82 INSECTS, 



work, as in the higher animals, to receive impressions of sight. 

 The structure of these eyes is sufficiently intelligible, but our 

 admiration is greatly excited when we come to consider the large 

 convex organs of compound vision, and find that each of these 

 contains many thousands of eyes, all capable of distinct percep- 

 tion. The microscope reveals to us that the compound eye of an 

 ant contains fifty lenses, that of a fly four thousand, that of a 

 dragon-fly twelve thousand, that of a butterfly seventeen thou- 

 sand, and that of a species of mordella (a kind of beetle) the 



FIG. 73. EVES OF BEE. FIG. 74. COMPOUND EYE OK A DRAGON-FLY. 



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 its own iris and pupil and a per- 

 fect 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 we 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 main trunks which run along each side of the body ; 



