EXTERNAL STRUCTURE OF INSECTS. 25 



The organs principally to be noticed in the head are 

 the eyes, the antennae or " horns," and the mouth. 



The eyes of insects are of two kinds, simple and com- 

 pound. The reader must often have observed the large 

 convex brown eyes of the common house-fly, and those 

 lustrous bodies which form so conspicuous a feature in the 

 dragonfly, and the glorious golden or ruby eyes of the 

 delicate lace-fly. He may also have observed that these 

 eyes are immovable, and consequently, the motion of 

 the head being very limited, would be of little use either 

 in the avoidance of danger or in the pursuit of prey, 

 were they constructed like our own and able to see in 

 one direction only. That the vision of the fly is not so 

 limited will be amply proved by a few attempts to " get 

 on the blind side " of one. Approach him from above, 

 from below, from before, from behind, from either 

 side, or from round the corner, he perceives and avoids 

 the danger. How is this ? The large eye which we 

 observe on either side of the head is in fact a cluster of 

 eyes, or, to speak more properly, is a compound eye. 

 Looking closely at this eye in one of the large insects, 

 the reader will observe the surface marked out into 

 hexagons (fig. 1). Each of these is the 

 surface of a true eye, and the hexagonal 

 form is such as a number of cylindrical 

 or conical eyes would naturally assume 

 if pressed together. It will be seen that 



Small portion of 



such a cluster of eyes, if arranged so as eye highly magni- 

 to form a semi-globular surface, would fied ' 

 be able to see all objects on one side of an insect, whilst 

 the fellow eye could receive impressions from objects on 

 the other side. These eyes then are so arranged. 



It has been said above that the compound eye is 



