LARVÆ AND IMAGO OF CASE FLY.
The movements of these creatures are as comical as their specimens of tailoring. We see them mounting a stem or leaf with great gravity, when suddenly up goes the tail, the legs hold tight, and the case turns completely over, as if on the first of May, Jack-in-the-green were to dance on his head. When the creature is hidden, and the case sways to and fro like a buoy attached by too short a rope, the sight is very curious. This case-maker is the larva from a fly which bears resemblances to the two families which stand on either side of it—the Lepidoptera, or true butterflies, and the Neuroptera, of which the dragon flies and other membranaceous winged insects are members. As soon as he enters the world, he begins to show his skill in tailoring, and by means of silken threads and gluten constructs his case of bits of stick, straw, dead leaves, or shells, in fact, whatever he can get, and as long as he retains the worm-like form the case is his castle, and he can defy the world. The case outside is generally a rough affair, but if you draw out the cad you will see that inside it is perfectly cylindrical, smooth, and polished, and around the doorway, through which the larva makes acquaintance with the world, it is neatly finished with a very circular rim. When you have removed a cad, if you throw him into a tank you will learn in an instant what is the use of his case, for his soft nakedness is no sooner exposed, than the minnows finish him, and find the flavour excellent. But to see a cad in his proper uniform molested is a very rare sight indeed. He passes his larva life innocently, and is an amusing fellow; when he feels the numbness of death creeping over him, the cad draws in his six legs, and sets to work inside to weave a winding-sheet and to shut the shutters, for he knows that his time is come, and there is no one to do such melancholy offices for him. All alone in his solitary cell, the hermit works day and night, and hourly his fingers grow more feeble. We look and find the shutters closed, and by this time the larva has changed into a pupa.
GRATING OF CASE-WORM, MAGNIFIED.
The mode in which the worm closes its cell is curious enough. Over its entrance it weaves a grating of silk, which hardens in water and remains insoluble. It may be seen very plainly by the naked eye, but under a good lens increases in interest. The grating is placed a little inside the margin of the opening, and fits exactly within it, and its object is to protect the pupa from invasion, and at the same time, to admit water for respiration. De Geer describes one of these gratings in which the pierced holes were disposed in concentric circles, as represented in the engraving. This, however, is not, as far as I am aware, the usual form of the grating, many that I have examined were formed in regular rays from a centre like the spokes of a wheel.
But the escape of the pupa, when about to undergo its last metamorphosis, is as interesting as the fact of its closing the shutters to announce its own death. It is provided with a pair of hooked mandibles, with which to gnaw through the grating, and no sooner have these accomplished their purpose than they fall off, and the pupa takes its last shape of a four winged fly, as represented in the cut.
COLEOPTERA.
The beetle tribe are distinguished from other insects by the possession of elytra, or wing-cases; which wing-cases are, with regard to the typical structure of an insect, to be regarded as really the first pair of wings hardened into a horn-like consistence to protect the others. The wing-cases are of little or no use in flight, this action being accomplished by means of the second pair, or the true wings, which are generally of large size, and when not in use are neatly folded up beneath the elytra. The division of the body into three parts—head, thorax, and abdomen—is very plain to the eye; but the segments, of which the several parts are composed, are frequently so consolidated that it is difficult to detect or count them. For instance, the thorax, theoretically, consists of three segments; but, practically, the first of these is usually so largely developed as to appear to constitute the thorax in itself. The (theoretical) nine segments of the abdomen are, in like manner, reduced to six or seven, in consequence of the last two or three being consolidated into one.