ORIGIN OF SILKEN LOOP 149 



afforded is equal ; but the pupal cuticle lends itself to 

 certain forms of cryptic defence much more freely and 

 completely than the walls of the cocoon : to the pro- 

 duction of angular shapes and of smooth or polished 

 surfaces, to the attainment of varied colours and the 

 perfect gradation of tints, above all to the power of 

 individual colour-adjustment. This latter culminating 

 effort of adaptation so commonly possessed by larvae 

 and exposed pupae is apparently extremely rare in the 

 cocoon. Indeed the only positive evidence of its existence 

 is supplied by Hylophila (H alias] prasinana, 1 and even 

 in this case it would be satisfactory to repeat the experi- 

 ments on a far larger scale than has been as yet attempted. 

 The transition is easy from a loose and open cocoon with 

 apertures through which the cryptic colours of the enclosed 

 pupa could play their part in defence, through stages in 

 which the latter element becomes more and more im- 

 portant as the cocoon progressively diminishes, to the 

 climax when the almost invisible remnants of the silken 

 covering are retained as supporting structures merely. 

 In all except small and light pupae a point would be 

 reached, at a greater or less distance from the climax, 

 when some special strengthening of the cuticle exposed 

 to the strain became the indispensable condition of further 

 advance. Thenceforth further reduction and further 

 strengthening would proceed together, the existing groove 

 and thickening being but the concentration of the broader 

 band of pupal tissue specially prepared to meet the 

 pressure when it first became a danger. 



Comparison with the pupae of some of our common 

 British Geometrae supports the hypothesis set forth above ; 

 for it is seen that very similar changes have independently 

 occurred, and occurred so recently that the essential stages 

 are still preserved. Furthermore, they are invariably met 

 with in species which have a short pupal period passed 

 in the warmer months of the northern year. Eugonia 

 quercinaria (the ' August Thorn Moth ') spins a loose 

 and open cocoon, within which the chrysalis, as well as 

 the larva before pupation, develops an effective cryptic 

 1 Trans. Ent. Soc. Lond., 1892, pp. 448-51. 



