Page:The Book of the Aquarium and Water Cabinet.djvu/142

130 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 width 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.

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