457 



13- — The caudal tracheal gills of Zygoptera must be considered a 

 simple or stem character. This view is supported by much evidence 

 from embryological studies and the presence of one or two living 

 forms in which the gills are decidedly cerciform and cylindrical. The 

 modification into flat plates is undoubtedly specialization, but the re- 

 duction of the abdominal appendages in the Anisoptera indicates 

 further specialization of a different kind. Changes in shape of the 

 zygopterous appendages are probably due to a change from terrestrial 

 to aquatic habits very early in the history of the group. If we con- 

 sider that the anisopterous appendage has been derived by progres- 

 sive reduction, the following should be the order of development : 

 cylindrical cerci, flattened cerci, and reduction of cerci to short ap- 

 pendages similar to those in all Anisoptera. If, however, the gills be 

 regarded as derived from shorter caudal appendages, the Anisoptera 

 have the primitive types and the Zygoptera are highly specialized ill 

 their elongate, flattened appendages. The presence of cylindrical 

 cerci as a primitive character seems to have the greatest amount of 

 embryological evidence to support it. 



14. — The abdominal gills of Cora and Euphea of the Zygoptera 

 also afford comparative evidence as to the age of this suborder. Here 

 there are remnants of lateral, cylindrical gills on the abdominal seg- 

 ments. There seem to be embryological data sufficient to prove that 

 these lateral gills represent the appendages of forms more primitive 

 even than the Insecta. 



15. — The oldest fossil Odonata showing ovipositors had the char- 

 acters of both Zygoptera and Anisoptera, and it is probable that the 

 stem forms had true « ovipositors. The simplest type of ovipositor 

 among living Odonata is found in the nymphs of Zygoptera and 

 consists of a number of similar valves. The reason for the reduction 

 of the ovipositor in the adults of Anisoptera lies in the acquisition 

 of the aquatic habit and the consequent difficulty of depositing eggs 

 in plant tissue. It is reported that some Zygoptera do not insert the 

 egg in the plant but merely press it against the plant and allow it to 

 drop to the bottom ; and this appears to be a transition stage from 

 the endophytic to the exophytic method of oviposition. Reduction 

 of the gonapophyses, then, means specialization, and the order would 

 be — Zygoptera, generalized; Anisoptera, specialized. 



Adult 



16. — So many different lines of specialization seem to have taken 

 place in the development of the head capsule of the adult that it is 



