INSECTS. 109 



an ant contains fifty lenses, that of a fly four thou- 

 sand, that of a dragon-fly 

 twelve thousand, that of a 

 butterfly seventeen thousand, 

 and that of a species of 

 mordella (a kind of beetle), 

 the amazing number of 

 twenty-five thousand. Every 

 one of these regular, polished, 

 and many-sided lenses, is 

 the external surface of a 

 distinct eye, furnished with FIG. n.-coMPouxD *YE OF A 

 its own iris and pupil and a 



perfect nervous apparatus, as may be seen in the 

 appended figure, representing the eye of a dragon-fly 

 cut perpendicularly through the middle. As the eyes 

 of insects are immoveably fixed in the head, it is 

 probable that this great number of lenses and visual 

 tubes is needful to see different objects, some or 

 other of the component eyes being turned towards 

 every point. 



The respiratory system of insects appears to be 

 constructed with a view to insure a perpetual renewal 

 of the vitality of the blood, combined with the utmost 

 lightness, so needful for animals of which the great 

 majority are denizens of the air. Hence we find 

 neither lungs nor gills, but a series of tubes pervading 

 every part of the body, by which the vital oxygen is 

 carried to the blood. If wo examine a beetle, a 

 grasshopper, or a caterpillar, we shall observe a row 

 of oval openings on each side, capable of being closed 

 by thickened lips (Fig. 75). These are the spiracles 

 or breathing apertures, for no insect breathes through 

 the mouth ; they admit the air into a main pipe which 

 runs along each side of the body; these are connected 

 by smaller branch pipes, which run across the rings 

 of the abdomen, and distribute an infinite number of 

 smaller tubes to every part of the interior. In 

 insects of great powers of flight, there are likewise 

 reservoirs of air ; these are particularly large in the 



