188 ANATOMY AND PHYSIOLOGY 



clearly seen closely, and more indistinctly at a dis- 

 tance. Now, if each facet of the eye can survey bul 

 one small portion of the field of vision, yet will the 

 entire eye be able to survey a field the larger in pro- 

 portion to the size of the segment of the circle i 

 forms, and to the convexity of its arch. In accord- 

 ance with this view, v> e find the eyes of predacious 

 insects, and such as require powerful vision, promi- 

 nent 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 impres 

 sion it has received from a ray of light to the nervou 

 filament with which it is supplied ; and as the latte 

 are all united in the great optic nerve, a common am 

 distinct image is ultimately produced. 



Compound eyes are almost invariably two in num 

 ber; frequently, however, they are partially or even 

 completely divided by the antennae being insertec 

 in the middle, and in such cases four appear to be 

 present. Whirl wig beetles (Gyrinus), and certain 

 Ephemerae may be said to have an upper and unde 

 pair. Their degree of projection from the heat 

 generally depends merely on their greater or les 

 convexity, but in not a few instances they are placec 

 on pretty long footstalks. The size of the eyes 

 often a mark of sexual distinction, as we shall hav 

 occasion to mention more particularly in the genera 

 history of the order Diptera. 



REPRODUCTIVE OR GENERATIVE SYSTEM. Among 

 insects the sexes are always distinct, and the organs 





