1580 THE BUTTERFLIES OF NEW ENGLAND. 



affected by that part of the surroundings which were made up of the 

 dark skins of its neighbors. In endeavoring to discover whether the sensi- 

 tiveness of the chrysalids to their surroundings was due to the general 

 surface of the skin as a whole or only to that of one portion, expeiiments 

 were made in tubes, part of which were colored black and part gilt, and 

 the two parts separately by a diaphragm only permitting the body to pass 

 through ; by reversing the condition and making experiments with a large 

 number of chrysalids, it became plainly evident that the color influence 

 acted on some element of the larval skin and that the larger the area of 

 the skin exposed to any one color the more the chrysalis followed its influ- 

 ence. 



The nature of the effects produced is thus described byPoulton : — "The 

 colouring matter of the dark pupae is contained in a thin supei-ficial layer 

 of the cuticle ; below this is a thicker layer divided into exceedingly deli- 

 cate lamellae, between which fluids are present, and the latter form the thin 

 plates which, by causing interference of light, produce the brilliant metal- 

 lic appearance. The thinner upper layer, being dark, acts as a screen in 

 the dark pupae. Precisely the same metallic appearances are caused by 

 the films of air between the thin plates of glass which are formed on the 

 surface of bottles long exposed to earth and moisture. Both have the 

 same spectroscopic characters and the same transmitted colours (comple- 

 mentary to those seen by reflection). The brilliancy of the cuticle can be 

 preserved in spirit for any length of time ; it disappears on drying, but can 

 be renewed on wetting (this had been previously known), and the colours 

 are seen to change during the process of drying, and when the cuticle is 

 pressed, for the films are thus made thinner. The same laraellated layer 

 exists in non-metallic pupae of other species, and is used as a reflector for 

 transparent colouring-matter contained in its outer lamellae. Thus the 

 structure which rendered possible the brilliant effects due to interference 

 probably existed long before these special effects were obtained, and was 

 used for a different purpose." 



It has long been known that many of the chrysalids of the Vanessidi 

 which yield parasitic Ichneumonidae in place of their proper inhabitants 

 are frequently gilded ; which is to be explained, in Poulton's view, from 

 the abnormal state of the larva, which prevents the formation of pigment 

 in the chrysalis. In this instance the gilded appearance is preservative 

 not of the creature itself but of one of its foes, and does not appear a very 

 wise provision of nature. 



These experiments were made principally upon Aglais urticae ; others 

 upon the different species of Pierinae were quite similar, the influence of 

 black surroundings being to produce dark chrysalids and the greater the 

 illumination the darker the chrysalids, this last result being the reverse of 

 that obtained in the Vanessidi ; white produced light chrysalids ; and 



