera. 



10. — Another modification is found in the disappearance in the 

 Libellulidae of the metafurcal invaginations. The primitive condi- 

 tion or stem form is unknown, as is also the cause of the disap- 

 pearance. It is probable, however, that the type with distinct in- 

 vaginations is the more generalized, which places the Zygoptera, the 

 Aeshnidae, and the Gomphidae much below the Libellulidae in position. 



11. — In the shape of the wing-pads, the Anisoptera show more 

 conformity to the generalized types occurring in Plecoptera and Or- 

 thoptera than do the Zygoptera ; and they must be regarded as gen- 

 eralized in this respect. 



12. — The simplest abdomen, judging from embryological studies, 

 is a cylindrical portion of about the same diameter as the thorax. 

 The abdomen is much modified in all Anisoptera, where it is enlarged 

 and the venter flattened. The Zygoptera are generalized in this re- 

 spect, and a series showing progressive specialization in this single 

 feature would be as follows : Agrionidae, Coenagrionidae, Aeshni- 

 dae, Gomphidae, Libellulidae. 



