More singular still are the circumstances which mark the exit of the insect from the pupa case in other instances; and yet more strikingly than those narrated, do they exhibit to us the amazing exercise of the Divine attributes of wisdom and forethought in the case of these humble beings. The larva of a species of moth, which dwells in a wooden cell scooped out of the poplar tree, to which there is no door by which it can escape readily, gnaws away the wood until it leaves only an extremely delicate layer between it and the outside of the tree, which is as thin as writing paper. This done, it enters into the pupa state. Its time in that condition being accomplished, it moves itself by the same contrivance as the _cossus_, and actually pushes through the thin layer, and appears on the outside of the tree, thus making its escape from prison by pushing down a part of its prison wall!
An instance described by the naturalist Bonnet is yet more ingenious in the arrangements by which the insect escapes. While in the larva form it takes up its abode inside the leaf of an ash, curiously rolled up into a cone; and then, after a time, it becomes a pupa, forming a silken cocoon of a very slight texture, and, therefore, easily ruptured by the insect, which it suspends like a hammock in the midst of its habitation. It is the closely joined sides of its leafy dwelling that form a barrier which, were it not for the precaution of the larva, would be impenetrable to so small and weak an animal. But, like the last-mentioned, this larva seems to be aware of the feebleness of its next condition, and gnaws in the leaf a round opening, taking care not to cut through the exterior thin layer of tissue, or _epidermis_. This door is to serve the insect for its exit in due time. But in proportion to its bulk, its green chamber is of considerable size. How, then, shall the insect know the exact place where its portal is situated? How, without a clue, shall it discover in its dark abode the precise circle which requires only a push to throw open its gate? Even this is foreseen and provided for. Out of all other positions in which the little hammock, of which we spoke, might have been hung, and they are numerous, the larva has been directed so to place it, that the silken cord which suspends the head is fastened close to the side of the door which it has previously constructed; and the insect, when it emerges from the pupa, guided by this thread, like Theseus, makes its way out of an apartment which, but for this contrivance, might have been to it a labyrinth as inextricable as that of Minos. Other insects adopt the same precaution of gnawing a doorway for the escape of the perfect insect, only leaving a sufficient thickness of outside tissue to protect the helpless pupa within from the invasion of enemies from without.