256 MR. E. B. POULTON ON THE EXTERNAL 



surface from the very beginning of tlie pupal period and long before the imaginal organs 

 have begun to appear. The line which corresponds to the future imaginal hind margin 

 is especially distinct, and is separated from the hind margin of the pupal wing by a very 

 wide interval. 



These facts were shown in the plate accompanying my paper in the Phil. Trans. 

 Roy. Soc. vol. 178 (1887), B, pp. 311-441, and were briefly alluded to in the description 

 of fio-ure 11 (p. 410), which represented a dark variety of the pupa of Vanessa lo, on 

 which the imaginal hind margin is very distinct. 



These facts are probably true of the hind wing as well as the fore wing, but they 

 have only been studied in the latter. 



Over the part of the pupal fore wing beneath which the imaginal wing will develop, lines 

 which correspond to the future venation are more or less plainly visible. These lines 

 cease at the limits of the area over which the wing will be formed (viz. the line H'.M'. 

 in figs. 18, 22, &c. on Plate XXVIL). Sometimes, however, their direction is continued 

 by irregular lines of pigment across the interval between the pupal and future imaginal 

 hind margins (viz. between H.'M.' and H.M.). These irregular continuations are, how- 

 ever, very different in character from the more defined appearance of the lines which 

 represent the venation. When the latter are studied in especially favourable species 

 {e. g. in Vanessa Atalanta, fig. 18, Plate XXVII.), they are seen to correspond exactly 

 with the future venation of ttic imaginal wing. This is all the more remarkable 

 inasmuch as the main tracheae within the pupal wing, which will ultimately be enclosed 

 as important elements in the imaginal veins, possess at this time an arrangement different 

 from that which they will then assume. Thus fig. 26, Plate XXVII., shows the arrange- 

 ment of the main trachea? in the left hind wing of the pupa of Fapilio Machaon, as seen 

 from within, an arrangement very different in detail from that which will be subse- 

 (p\ently assumed and is already mapped out (in the case of the fore wing) upon the 

 pupal cuticle. 



It has been already found that indications of earlier imaginal conditions may be traced 

 in the structure and form of the pupa. It was therefore of interest to investigate from 

 this point of view the relation of these pupal markings to the form and ajipearance of 

 the imaginal Aving. The point which I selected as a test was the indented and irregular 

 form of the hind margin of the fore Aving in Vanessa and allied genera. There is every 

 reason for the belief that such an outline has been derived from the more usual smooth 

 and continuous form of margin. A careful comparison of the imaginal hind margin 

 with that marked out on the pupa supported this conclusion. 



The hind margin of the fore wing of the imago of Cynthia cardui presents a slight bay 

 in its central part (fig. 17, Plate XXVII.) ; but the bay is even less marked in the cor- 

 responding line upon the pupa (H'.M'., fig. 16). In other words, the latter is more normal 

 and is now in a stage through which the imaginal hind margin has passed. Some indi- 

 cations of the black-and-white fringe on the imaginal hind margin (fig. 17) can be made 

 out in the disposition of the pupal cuticular pigment along the corresponding line 

 (H'.M'., fig. 16). Similarly in Vanessa Atalanta the bayed hind margin of the imago is 

 less pronounced on the corresponding part of the pupa (compare fig. 19 with fig. 18, 



