455 



dal egg as the primitive type, the series from lowest to highest would 

 be something like the following : Libellulidae, Gomphidae, Aeshni- 

 dae, Agrionidae, and Coenagrionidae. 



Nymph 



2. — The most striking differences in nymphal characters are 

 found in the shape of the body. Zygoptera are without doubt near- 

 est the primitive Campodea type, and Anisoptera show a marked 

 deviation which is possibly due to the habits of life. This interpreta- 

 tion is supported by the embryonic stage, in which the body shape is 

 essentially campodeiform in both suborders. 



3. — The compound eyes of all forms are specialized, but the line 

 of descent is not difficult to follow. The primitive type is found in 

 the embryo, which has small circular eyes on the lateral aspects of 

 the head. The nearest approach to this is found in the eyes of zygop- 

 terous nymphs ; the farthest away from it, in the Anisoptera, where 

 the eyes show a tendency to become dorsal in position. The cause of 

 the modification is unknown, but may be due in part to their habits, 

 the Anisoptera being mud-inhabiting to a large extent and needing 

 eyes on the dorsum of the head. Another cause may possibly be 

 found in the accelerated development of the greatly enlarged eyes of 

 the adult. In respect to shape and position of the compound eyes, 

 then, the Anisoptera should be regarded as the more highly special- 

 ized group. 



4. — The antennae show important lines of development. The 

 primitive antennae of the embryo consist of three segments, the sec- 

 ond segment being the longest. A great lengthening of the first seg- 

 ment is the main line of specialization, and this occurs only in 

 Zygoptera in the family Agrionidae. The antennae nearest the em- 

 bryonic type are found in the Gomphidae ; next in order are the 

 Aeshnidae, then the Libellulidae, and, finally, the Coenagrionidae and 

 the Agrionidae. 



5. — The labium shows the more primitive condition in Zygoptera, 

 where the median lobe is deeply cleft in the family Agrionidae. Gra- 

 dations in complexity are found in a reduction in the depth of the 

 cleft, and the line of specialization may be followed through the fol- 

 lowing series, beginning with the least specialized : Agrionidae, 

 Coenagrionidae ; Gomphidae, Aeshnidae and Libellulidae. 



6. — Mental setae are lacking in the embryo and also in the nymplis 

 of Aeshnidae, Gomphidae, Agrionidae, and a few Coenagrionidae. 

 The cause of the production of mental setae is unknown. There 



