26 THOMAS SAY FOUNDATION 



crossveins were counted in all the spaces between the tips 

 of veins Rs and M in all the wings of these 75 specimens 

 and only in a minority of the specimens did they occur 

 singly. Most of them were between the branches of 

 radius, the first, second, third and fourth longest inter- 

 space containing 27, 34, 11 and 1 crossveins respectively, 

 and the space between Rs and Rl having 4. The fore- 

 most interspaces between the branches of the cubital vein 

 and the one next behind it contained 11 and 6 extra cross- 

 veins respectively in all. The crossvein conjoining the two 

 branches of the cubital vein and the one next behind it 

 contained 11 and 6 extra crossveins respectively in all. 

 The crossvein conjoining the two branches of vein M 

 (with the branches wide apart) was absent in about 8 per 

 cent of the wings, and was absent from all four wings of 

 one single specimen. It was absent from at least one 

 wing of 19 specimens. In addition to this it had disap- 

 peared by fusion of these two branches together in 44 

 cases, the extent of the fusion equalling usually the nor- 



Cu ^ 



IstA 



3dA ""V,. V "\ lstA V,. \ 3dA 



2 d A 3dA 2d A 3dA 2dA 



3 2 3 4 



Fig. 9. Diagram of the relation between the cubito-anal crossvein 



(cu) and the anal cell (a): 1, in Perla bilobata; 2, in 



Pcrlodes irregularis; 3, in Peltoperla anna; 4, in Perla 

 capitata. 



mal length of the crossvein when free, but sometimes run- 

 ning to four or even five times that length. 



The three regular crossveins that enter into the compo- 

 sition of the cord, r, r-ni, and m-cu, are always present or 

 replaced by fusion of adjacent veins. The foremost one, 

 r, is most constant and does not disappear by fusion nor 

 vary greatly in length or position. The second, r-m, was 

 present in all 4 wings of 38 specimens and in at least two 

 wings of all the specimens. The third crossvein, m-cu, 

 was present in 40 wings, and fused out in all the others, 

 the extent of the fusing ranging up to four times its nor- 

 mal length when present. 



Conclusion. — All characters are good characters if used 

 with discretion. Though crossveins and branches vary in 

 number, there is a range and a disposition of both that is 

 constant for the species. But more dependable than these 

 are the relative positions of parts at the principal points 



