2 THE INSECT WORLD. 



cations of the head which are presented in the immense class of 

 insects. 



The eyes of insects are of two kinds. There are compound eyes, 

 or eyes composed of many lenses, united 

 by their margins and forming hexagonal 

 facettes ; and there are also simple eyes, 

 or ocelli. 



The exterior of the eye is called the 

 cornea (Fig. 2), each facette being a cornea ; 

 and the facettes, which vary in size even 

 in the same eye, unite and form a common 

 r, g .2.-A compound Cornea. cornea> wh i ch is represented by the entire 



figure. 



In order to show the immense number of the facettes possessed 

 by many insects, we give the following list : 



In the genus Mordella (a genus of beetles) the eye has 25,008 facettes. 



In the Libellula (dragon-fly) 12,544 



In the genus Papilio (a genus of butterflies) . . 17,355 



In Sphinx convolvuli (the convolvulus hawk -moth) . 1,300 



In Bftnbyx mori (the common silkwonn moth) . 6,236 



In the house-fly 4,000 



In the ant 50 



In the cockchafer 8,820 



The facettes appear to be most numerous in insects of the genus 

 Scarabceus (a genus of beetles). They are so minute, that they can 

 only be detected with a magnifying glass. 



Looked at in front, a compound eye may be considered an 

 agglomeration of simple eyes ; but internally this is hardly correct. 



On the under side of each facette we find a body of a gelatinous 

 appearance, transparent, and usually conical; the base of this 

 occupies the centre of the facette in such a manner as to leave 

 around it a ring to receive some colouring matter. This body 

 diminishes in thickness towards its other extremity, and terminates in 

 a point where it joins a nervous filament proceeding from the optic 

 nerve. These cones, agreeing in number with the facettes, play the 

 part of the crystalline lens in the eyes of animals. They are 

 straight and parallel with each other. A pigment fills all the spaces 

 between the cones, and between the nervous filaments, and covers 

 the under side of each cornea, except at the centre. This pigment 

 varies much in colour. There are almost always two layers, of 

 which the exterior one is the more brilliant. In fact, these eyes 

 often sparkle with fire, like precious stones. 



