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. Reaumur says that this gyration 

 usually throws off the slough, in consequenee 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 



bv- Besides, the 



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

 Reaumur, follow^ed 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 sa- 

 tisfied 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 

 observed that the insects which change into chrysalides. 



a, h, front and side view of chrysalides of Vanessa UrticcB sus- 

 pended by their anal hooks, c, old skin fallen off. d, anal 

 hooks macnilied. 



* J. R. 



