186 ANATOMY AND PHYSIOLOGY 
only two, and in a few instances only a single ocellus 
has been detected. Their internal structure renders 
it obvious that they are organs of vision. Muller is 
of opinion that their refraction must be very great, 
the convex cornea, the anterior convex surface of the 
lens, the posterior convex surface of the lens, and, 
finally, the convex surface of the glassy body itself, 
each having the power of refracting a ray of light. 
It is improbable that they are of any use in distant 
vision, but they are fitted to give a distinct view of 
objects close at hand, and are doubtless designed to 
facilitate the insect’s operations in the narrow passages 
tubular flowers, &c., where they are so often em 
ployed. 
When we look at a composite eye, it is found to 
present a reticulated appearance, which is occasioned 
by its being composed of an aggregation of minute 
hexagonal pieces. Each of these pieces is a distinct 
lens, and constitutes in itself a complete organ of 
vision. Their amount has been calculated in a variety 
of species, and in most cases found to be astonishingly 
great : 7000 have been counted in the eye of a com- 
mon fly; 12,544* in a dragonfly; 11,300 in the 
goat-moth ; 17,355 in a butterfly; 25,088 in a species 
of mordella; 1300 in the convolvulus sphinx; 50 in 
an ant,* and about the same number in Xenos. The 
eye consists of various layers ; first of a hard, transpa- 
rent, facetted membrane, which may be regarded as 
* A few species of foreign ants (Gen, Ponera , &c.) are among 
the very few insects with which we are acquainted which 
seem to be entirely destitute of eyes. 
