SHAPE AFFFXTED P.V GRAVITY 151 



The effect of Gravity upon the shape of S^cspended Pupae 

 s?tch as those of the Nymphaliiiae. 



Every naturalist who has watched the pupation of a 

 Vanessid must have observed the extraordinary mobility 

 of the abdominal region of the freshly-exposed chrysalis. 

 Movements of remarkable amplitude take place in every 

 direction, and especially in the dorso-ventral plane, these 

 latter being essential for the withdrawal of the posterior 

 segments from the larval skin and the remarkable feat of 

 attachment to the silken boss close to the point from 

 which the skin itself is hanging. Success is only rendered 

 possible by the remarkable contractile power of the inter- 

 segmental muscles along the median ventral area. These, 

 by their contraction, keep the rigid hook-armed apex of 

 the abdomen firmly pressed to the outside of the larval 

 skin up which it is being forced, and enable it to press 

 down or push aside any of the stiif spines which oppose 

 the movement : these finally bring it to the small silken 

 boss which alone provides a secure basis of attachment 

 for the terminal hooks. For this purpose the ventral 

 muscles require to be far stronger than those of any 

 other region, and we invariably find that they entirely 

 overbalance the dorsal intersegmental muscles in pupae 

 which have been produced on the floor of the breeding- 

 cage. In such pupae the abdominal segments are curved 

 round towards the ventral side, so that the long axis of 

 the apical part forms at least a right angle with that 

 of the thoracic region, and this attitude becomes stereo- 

 typed with the hardening of the pupal cuticle and the 

 consequent loss of all power of dorso-ventral move- 

 ment. These free pupae form a striking contrast with 

 the normal attached individuals in which the long axis 

 of the abdominal segments is nearly in the same line 

 with that of the thoracic. Suspension by the posterior 

 apical hooks and the assumption of a form in which 

 the long axis of the body is nearly in one line, is very 

 ancient, dating back to the common ancestor of a number 

 of closely-related species. For a countless number of 

 generations the soft and yielding Vanessid pupa has 



