84 
INTRODUCTION. 
caterpillar enters upon that state in July, produces 
the butterfly in thirteen days ; but when the chry- 
salis is formed in the end of autumn, the perfect in- 
sect is not evolved till the succeeding June. Such 
variations were conjectured by Reaumur to depend 
on the temperature to which the chrysalides are ex- 
posed ; and he proved this to be the fact by a series of 
very simple and conclusive experiments. By placing 
a variety of chrysalides in an atmosphere artificially 
heated, he succeeded in bringing out several broods 
of butterflies in the very middle of winter, which, if 
left to natural influences, wrnuld not have appeared 
till the ensuing summer. He found that when the 
temperature was rather high, the chrysalides made 
as much progress to maturity in five or six days as 
they would have done in ordinary circumstances in 
an equal number of weeks. Having thus proved 
the influence of heat in hastening the exclusion of 
these insects, he next tried the effect of cold in re- 
tarding it ; and the result was equally satisfactory. 
He preserved several pupse from heat, by keeping 
them during summer in an icehouse, in consequence 
of which the butterflies were not disclosed till a year 
after their ordinary and natural time. * 
When the butterfly is fully matured, it extricates 
itself from the puparium, by bursting that portion of 
it which covers the thorax, an operation which is ea- 
sily accomplished, as the membrane has by that time 
become weak and friable. On its first exclusion, it 
• Reaumur, ii. 10. 
