16 BRITISH BUTTERFLIES. 



centre of thorax to apex of forewing x 2), these are indeed minutissimus, 

 the smallest J measures only 23mm. His largest British J is 41mm., his 

 largest £ 39mm. There is considerable sexual difference in the coloration 

 of the underside of this species, that of the^ being, as a rule, lighter than 

 that of the $ , yet it might be difficult, in particular cases, to distin- 

 guish one from the other, so closely do the $ s in their darker forms 

 approach the paler $ s ; on the whole, however, the underside of the 

 $ s incline to whitish or wbitish-grey, tinged with fawn or grey-brown, 

 whilst that of the 2 is pale brown, more heavily shaded with darker 

 brown. The palest undersides of the^ are of a clear chalky-white, the 

 hindwings with the faintest possible tinge of fawn (ab. pallida, Tutt); 

 just in advance of these the ground colour is whitish -grey, the hind- 

 wings tinged with greyish- or yellowish-fawn (r/risescotH, n . ab.) ; sometimes 

 the shading becomes strongly tawny (fulvescens t n. ab.), and it may be quite 

 dark grey, the hindwings distinctly tinged with brown (fnxcescnis, n. ab.); 

 this latter form is common to both sexes, the grey ground colour of 

 the forewings in the 2 being usually due to the development of the 

 pale ground along the nervures. The ground colour of the underside 

 of the $ is, however, usually brown rather than grey-brown, generally 

 with more fully brown hindwings (brunnescem, n. ab.), and, in more 

 extreme cases, becomes so dark that the ground of the hindwing tends to 

 coffee- or even chocolate-brown (castanea,n.a,h.). The difference between 

 the tint of the fore- and hindwings may be very slight, on the other 

 hand, it is sometimes markedly different. It is also to be noted that 

 the colouring of the underside is often racial, and the specimens are 

 apparently whitish when the insect occurs on steep mountain or hill slopes 

 of limestone with sparse herbage. We notice them as particularly white 

 on all the slopes in the Trafoi -thai between Gomagoi andFerdinandshohe, 

 between St. Anton and the top of the Arlberg Pass, on Mt. Salvatore, 

 above Piora, at Arolla, at Bourg d'Aru in the Dauphiny Alps, etc. ; where 

 the insect occurs on thickly-clad downs, etc., the ground colour is 

 generally, in both sexes, darker, hence one suspects the colour has some 

 distinct protective value. The spotting of the underside shows 

 considerable variation both as to position, direction, shape, size, 

 and number. Roughly, the spots on the forewing consist of the 

 basal (normal number two), the discoidal lunule, the submedian 

 series (7, very rarely 8), and the marginal series of lunules sur- 

 mounted by chevrons (7) ; on the hindwing they consist of the 

 basal (3), the discoidal, the submedian (9), the marginal also sur- 

 mounted by chevrons (9). The position of the submedian series of 

 forewings usually lies almost midway between the discoidal lunule 

 and marginal series, whilst the direction is generally that of a curve 9 

 with spots 1 and 5 furthest towards the centre, and 6 and 7 turned 

 somewhat outwardly, sometimes they are thrown back from the 

 discoidal in an almost straight line against the marginal series (ab. dis- 

 creta,n. ab.), at other times the series is brought quite close to, and with 

 the basal series forms a sort of semicircle around the discoidal lunule 

 (ab. glomerata, n. ab.). [Both these 1 forms (discreta and glomerata) occur 

 in A. thetis as well as in Polyommatus icarus, Plebeius an/us, and Aricia 

 asirarclif.and are commoner in the last-named species than in the others.] 

 Similar modifii at ions occur in the position of the spots on the hindwing. 

 The spots vary in shape and may he rounded, oval, or elongate, the 

 two latter sometimes occurring in the same example; the extreme 



