PROTECTIVE RESEMBLANCE 135 



manner. The underside of the caterpillar is somewhat 

 flattened, so that it is in contact with a small part of the 

 circumference of the branch, and the furrow on each side 

 is partially rilled up, at any rate in certain species, by a 

 number of fleshy tubercles. The shadow which would 

 betray the furrow is also neutralised by the light colour 

 of the tubercles." 



Professor Poulton also emphasized, in the following 

 passage, the protective value of appropriate shading in the 

 case of the large green pupa of the purple emperor butter- 

 fly (Apatura vis), which closely resembles a leaf of its 

 food-plant, the sallow. " The most extraordinary thing 

 about this resemblance is the impression of leaf-like flat- 

 ness conveyed by a chrysalis, which is in reality very far 

 from flat. In its thickest part the pupa is 8*5 mm. across, 

 and it is in all parts very many times thicker than a leaf. 

 The dorsal side of the pupa forms a very thin sharp ridge 

 for part of its length, but the slope is much more pro- 

 nounced in other parts and along the whole ventral side. 

 But exactly in these places, where the obvious thickness 

 would destroy the resemblance to a leaf, the whole effect 

 of the roundness is neutralised by increased lightness, so 

 disposed as just to compensate for the shadow by which 

 alone we judge of the roundness of small objects. The 

 degree of whiteness is produced by the relative abundance 

 of white dots and a fine white marbling of the surface, 

 which is everywhere present mingled with the green. 

 The effect is, in fact, produced by a process exactly analo- 

 gous to stippling. The degree of lightness produced in 

 this way exactly corresponds to the angle of the slope, 

 which, of course, determines the depth of the shadow. 

 By this beautiful and simple method the pupa appears to 

 be as flat as a leaf which is only a small fraction of 1 mm. 

 in thickness." 



