2 THE INSECT WOELD. 



having to encounter hard substances, must be strongly supported. 

 The exception to this rule is among insects which live by suction. 

 It would be out of place here to mention the numerous modifi- 

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

 the class of insects. 



The eyes of insects are of two kinds, called compound eyes, or 

 eyes composed of many lenses, united by 

 their margins and forming hexagonal 

 facettes ; and simple eyes, or ocelli, called 

 also stemmata. 



The exterior of the eye is called the 



Fig. 2.-A Compound C0rn6a (*%' 2 )> 6ach faC6tte ^g & 



cornea, cornea ; but the facettes unite and form 



a common cornea, which is represented by the entire figure : these 

 facettes vary in size even in the same eye. 



In order to show the immense number of these 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 Libettula (dragon-fly) 12,544 



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

 In Sphinx convolvuli ^ (the convolvulus ha wkmoth) . . 1,300 

 In Bombyx mori (the common silkworm 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 

 scarcely be detected with a glass. 



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

 agglomeration of simple eyes ; but internally its structure is 

 altogether different. 



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

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

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

 around it a ring to receive the pigment. 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 



