Graded Mutations in Wings of a Stonefly

Abstract

MORE than two hundred definable variants have been recognized in the wings of the variable stonefly, Allocapnia pygmœa Burmeister. Some of them were mentioned in a former paper¹. The three following deviations are new and merit special reference. Figs. 1 and 2 are left and right forewings of the same female, the former nearly normal, the latter highly exceptional ; Fig. 3 is the left forewing of another and Fig. 4 that of a third. Left wings are seen from below. In a typical plecopterous forewing (Fig. 1), the radial sector (2) arises directly from the radius (1) by its own radicle ; it is separated from the arculus or basal anastomosis (4) by a radial internode (5) and from the median stem (3) by the basimedian passage (6). The hindwing is characterized, typically but not invariably, by a partial fusion of radial sector and median vein, which leave the arculus by a common mediosectoral pedicel. In A. pygmœa these veins issue separately from the arculus on an average of 8 per cent in five hundred hindwings.