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 pupæ 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 pupæ 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.[43] [43] 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. 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 insects 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 it is then placed in a box, the metamorphosis may be observed. To study this last evolution more at his ease, Réaumur covered a large extent of the wall of his study with pupæ of the _Vanessa polychloros_ and other species. [Illustration: Fig. 128.--Chrysalis of the Large Tortoise-shell Butterfly (_Vanessa polychloros_) whose different parts have been opened before they were fastened down. (_a_, wings; _b b_, antennæ; _t_, trunk, or proboscis.)] When the parts of the body of the insect have attained a certain degree of solidity within the envelope, it has no great difficulty in making the thin and friable membrane which surrounds it split in different places. If it even distends itself or moves, a small opening will be made in the dried skin. If the movements persist, 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 fixed, and the insect can take advantage of the opening which is made, and escape. Little by little also it advances its head. The head is the first out of the old skin, and the insect sets itself entirely free. This occupies rather a long time; for we must remember that, under the pupal envelope, its legs, its antennæ, its wings, and many other parts, are enclosed in special cases. These peculiar circumstances show that the animal has much trouble and must employ some time in setting free all the parts.