130 THE WATER CABINET. 
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 repre- 
sented 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 
enaw 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 repre- 
sented in the cut. 
CHAPTERS A, 
COLEOPTERA. 
Tue 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 seg- 
