INSECTS: THEIB EABS AND EYES. 169 



can present to the sensorimn but the idea of a single, 

 undistorted, unconfused object, probably on somewhat of 

 the same principle by which the convergence of the rays 

 of light entering our two eyes gives us but a single stereo- 

 scopic picture. 



The soft blue colour of this Dragon-fly's eyes, as also 

 the rich golden reflections seen on the eyes of other 

 insects, as the Whame-flies, and many other Diptera, 

 is not produced by the pigment which I have alluded to, 

 but is a prismatic reflection from the cornea. 



You would suppose that, having 24,000 eyes, the 

 Dragon-fly was pretty well furnished with organs of vision, 

 and surely would need no more ; but you would be 

 mistaken. It has three other eyes of quite another 

 character. 



If you look at the commissure or line of junction of the 

 two compound eyes on the summit of the head, you will 

 see, just in front of the point where they separate and 

 their front outlines diverge, a minute crescent-shaped 

 cushion of a pale-green colour, at each angle of which is 

 a minute antenna. Close to the base of each antenna 

 there is set in the black skin of the head that divides the 

 green crescent from the compound eyes, a globose, 

 polished knob of crystal-like substance, much like the 

 " bull's eyes " that are set in a ship's deck to enlighten 

 the side-cabins. On the front side of the cushion there 

 is a third similar glassy sphere, but much larger than the 

 two lateral ones. What are these ? 



They are eyes, in no important respect differing from 

 the individuals which compose the compound masses, 

 except that they are isolated. The shining glassy sphere 

 is a cornea of hard transparent substance, behind which 

 is situated a spherical lens, lodged in a kind of cup formed 

 by an expansion of the optic nerve, and which is sur- 

 rounded by a coloured pigment-layer. 



You may study these simple eyes, or stemmata as they 



