OF INSECTS. 17 



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, (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 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 

 Aiuch 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 

 au^er order, Nepa cinerea, possesses two respir- 



