LEPIDOPTERA. 167 
fetters; it flies away, brilliant and free, and its many-coloured 
wings glitter in the sun. 
The duration of the pupa state is variable, according to the 
species and the temperature. Réaumur placed in a hot-house, 
in the month of January, some pup which, in the ordinary 
course of things, would not have been hatched till the month 
of May, and a fortnight afterwards the imagos had appeared. 
On the other hand, he shut up some pupe in an ice-house during 
the whole of a summer, and thus retarded their being hatched 
by a whole year. The influence of the temperature on the period 
of emerging, and, consequently, the influence of the seasons on 
the length of this period, are completely brought to light by 
these experiments.* 
We will now see how the insect delivers itself from the last 
skin. To quit the pupa case is not so laborious an operation as 
it was for the same insect to quit the caterpillar’s skin. This is 
because the pupa case is drier; it does not adhere to every part of 
the body, and is brittle. Those which are enclosed in a cocoon free 
themselves of the pupa envelope in the cocoon itself. To witness 
the last operation, the cocoon may be opened, and the pupa 
drawn out of it with care. If then placed in a box, one sees the 
metamorphosis take place. To study this last evolution which 
1s now occupying our attention more at his ease, Réaumur covered 
a large extent of the wall of his study with pupe of the Vanessa 
polychloros and other species. 
When the parts of the body of the insect have attained to a 
certain degree of solidity within the envelope, it has no great dif- 
ficulty in making the thin and friable membrane which surrounds 
it split in different places. If it even distends itself or moves, a 
small opening is made in the dried skin. If it reiterates its move- 
ments, the opening increases in size, and very soon allows the 
imago to emerge. It is on the middle of the upper part of the 
thorax that the envelope begins to split. The split extends over 
the middle of the forehead and back. The pieces of the thorax 
open, separate themselves from the other parts to which they were 
* They hardly seem from later experiments to be so fully explained. It is a well- 
known fact that many insects remain in this state a variable time—the Small Eggar 
(Bombyx lanestris) sometimes as many as seven years.—ED. 
