ARTHROPOD A— INSECTA. 449 



Colorado. Here half a hundred species of wasps (Terebrantia) 

 are represented by hundreds of individuals. They include the leaf- 

 wasps or saw flies (Tenthredinidce) , of which Atocus defessus 

 Scudder (Fig. 1751), is an ex- 

 ample, the ichneumon flies {Ich- 

 neiimonidcB ex. : Protostephanus 

 ashmeadi Cockerell), the gall 

 flies (Cynipidce) , etc. The ants 

 (Formicidce) are exceedingly 

 abundant here, thousands of 

 individuals having been obtained, 

 while the true wasps (Vespidce) Fig. 1751. Aiocus defessus, complete, 



and the bees (Apidce) are not X 3. and antenna much enlarged. Oligo- 



cenic, Florissant, Col. (After Scudder.) 

 uncommon. » ' v / 



Other American localities where Hymenoptera have been found 



are the Green River beds of Wyoming (Decatoma antiqua Scud- 



aer, White River of Colorado (Ichneumon petrinum Scudder) and 



the Tertfary beds of British Columbia (Aphaenogaster longaeva 



Scudder, Dolichoderus obliteratus (Scudder) and Formica arcana 



Scudder). 



Order ODONATA Fabricius. 



(Dragon- flies.) 

 Often large insects (length of wing in one form 122 mm., in 

 another 378 mm., with a spread of wings of 750 mm.), slender, 

 with highly developed compound wings, a free movable head, with 

 large and highly developed compound eyes; wings nearly equal, 

 very delicate and transparent, a nodus or faint contraction of the 

 costal area marks the end of the subcosta at the oblique cross- 

 vein or subnoduSj uniting R and M^, C marginal, R simple, Rs 

 crossing anterior branches of media (M^, 2) and ending on pos- 

 terior margin, generally between M^ and M^. In the adult the 

 crossing is indicated by an oblique cross-vein originating below 

 and more or less in front of the fork between M^^ and M^. The 

 backward continuation of the Rs, behind the oblique cross-vein 

 in the adult, is a later addition and forms the "' bridge/' This ob- 

 scures the course of the Rs, Cu often making an abrupt bend, just 

 behind the arculus, this bend forming the base of the ''wing- 

 triangle'' (Anisoptera). Cu^, immediately after leaving Cw^, fuses 

 29 



