Insects 155 



The dragon-flies of Colorado are easily separated into two 

 great groups or suborders, the Anisoptera and Zygoptera, the 

 latter being often called damsel-flies. The Anisoptera, or dragon- 

 flies proper, are usually larger and more robust, with more power- 

 ful flight, and the hind wings are more or less broadened at the 

 base, in some species forming a prominent angle. When at rest, 

 the wings are carried horizontally, as in flight. In the damsel- 

 flies the body is slender, and the wings taper to the base, the 

 anterior and posterior ones being very much alike. With very 

 few exceptions, the wings are held backward over the abdomen 

 in repose. The larvae of the Zygoptera are known by the three 

 conspicuous appendages, sometimes leaf-like, known as the 

 caudal gills. Their importance for respiration is not altogether 

 evident, as when several larvae are confined together, they will 

 eat each other's appendages, and individuals thus mutilated go 

 on living. It has generally been assumed that of the two sub- 

 orders, the Zygoptera are the older, on the ground that the first 

 flying insects doubtless had two pairs of wings alike. But as 

 Tillyard points out, the very ancient primitive dragon-flies known 

 from fossils, in some cases over two feet across the wings, were 

 evidently powerful flyers, with a build suggesting the Anisoptera, 

 not at all the damsel-flies. A very beautiful Eocene fossil (Eodi- 

 chroma mirifica) recently discovered in Texas has very broad 

 wings, yet from its structure is evidently one of the Zygoptera. 

 Thus it seems reasonable to suppose that the principal features 

 of modern damsel-flies are not really primitive, but represent an 

 adaptation to their special mode of life. 



Fossil dragon-flies are found in Colorado both in Eocene and 

 Miocene rocks. The most interesting and remarkable form, which 

 has been exhaustively discussed in a memoir (Proceedings of 

 Philadelphia Academy, 1913) by Calvert, is the genus Phenaco- 

 lesles from Florissant. This is one of the Zygoptera, and the 

 wings found are some of them almost perfectly preserved, showing 

 the dark cloud across the apical half. A striking feature, in which 

 it differs from most of its near allies, is the presence of several 

 antenodal cross-veins. These are the little cross-veins from the 

 upper edge before the slight depression (at the end of the arched 

 part) known as the node. This is thought to be a primitive 

 character, and is found also in a genus from the Eocene rocks of 



