( 19) 
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 tne 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 larvee and exposed pupze—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 pups 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. 
