U 8 INSECTS AND HEREDITY 



position they were compelled to pupate. The condition 

 of the resulting pupae clearly refuted the hypothesis of 

 a mechanically-created groove and thickening, caused by 

 the cutting into and pressure upon the soft yielding 

 cuticle. For in the vertical position with head down- 

 ward the pupa slips through the silken loop beyond the 

 position of the groove, so that the pressure has to be 

 borne by an unprepared part of the cuticular surface. 

 Upon the mechanical hypothesis, we should expect that 

 the fresh surface would gain some measure of resistance 

 from the strain ; but on the contrary the pupae were all 

 hopelessly deformed and the imagines — if indeed they 

 could have emerged at all — would have been incapable 

 of flight. 1 It is evident that from the very beginning 

 the loop has been accompanied by a sufficient strengthen- 

 ing of the part of the surface exposed to its pressure as 

 soon as the larval skin is thrown off. 



The silken loop together with the attachment of the 

 posterior extremity of the pupa is in all probability the 

 persistent trace of a vanished cocoon, and we may imagine 

 the selective process which made good each step on the 

 road of gradual 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 concealment 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 move- 

 ments in the period before pupation are greatly reduced. 

 These benefits are conferred when the concealment 



1 This experiment has not been published hitherto. It was, however, 

 described and the pupae exhibited in the discussion in Section D of the 

 British Association at Manchester, on Monday, Sept. 5, 188?. See 

 Report, p. 755. 



