Evolution and Taxonomy 57 



is we must select what seems to be the more generalized types 

 and tr)' to eliminate from these the results of sidewise develop- 

 ments. 



What is gained by the study of carboniferous insects, I be- 

 lieve, is the demonstration that certain characters which have 

 been commonly considered primitive did not exist with these 

 insects, and consequently must be considered as secondary 

 developments. 



A comparative study of these insects show that in the Palae- 

 odictyoptera the two pairs of wings were very similar in 

 structure both being membranous and furnished with a simi- 

 lar framework of veins. "^ Each wing possessed six principal 

 veins or groups of veins which are clearly homologous 

 with the costa, subcosta, radius, media, acbitus, and anal 

 veins of modern insects. The wings when not in use were 

 closed over the abdomen. They were sometimes broadly 

 folded but were never plated, as are the hind wings of the Acri- 

 didae for example. This feature being with little doubt a com- 

 paratively late development. (See Scudder, Pretertiary In- 

 sects, p. 49.) 



It seems to me probable that the Palephemeridse presented 

 an exception to the general statement given above, in that the 

 two pairs of wings were not of equal size. The shape of the 

 fore wings of Platepheinera ajitiqtia (Fig. 6) is such as to in- 

 dicate that the hind wings were small. And I am informed 

 by Mr. Scudder that the only paleozoic may-fly known that 

 shows the hind wings at all {^Palingenia feistmanteli^ shows 

 that they must have been broad and in all probality much 

 shorter than the fore wings, f 



* We find here an iutermembral homology analogous to that which 

 exists between the fore and hind limbs of Vertebrates. See an exhaus- 

 tive paper by Wilder on Interinenibral Homologies, Proc. Bost. Soc. of 

 Nat. Hist. vol. XIV, p. 154. 



fin fact I do not believe that the living May-flies are so primitive as 

 regards their wing structure as has been commonly supposed. They 

 have attained a high degree of cephalization of the function of flight , 

 as shown by the reduction in size of the hind wings, and the large de- 

 velopment of the mesothorax. The subcosta and radius are nearly 

 parallel with the costa and closelj- approximated to it ; the wings are 

 well corrugated ; and an extensive system of concave veins have been 

 developed. Each of these characteristics is discussed later. 



