188 
ANATOMY AND PHYSIOLOGY 
clearly seen closely, and more indistinctly at a dis- 
tance. Now, if eacli facet of tlie 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 pro- 
portion to the size of the segment of the circle it 
forms, and to the convexity of its arch. In accord- 
ance with this view, w 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 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 arc almost invariably two in num- 
ber ; frequently, however, they are partially or even 
completely divided by the antennae being inserted 
in the middle, and in such cases four appear to be 
present. Whirlwig beetles ( Gi/rinus), and certain 
Ephemerae 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 
