XV 



INSECTA : LEPIDOPTERA 



221 



pigment is generally darker in the summer butterflies than 

 in those which emerge in May. 



On the under side of their wings the two sexes are much 

 more alike. In both there are two dark spots on the front 

 wings, and their tips are yellow, whilst the hind wings are 

 entirely yellow, and are covered with minute scattered black 

 specks, with an inconspicuous black smudge at the centre 

 of the front margin. 



The colouring of the wing is due to pigment, or 

 Structure * n ^ e ca se of some other butterflies to the 

 striation of many thousands of little scales, which 

 cover its surface and come off on the finger as a fine dust, if 

 the wing is gently 

 rubbed. Under the 

 microscope these 

 scales are seen to 

 vary considerably in 

 form, size, and tint. 

 They are modifica- 

 tions of hairs, and 

 transitional stages 

 may be seen (see Fig. 

 150). In their natural 

 position on the 

 wing, they overlap 

 in regular series like 

 the tiles on a roof, 

 each being fixed by 

 a short stalk into a socket in the membrane of the wing. 



If the scales are gently brushed off, the characteristically 

 few " veins " or " nervures " of the wings will be seen, running 

 from the base of the wing to its outer margin ; these branch 

 so frequently that the veins are far more numerous at the 

 margin than at the base. The cross nervures in the wings of 

 butterflies are few or are entirely absent, and so there are very 

 few areas closed on all sides by them. Such areas are called 

 " cells," and in the Large White Butterfly there is only one 

 "cell." In some butterflies, e.g. Vanessa, there is no "cell" 

 at all in the wing. 



The wing itself is formed of two delicate membranes held 

 apart by an irregular tissue. At intervals along definite lines, 



FIG. 150. Scales rubbed from the wing of 

 Pieris brassicae. 



