148 INSECTS AND HEREDITY 



position they were compelled to pupate. The condition 

 of the resulting pupae clearh* refuted the h) pothesis of 

 a mechanicall)-crealed groove and thickening, caused by 

 the cutting into and pressure upon the soft )ielding 

 cuticle. I'or in the vertical position with head down- 

 ward the pupa slips through ihn silken loop beyond the 

 j^osition of the groove, so that the pressure has to be 

 borne by an unprepared j)art 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.' 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 cxtremit) of the pupa is in all probabilit}' the 

 persistent trace of a vanished cocoon, and we ma)- 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 commonl)' 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 i)robably favoured by 

 a short pupal period falling wholl\- within the time of 

 least stress — summer or the wet season. When the 

 cryptic colouring of the bare i)Upal surface is as effective 

 for concealment as that of the cocoon, it presents certain 

 advantages over the latter. The secretion of a large 

 (juantity of material is unnecessary and tell-tale move- 

 ments in the period before pupation are greatly reduced. 

 These benefits are conferred when the concealment 



' Tliis ex|)erimciU ha.s not been publiblied liithcrlo. U was, however, 

 described and the juipae exiiibiied in ihe diseiission in Section D of the 

 Briiish Association at Manchester, on Monday, Sept. 3, 1887. Sec 

 I^t/>or/, p. 755. 



