1913.] NATURAL SCIENCES OF PHILADELPHIA. 253 



17a. The Number of Antenodal Cells between Mi and Cm. — This 

 character is in general dependent on the degree of retraction of the 

 nodus, as the more the nodus approaches the base of the wing the 

 smaller is the area available for these cells. This correlation, although 

 not absolute and exact in the genera here compared, holds in a rough 

 way. We may therefore regard reduction in the number of antenodal 

 cells as a specialization. There appear to have been 12 on the front 

 wing, 13 on the hind wing of Tarsophlebia eximia (counting only a 

 single row — there are two rows in part of this area) , almost three in 

 Steleopteron. A reduction in the number of antenodal cells may occur 

 independently of retraction of the nodus, however, by the greater 

 development of the quadrilateral distad. (Compare Table I, 

 Thaumatoneura and Paraphlebia, also pp. 233, 237 anted.) ■ 



18. The Number of Cubito-anal Cross-veins. — These veins are usually 

 small and weak in the Zygoptera, and they have not been preserved 

 well in the fossils, so that few data on their development in point of 

 geologic time are available. Perhaps the same kind of evidence 

 which Mr. Campion has secured^^ fQj. ^jie reduction of the antenodals 

 might be obtained for these cross-veins also. It is conceivable that 

 the ancestral Zygopteron might have had a moderate number and 

 that some of its descendants might have acquired more, others fewer. 

 While recognizing the paucity of evidence, I am inclined to view 

 reduction in their number as specialization, in harmony with the 

 reduction of antenodals. 



21. Petiolation of the Wing. — When the anal vein (A) separates 

 from the hind margin of the wing at some distance from the base, a 

 stalk-like appearance of the wing-base is produced, especially on the 

 hind margin, to which the term "petiolee" was long ago applied by 

 de Selys. Petiolation involves a reduction of the anal area and is 

 specifically mentioned by Prof. Needham^ as a fundamental develop- 

 mental tendency in the Agrionidse ( = Agrionin£e Selys). It is not 

 visible m Protomyrmeleon (Lias); in Tarsophlebia, Stenophlebia , 

 Isophlebia, or Anisophlebia (Jura). It seems to have been well- 

 marked in Steleopteron, while the shape of the wing-outline certainly 

 suggests it in Euphceopsis, although here A may have remained dis- 

 tinct from the hind margin.^* It seems justifiable to assume that the 

 farther distad petiolation has progressed, the more specialized the 

 insect. 



82 These Proceedings, p. 223, antea. 



« Genealogic Study, p. 748. Cf. Ris, 1912, p. 45; Tillyard, 1913, p. 444. 



«* Handlii-sch, Foss. Ins., Taf. XLVII. 



