PLECOPTEBA OF XOBTH AMEBICA 47 



this appendage in P. regularis seems to spread out and be- 

 come lost on the dorsum of that segment. 



This scoop varies greatly in width in the specimens be- 

 fore us. The widest form was taken by Miss Smith for 

 the then unknown male of P. regularis and the narrower 

 form was taken for a distinct species named P. torosa. 



Female. Genital plate blackish, a little more recurved 

 within the apical area of the eighth sternite than in P. 

 regularis, its edge usually falling a little short of the level 

 of the apex of segment, its border typically smoothly 

 rounded, but sometimes truncate, or even a little refuse, 

 but never with an acute middle notch. Another variant 

 in form was taken by Miss Smith to be a distinct species 

 and named P. triloba (PI. 7, fig. 16), but we have not been 

 able to find any good line of demarcation for such a species. 



Distribution. — Bridger Basin, Wyo. ; Cache Valley, Ut., 

 Logan River, Ut. (I. M. Hawley) ; Bozeman, Mont., June 

 18, and Bridger Mts., Mont., 5,000 ft., June 19 (R. A. 

 Cooley) ; Provo, Ut. (E. C. VanDyke) ; Crane, Ariz., Apr. 

 27 (W. J. Chamberlain) ; Watertown Lakes, Alta., Canada, 

 July 1 (J. McDunnough) ; Milton, Ore., Apr. 5; Hood 

 River, Ore., Apr. 7 (J. C. Bridwell). 



A considerable number of adults of both sexes taken 

 together in Logan Canyon on July 12, 1922, by the senior 

 author and preserved in alcohol, has enabled us to learn 

 something of the variability of this species. These adults 

 were of all ages : some were emerging from their nymphal 

 skins on the leaf drifts at the eclges of the stream and 

 were still pale ; some were mature and well colored ; and 

 others were old and black. They were found clinging low 

 in the willows and on the weeds by the waterside, and, be- 

 ing indisposed toward flight by day, were picked with the 

 fingers. 



Family PERLIDAE. 



This is a large family, containing many of the most 

 familiar genera and species of stoneflies. The largest 

 forms are members of the genera Acroneuria and Perla. 

 A number of diurnal, foliage-inhabiting, greenish forms 

 occur in the genera Alloperla, Chloroperla and Isoperla. 

 All are stream-side dwellers but a few, whose nymphs 

 have the most abundant development of gills, such as Pcr- 

 linella drymo and Perla capitata, may be found about the 

 stony borders of lakes and ponds. 



Wingless forms are unknown, but brachyptrous males 

 occur in one or more of our Western species (Perla lan- 

 guida). In all the terminal segments of the palpi and the 



