458 



almost impossible to arrive at any conclusion as to its simplicity or 

 complexity. Suffice it to say that paleontological and embryological 

 data prove that there are primitive types in both Zygoptera and An- 

 isoptera. In the holoptic condition of the compound eyes, however, 

 there is a more definite character. As already stated, the primitive 

 type is dichoptic; and beginning with this condition, which we find 

 most closely approximated in the Zygoptera, there are all degrees of 

 dichoptic and holoptic states. The cause of the modification is prob- 

 ably due, in the adult, to the increased power of vision made neces- 

 sary by the greatly increased powers of flight and the fact that the 

 insect captures its prey while on the wing. An excellent series of 

 specializations is to be had in the following families, the Zygoptera 

 being the more generalized : Agrionidae, Coenagrionidae ; Gomphi- 

 dae, Aeshnidae, and Libellulidae. 



17. — The antennae, as already noted, show marked reduction in 

 size from those of the nymphs. The nearest approaches to the primi- 

 tive, seven-segmented condition are found in the Libellulidae and 

 some of the Aeshnidae, where six segments are often encountered. 

 Most representatives of the remaining families have the segments 

 quite consistently reduced to four. The adults of the Agrionidae 

 have the most highly specialized antennae; and in a series showing 

 increasing specialization the Libellulidae would be the more gener- 

 alized. The following is such a series based upon antennal structure : 

 Libellulidae, Aeshnidae, Gomphidae, Coenagrionidae, and Agri- 

 onidae. 



18. — The front shows great deviation from the simpler forms in 

 the majority of the Anisoptera, and the mound-like elevation of this 

 part is characteristic of most families of this suborder. 



19. — The mandibles of the adult have apparently undergone no 

 modification of importance in the different families. They are so 

 nearly alike in all groups that a comparison will not be attempted. 



20. — The maxillae of the adult have likewise undergone little 

 modification in the different families, but the form nearest the primi- 

 tive type present in Plecoptera nymphs is found in the Gomphidae. 



21. — The labium shows the same deviations from the primitive 

 condition as were described for the nymph. Looking upon the depth 

 of the median cleft as a measure of generalization, the Agrionidae 

 would be considered as the more generalized. Next in order are the 

 Coenagrionidae and, following these, the Aeshnidae, Gomphidae, and 

 Libellulidae. The labial palpi retain about the same degree of 

 specialization that occurs in the nymphs; and the same sequence of 

 family specialization as has been described for the nymphs is present 

 in the adults. 



