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. M tiller 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 wo are acquainted which 

 seem to be entirely destitute of eyes. 



