[Illustration: _Swelling of the Head of the Fly._]
But other pupæ, although not, perhaps, quite so arduously placed as in the last instance, nevertheless present us with an extremely difficult puzzle, as to how the included insect is to be extricated from its swathing bands. The common flesh-fly, or blow-fly, for instance, in the pupa state is shut up in a membranous case, out of which there seems no escape; but there is a way, and the manner in which the insect gets out of its prison is a remarkably curious one, well repaying the trouble of a little close observation. At the larger end, under which the head of the fly lies, and from which it always issues, there is commonly a sort of lid which can be pushed off like the lid of a box, and the insect can then walk out at pleasure. But in the case of the pupa of the blow-fly this lid is not very easily removed, and the fly, therefore, is furnished with a most ingenious method of thrusting it off. On opening the larger end of such a pupa, if the fly within is ready to come out, a most curious phenomenon will be seen. The insect moves towards the lid, and there begins to blow out its head in the most extraordinary manner, swelling it to twice its natural size; a moment after it will resume its natural size; then again it will puff it out, making its two eyes to start asunder, and its head to assume several different shapes in succession! Two representations are annexed of the striking figure of the insect's head when it thus causes it to swell out. After repeating this action several times, the fly emerges from the pupa. The cause of this remarkable dilatation is the filling of a membrane, situated at the middle part of the head, with air, by which it is blown out into a sort of bladder as large as the head itself. This acts as a kind of lever, and eventually pushes up the lid of the pupa case, allowing the insect to make its exit unmolested. This part generally disappears afterwards, and the head becomes alike firm and unyielding in all its parts; but it may, in at least its rudiments, be seen even in the head of the adult fly, by slightly pressing its head between the fingers, when it appears as at _x_ in the cut.
[Illustration: _Head of a Fly magnified. It shows opposite the letter x the remains of the membranous bag._]