THE WINGS OF PLECOPTERA 249 



the secondary branches of media. In this region of the wings there are 

 frequently striking differences between the wings of the two sides of 

 individual insects. 



I am convinced, as already stated in the discussion of the special 

 features of the wings of the Plecoptera, that only the first forking of the 

 radial sector, the division of this vein into veins R2+3 and R4+5, is primi- 

 tive; and that in those cases where the radial sector is more than two- 

 branched, the additional branches have been developed secondarily. 



It is also evident that only the first forking of media, the division of this 

 vein into veins M1+2 and M3+4, is primitive, for the farther branching of 

 these veins is too inconstant and erratic to be considered primitive. 



The result of these conclusions is that forms that have only a two- 

 branched radial sector and a two-branched media, like A^eniozira for example, 

 are believed to resemble more closely what must be regarded as the primi- 

 tive type of the order than do those in which these veins are more than two- 

 branched, even though among them individuals may be found that much 

 more closely resemble the hypothetical primitive type. In other words the 

 known facts indicate that the Plecoptera have been evolved from a form 

 in which the radial sector and the media where each only two-branched. 

 In some of the descendants of this primitive stone-fly the reduction of these 

 veins has been carried still farther, in others, additional branches have been 

 developed upon these veins, but in a very erratic manner. 



As yet paleontology affords little data as to the form of the primitive 

 members of this order, but what data we have tends to confirm the con- 

 clusions stated above. I do not, however, place much weight on the 

 paleontological evidence; for so few ancient Plecoptera are known that we 

 can not claim to have an adequate conception of the ancient plecopterous 

 fauna. Remains of two nymphs and one adult have been found in Jurassic 

 deposits; and this is all the data that we have from times preceding the 

 Te^tiar3^ 



The adult from the Jurassic is Mesonemottra Maaki Brauer. The fore 

 wing of this species is quite well -presented and is quite similar to a wing of 

 the recent genus Nemoura. 



Less than a score of species are known from the Tertiary; all of these 

 belong to existing genera, and the greater nttmber of them belong to the 

 Nemouridae ; the other species are placed in the genus Perla. 



Returning to an examination of recent forms, from a study of which we 

 must draw our conclusions, we find that the type of wing that I have 

 concluded represents best the primitive plecopterous wing exists practically 

 unmodified in one or both pairs of wings of representatives of four of the 

 five families of the order; and in the fifth family, the Pteronarcidae, where 

 extended specialization by addition has taken place, the modification of 

 it is comparatively slight. 



