OF INSECTS. 
157 
fig. 6*,) which appear when open, like a membrane 
torn longitudinally not far from one of the sides, 
leaving jagged and deeply divided edges, beset with 
pencils of hair. In other cases the aperture is filled 
by a cellular membrane drilled with small holes for 
the passage of the air, as in the common cockchaffer, 
(PI. III. fig. 7). In the larva of the water-beetle 
mentioned a few lines above, a circular membrane 
is stretched over the valves, ornamented with con- 
centric zones of different colours, and having a cir- 
cular aperture in the centre, (PI. III. fig. 8.) Other 
peculiarities presented by these pneumatic orifices are 
to he found in species belonging to the different orders. 
Many aquatic insects have a special provision for 
introducing the air more readily into their system, 
which is rendered necessary by the difficulty they 
would experience in bringing their spiracles into con- 
tact with it. This generally consists of an elongated 
anal tube, sometimes naked at the extremity, but 
more frequently surrounded with hairs or branched 
rays, often exhibiting a very beautiful radiated ap- 
pearance. It is pushed upwards to the surface 
while the body of the insect continues submerged, 
and admits the air at its extremity, an operation 
much facilitated by the rays alluded to, which have 
the power of repelling water, and diverge on all sides, 
thus leaving a free space for the influx of the air. 
Appendages of this nature are chiefly found among 
the Diptera, and will, therefore, be further noticed 
in describing that order. A common species of 
another order, Nepa cinerea , possesses two respir- 
