368 



interior part of the forewing becomes visible, the line of separa- 

 tion corresponding to the diagonal bar on the wing-case of the 

 pupa. This difference is caused by the amount of opaque 

 matter in the epidermic cells : the hghter-coloured field con- 

 taining more of it, as can be proved by observing the wing in 

 transmitted light, when we see the contrast between the two 

 wing-areas changing into the opposite effect, the hghter part 

 becoming darker, and vice versa (figs. 12 and 13). 



Very soon after the first appearance of pigment-substance 

 in the cells, the often-mentioned series of light spots parallel 

 to the outer border of the forewing becomes visible. The spots 

 do not arise all at one time. Nos. 4 and 5 being the first to become 

 visible. They owe their distinctness to the same phenomenon 

 which caused the contrast between the anterior and posterior 

 wing-fields : accumulation of non-transparent pigment-matter 

 in the cells of the epidermis. This matter reflects light more 

 strongly than the surrounding wing-part, thereby calling forth 

 the impression of a light spot in a darker field. 



At the inner side of the anterior spots a darker area, of a 

 more or a less triangular shape, can be observed : here the trans- 

 parency of the wing has increased instead of being diminished. 



The hindwings remain far behind the forewings in the develop- 

 ment of a pattern, and do not pass through a stage of diagonal 

 division into a lighter and a darker field. Their transparency 

 lasts longer than that of the forewings, and passes very gradually 

 into a general coloration of a cinnamon-brown tint. In their 

 marginal interveinal spaces a series of light spots becomes 

 visible, some days after those of the forewings, but of the same 

 nature as these. 



This primitive pattern remains in existence during almost 

 the whole period of the chrysalid stage, without undergoing 

 important modification, and then changes rapidly into the 

 imaginai pattern. The modifications shown before the final 

 alteration consist chiefly of a gradual diminution and final 

 disappearance of the contrast between the two wing-fields, 

 hand-in-hand with the development of a well-pronounced brown 

 ground-colour over the whole surface of the wing. This ground- 

 colour, however, only fills in the areas of the interveinal spaces : 



