322 INSECT ARCHITECTURE. 



wax. The cocoon of the emperor-moth, though 

 thus in some measure impenetrable from without, is 

 readily opened from within ; and when the moth 

 issues from its pupa case, it easily makes its way 

 out without either the acid or eye-files ascribed to the 

 silk-worm. The elastic silk gives way upon being 

 pushed from within, and when the insect is fairly 

 out, it shuts again of its own accord, like a door with 

 spring hinges, — a circumstance which at first puzzled 

 Roesel not a little when he saw a fine large moth 

 in his box, and the cocoon apparently in the same 

 state as when he had put it there. Another na- 

 turalist conjectures that the converging threads are 

 intended to compress the body of the moth as it 

 emerges, in order to force the fluids into the nervures 

 of the wings ; for when he took the chrysalis pre- 

 viously out of the cocoon, the wings of the moth 

 never expanded properly.* Had he been much 

 conversant with breeding insects, he would rather, 

 we think, have imputed this to some injury which 

 the chrysalis had received. We have witnessed the 

 shrivelling of the wings which he alludes to, in many 

 instances, and not unfrequently in butterflies which 

 spin no cocoon. The shrivelling, indeed, frequently 

 arises from the want of a sufficient supply of food to 

 the caterpillar in its last stage, occasioning a defi- 

 ciency in the fluids. 



The elasticity of the cocoon is not peculiar to the 

 emperor-moth. A much smaller insect, the green 

 cream-border-moth (Tortrix chlorana) before men- 

 tioned (page 110), for its ingenuity in bundling up 

 the expanding leaves of the willow, also spins an 

 elastic shroud for its chrysalis, of the singular shape 

 of a boat with the keel uppermost. Like the cater- 

 pillar of Pyratis strigulalis (page 198), whose build- 



* Meinecken, quoted by Kirby and Spence, iii. 280. 



