28 A VENATIONAL STUDY OF THE ZYGOPTERA 



It should be remembered of course that such a table is of value 

 in a general way only, since there is great variation in some sub- 

 families. Epiophlebia is a member of a lower suborder, yet its 

 supplementary sectors are not so numerous as those of many 

 Zygoptera. Thaumatoneura (fig. 51) is placed in the Megapoda- 

 grioninae, but its many sectors indicate a primitive condition, or 

 possibly they may represent a secondary acquirement. Neverthe- 

 less, this table shows that the highest subfamilies have the fewest 

 sectors. 



Dr. Calvert has shown that in a general way the reticulation of 

 the oldest fossil order related to Odonata, the Protodonata, was 

 rich in supplementary sectors. In Anisozygoptera some wings 

 were richly veined, others less so. He concludes, that we have, 

 therefore, "only the general tendency to guide us when we assume 

 that reduction in density of venation means specialization." 



2. Reduction of the veins Cuo and Cui. — This tendency appears 

 in three widely removed groups; by it are derived Disparocypha 

 (fig. 37) from the Epallaginae, Lestoidea (fig. 45) from the Lestinae, 

 and the Protoneurinae from the Coenagrioninae. As was shown 

 in the discussion of the Protoneurinae, Cui follows Cu2 in its reduc- 

 tion until nothing remains for both except the cross vein forming 

 the lower part of the medio-anal link. This condition is the one to 

 be found in Selysioneura (fig. 153) and is approached by a number 

 of forms, such as Nososticta (fig. 151) and Isosticta (fig. 150). 



3. Decrease in the number of antenodal cross-veins. — This very 

 apparent tendency needs but little discussion, particularly after 

 the thorough treatment given it by Campion (191 3). The number 

 of cross-veins in the lower Agrionidae is very high; in the higher 

 Epallaginae it is reduced to seven, eight, and nine; in the Coenagri- 

 onidae, only Thaumatoneura (fig. 51) has over three. Neurolestes 

 (fig. 62), Trineuragrion (fig. 54), and Neuragrion have three gener- 

 ally, in one wing at least; and Chlorolestes (fig. 66), Podopteryx (fig. 

 53), Paraphlebia (fig. 64), Heteragrion (fig. 68), and Pyrrhosoma 

 (fig. 89) have occasionally been observed to have three ; all others 

 commonly two. Dr. Calvert cites evidence to show that this 

 reduction is not in proportion with the retraction of the nodus, but 

 is apparently an independent and recent acquisition. 



