206 BRITISH BUTTERFLIES. 



the name argus differently from the way in which it was used by 

 Schiffermiiller. But the difficulty of separating the species has also 

 proved troublesome, although, except in some individual cases, the 

 difficulty is not as a rule great, and mistakes are rather the result of 

 hurry or want of training than anything else, and one can usually 

 discriminate the $ s at a glance. It is difficult, however, to define by 

 words the differences we see. In colour P. argus (aegon) is more violet- 

 or purplish- blue, i.e., has a more pronounced tinge of reddish in its 

 tone, the apex of the forewings is rather rounder, the outer margin 

 also rounded, the costa of the forewings more broadly white, the dark 

 marginal border wider and more decided ; the hindwings have a very 

 black costal margin (to second nervure, i.e., to second branch of sub- 

 costal nervure), extending to the outer margin; those of argyrognomon 

 are only dark to the first branch ; the dark margin of the hindwings 

 also is more pronounced in argus, whilst, in argyrognomon, the margin is 

 represented rather as interneural spots ; in all the wings the fringes of 

 argyrognomon usually appear quite dull grey compared with the snowy- 

 white (externally) fringes of argus. On the undersides the ground 

 colour of argus is bright silvery-grey (tending sometimes to w T hiter, at 

 others to greyer than normal) with bright blue bases to all the wings ; 

 in argyrognomon the ground colour is dull grey (almost with a tinge of 

 ochreous), and the base of the forewings is only slightly, of the hind- 

 wings more strongly, sprinkled with greenish-blue ; sometimes the 

 ground colour of argus has a tinge of red in it, but the more variegated 

 appearance of the underside of argus as a whole is usually very marked 

 when compared with the uniform sameness of that of argyrognomon ; 

 the spots of the angulated row on the underside of the forewings are 

 larger, more conspicuously and more strongly ringed with white, in argus, 

 whilst the direction is different, there being a much stronger angula- 

 tion in argus centrally ; the discoidal spot also is much nearer this 

 row in argus. On the hindwings, again, the black spots are more 

 conspicuous, comparatively large, and more strongly ringed with 

 white, and again the spots run up nearer the discoidal. The marginal 

 orange spots incline rather to red-orange in argus, to yellow in argyro- 

 gnomon ; the metallic scales in these marginal orange (yellow) spots 

 are often much brighter and of a more brilliant blue in argus than in 

 argyrognomon, where they are inclined to greenish. The tips of the 

 palpi appear to have a black pointed terminal spine in argyrognomon, 

 but are more slender, black with a white terminal point, in argus. 

 The eyes of argus are surrounded with blue-white scales, those of 

 argyrognomon with white scales. The tips of the antenna) of argus are 

 strongly marked with brown beneath; the long, black, antennal 

 segments, and white intersegmental rings appear also to be much 

 more sharply marked in argus than in argyrognomon. Apart from these 

 differences, four structural characters are absolute, viz., (1) the presence 

 of a well-developed spine on the front tibia of argus (aegon), and its 

 absence in argyrognomon. (2) The great difference in the $ genitalia 

 is remarkable ; Chapman notes it as almost greater than that between 

 any other two Plebeiids (see antea pp. 155-G). (8) The androconia of 

 argus are somewhat elongate, crossed by eight to nine longitudinal 

 rows of points fairly separate ; those of argyrognomon are almost 

 circular, with ten to twelve longitudinal rows of points. (4) 

 The conspicuous white hair-scales, so abundant in argyrognomon, are 



