463 



been reduced from a primitive form similar to that of Zygoptera. This, 

 together with the evidence furnished by extinct species where adults 

 with wing venation similar to that of the Anisoptera had ovipositors, 

 proves fairly conclusively that the extant species without ovipositors 

 have undergone specialization by reduction. 



34. — One of the most complete lines of specialization has been de- 

 termined by Ris ('96) for the structure of the proventriculus. He 

 found what he considered a primitive condition in the Zygoptera 

 (Agrionidae) in which there are sixteen internal folds. Specializa- 

 tion takes place by reduction, and there are eight folds in the Lestinae, 

 four in Gomphus and Aeshna, and none in Libellulidae, there being 

 instead four large symmetrical teeth. 



35. — Specialization among the Anisoptera seems to be still further 

 indicated by the habits of the group, especially their habits of migra- 

 tion. The mere fact of migration is not important; but the method 

 of flying in companies and particularly of so flying that there are reg- 

 ularly spaced intervals between the individuals is something which, if 

 true, is unique in this order and in the class Insecta. 



Considering the preceding characters as a whole, it will be found 

 that there are two orders of specialization which apparently proceed 

 in opposite directions. One of these begins with the Agrionidae of 

 tlie suborder Zygoptera and ends with the Libellulidae of the Anisop- 

 tera ; and the other begins with the Libellulidae and ends with the 

 Agrionidae. The characters mentioned in the various paragraphs 

 will now be assembled for a comparison of the number of generalized 

 features in each family. The families are listed below, and are usually 

 or frequently generalized in the characters discussed in the paragraphs 

 the numbers of which are placed opposite. 



Agrionidae. — 2, 3, 5, 6, 8, 9, 10, 12, 13, 14, 15, 16, 18, 21, 22, 

 23, 24. 25, 28, 29, 30, 31, 32, 33, 34, 35. 



Coenagrionidae. — 2, 3, 5, 8, 9, 10, 12, 13, 14, 15, 16, 18, 21, 22, 

 23, 24, 26. 29. 30, 31, 32, 33, 34, 35. 



Aeshnidae. — i, 4, 6, 10, 11, 15, 17, 25, 27, 28. 



Gomphidae. — i, 4, 6, 10, 11, 17, 20, 27, 28. 



Libellulidae. — i, 11, 17, 27, 28. 



From this it will be seen that the most generalized family is the 

 Agrionidae. The evidence is such that it can not be doubted, and 

 it points to some form of the Agrionidae or related family as the stem 

 type. The following genealogical tree, based partly on Handlirsch 

 ('o6-'o8), has been constructed after taking into account all existing 

 evidence. Distance to the right indicates specialization; vertical dis- 

 tance, time. 



