( cxiii ) 



transformation. A cocoon is one form of passive defence, cryptic 

 colouring is another, although the two are commonly combined, 

 especially in cocoons built to endure for comparatively long 

 periods, including the times of special stress, — the winter of 

 the northern belt, the dry season of more southern latitudes. 

 The original decline of the cocoon was probably favoured by 

 a short pupal period falling wholly within the time of least 

 stress, — summer or the wet season. When the cryptic 

 colouring of the bare pupal surface is as effective for con- 

 cealment as that of the cocoon, it presents certain advantages 

 over the latter. The secretion of a large quantity of material 

 is unnecessary and tell-tale movements in the period before 

 pupation are greatly reduced. These benefits are conferred 

 when the concealment 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 

 production 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 larvse and exposed pupae — is apparently 

 extremely rare in the cocoon. Indeed the only positive 

 evidence of its existence is supplied by Hylophila {Halias) 

 prasinana* and even in this case it would be satisfactory to 

 repeat the experiments 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 

 important 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 

 * Trans. Ent. Soc. Lond., 1892, pp. 448-451. 

 PROC. ENT. SOC. LOND., V. 1904. J 



