clearly seen closely, and more indistinctly at a distance. Now, if each facet of the eye can survey but one small portion of the field of vision, yet will the entire eye be able to survey a field the larger in proportion to the size of the segment of the circle it forms, and to the convexity of its arch. In accordance with this view, we find the eyes of predacious insects, and such as require powerful vision, prominent and globular; and the reverse is witnessed in parasitical species, and others to which the perfection of this sense is less essential. Each separate cone or lens, must be understood as conveying the impression it has received from a ray of light to the nervous filament with which it is supplied; and as the latter are all united in the great optic nerve, a common and distinct image is ultimately produced.
Compound eyes are almost invariably two in number; frequently, however, they are partially or even completely divided by the antennæ being inserted in the middle, and in such cases four appear to be present. Whirlwig beetles (Gyrinus), and certain Ephemeræ may be said to have an upper and under pair. Their degree of projection from the head generally depends merely on their greater or less convexity, but in not a few instances they are placed on pretty long footstalks. The size of the eyes is often a mark of sexual distinction, as we shall have occasion to mention more particularly in the general history of the order Diptera.
Reproductive or Generative System.—Among insects the sexes are always distinct, and the organs
