48 BKITISH LEPIDOPTERA. 



Parallel with the outer margin is a more or less distinct transverse 

 line, which has been already referred to as " Poulton's line," so called 

 because Poulton first drew attention to its presence and probable 

 meaning. Of this line, Poulton writes : — " Although the wing of the 

 imago expands into a size far beyond that reached by the pupal organ 

 within which it is developed, the former does not entirely fill the latter 

 before emergence. The margin of the imaginal wing lies well within 

 the corresponding margin of the pupal wing which encloses it. Not 

 only are the two margins separated by an interval, but their contours 

 are frequently very different. Furthermore, in many species, the 

 position of the future margin of the imaginal wing can be distinctly 

 made out on the pupal surface from the very beginning of the 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." Without committing ourselves to the 

 generalisation involved in the statement that "the imaginal wing 

 does not entirely fill the latter before emergence," the facts of the 

 actual meaning of " Poulton's line " appear, from our subsequent 

 observations, to be fairly accurately set forth. Poulton's line appears 

 to be the line at which the development of the outer margin of the 

 wing takes place, but, subsequently, as the wing grows, it pushes itself 

 out (at least in many species) to the fullest limit permitted by the 

 pupal wing. Poulton further considers that a " careful comparison of 

 the imaginal hind-margin with that marked out on the pupa, supports 

 the conclusion that the angulated outline of the imaginal wing in the 

 Vanessids and allied genera, has been derived from the more usual 

 smooth and continuous form of hind-margin," and states that " the 

 hind margin of the fore-wing of the imago of Pyrameis cardui presents a 

 slight bay in its central part, but the bay is even less marked in the 

 corresponding line upon the pupa. In other words, the latter is more 

 normal and is now in a stage through which the imaginal hind margin 

 has passed. Home indications of the black-and-white fringe on the 

 imaginal hind-margin can be made out in the disposition of the pupal 

 cuticular pigment, also the corresponding line. Similarly in P. ata- 

 lanta the bayed hind-margin of the imago is less pronounced in the cor- 

 responding part of the pupa. A similar relationship is witnessed in 

 Vanessa io although the difference is rather in the angularity and amount 

 of projection of the cusps than in the depth of the bay. In dark varieties 

 of this pupa the pigment is distributed along the lineh'm' (' Poulton's 

 line ') in such a manner as to suggest a former black-and-white fringe 

 which is now absent from the imaginal hind margin. ... A 

 comparison between the pupa and imago of Euyonia polychloros 

 similarly shows that the pupal line is rather less indented than that of 

 the imago. . . . An examination of the pupa of Polygonia r-albiim 

 supported, in the most complete manner, the conclusions already arrived 

 at, and it is clear that the hind margin of the imago is far more jagged 

 than that of the corresponding line upon the pupa, which, indeed, is 

 not much in advance of the condition found in the imago of V. io 

 or E. polychloros. Hence, we see that not only are the traces of lost 

 imaginal parts preserved, but the indications of ancestral forms and 

 markings are also fixed on the surface of the pupa." Although we 

 have made many observations on "Poulton's line" and its relationship 



