278 



INSECT TRANSFORMATIONS. 



the spines of the old skin ; and then giving itself a 

 sudden jerk, it spins itself rapidly round a dozen or 

 twenty times. Rt aumur says that this gyration 

 usually throws off the slough, in consequence of its 

 being farther from the centre of motion, and there- 

 fore exposed to a greater centrifugal force ; but un- 

 luckily for this refined philosophy, it is not the silk 

 button, but the chrysalis which spins round, and con- 

 sequently the old skin does not twirl at all, and only 

 moves like a pendulum; — the best method, evidently, 

 of disengaging the hooks it hangs by. Besides, the 

 threads of the silk are not broken by the gyration, as 

 Reaumur, followed by Kirby and Spence, asserts ; 

 otherwise the weight of the chrysalis would to a cer- 

 tainty break its threads, more easily than the sup- 

 posed centrifugal force would break those which 

 suspend the slough. Repeated observation has satis- 

 fied us, therefore, that the twirling of the chrysalis is 

 both for the purpose of disengaging the old skin and 

 strengthening its own hold.* Bonnet may be right 

 or wrong in thinking the stimulus of the spines of the 

 old skin is the cause of the twirling : we have ob- 

 served that the insects which change into chrysalides, 



fl, 0, front and side view of clirysalides of Vanessa urtica, sus- 

 pended by their anal hoolis. c', anal hooks magailied. <f, old 

 skin fallen off. |ft 



* J. R. 



