Page:Jardine Naturalist's library Entomology.djvu/163

Rh 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, (Pl. 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 concentric zones of different colours, and having a circular aperture in the centre, (Pl. III. fig. 8.) Other peculiarities presented by these pneumatic orifices are to be 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 contact 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 appearance. 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