TEUE MIMICRY 105 



thus exercised no influence upon the caterpillar. Nor has it affected 

 the pupa, which in both cases exhibits the very different and quite 

 characteristic form of the Danais pupa and the Limenitis pupa 

 respectively (PI. I, Fig. io6, and ii b). 



But even in the butterfly itself nothing is altered, except what 

 increases the resemblance to the model. All else has remained un- 

 changed, above all, the venation of the wings. Since the painstaking 

 and valuable work of Herrich-Schafer the venation has been made 

 the basis of the whole systematic arrangement of butterflies, and it 

 enables us, in point of fact, to distinguish with precision, not the 

 families alone, but often even the genera, for the course of the veins 

 in the different species of a single genus is the same, and that is true 

 for the mimetic species as well as for others. Thus the Danaid-like 

 Limenitis has the usual Limenitis venation, of the kind seen in our 

 own indigenous species of Limenitis, and the already described 

 Elymnias species of the African and Indian forests and grassy 

 plains have always the venation characteristic of this genus, whether 

 they be protected only by sympathetic colouring or imitate an 

 immune Euploea, a Danais, an Acroea, or a Tenaris. However 

 much the contour of the wing may vary, the venation is unaffected, 

 and we can distinguish model from copy by this means alone, so that, 

 even when there is the closest resemblance, no doubt is possible. In 

 its theoretical aspect this constancy of venation is obviously important, 

 for as nothing about the organism is incapable of variation, the veining 

 of the wings might have varied, as indeed it has varied from genus 

 to genus in the course of the phylogenetic history ; but as changes in 

 venation could not be detected by the butterflies' enemies, however 

 sharp-sighted, there has been no reason in these cases for variation 

 in this respect. 



In this connexion Poulton has brought forward interesting facts 

 showing that the mimics of one model, belonging to different genera, 

 often secure the same effect in quite different ways. Thus the glass- 

 like transparency of the wings in the Heliconiidae of the genus 

 Methona depends on a considerable reduction of the size of the scales, 

 which ordinarily cover both sides of the wing as thickly as the tiles 

 on a roof, and produce the colour. In another quite similar species, 

 also transparent-winged, the Danaid liuna ilione, the transparency 

 is due to the absence of most of the scales, and in a third mimic, 

 Gastnia linns, var. heliconoides, the scales are not altered either in 

 size or number, but have become absolutely unpigmented and trans- 

 parent. In a fourth mimic, a Pierid, Bismorphia arise, the scales 

 have not decreased in number, but have become quite minute, while 



H 2 



