396 SYNOPTIC COLLECTION. 



will reveal insect forms that are marked by Thysanuran 

 simplicity of structure, and by the possession of one or 

 two pairs of wing sacs. Until there is more light on the 

 subject we may suppose, as Packard l points out, that the 

 dorsal parts or nota of the two hinder segments of the 

 thorax grew out laterally in some running or leaping 

 insect, and that these expansions became of use in aiding 

 to support the body in its longer leaps. Then by contin- 

 ual use they would become articulated to the body and, 

 growing larger, would in time develop into permanent 

 flying organs or wings. 



Order 2. EPHEMEROPTERA. (EPHEMERIDA, Comstock.) 



One of the earliest winged insects found in America, 

 known as Xcnoneura antiquorum Scudd., was a synthetic 

 form which possessed affinities with several generalized 

 families, among them the Ephemeridae or mayflies. 

 Although the wings are the only parts of these ancient 

 insects preserved, yet nevertheless they have become 

 trustworthy aids in determining the relationships of the 

 insects once possessing them. 



The wing of Xenoneura (PI. 955, a composite drawing 

 from different specimens) exhibits primitive characters 

 and simple venation, indicating slight differentiation of 

 structure. 



Another Devonian insect, Platephemera antiqua Scudd., 

 was, with little doubt, an ancestor of the present Ephe- 

 meridae. The wings (PI. 956, the dotted lines indicating 

 the probable form of the wing) possess unmistakable 

 resemblances to the same organs of our present mayflies. 

 The drawing is made to show the expanse of the wings, 

 which was probably about 135 mm., proving that this 

 ancient mayfly was much larger than recent species. 



1 3d Rep. U. S. Ent. Comm., i88o-'82 (publ. 1883), pp. 268-271. 



