198 EVENINGS AT THE MICROSCOPE 



the rich golden reflections seen on the eyes of other in- 

 sects, as the Whameflies, and many other Diptera is not 

 produced by the pigment which I have alluded to, but is 

 a prismatic reflection from the cornece. 



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

 Dragon-fly was pretty well furnished with organs of vis- 

 ion and surely would need no more; but you would be 

 mistaken. It has three other eyes of quite another char- 

 acter. 



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 cush- 

 ion of a pale-green color at each angle of which is a mi- 

 nute 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" or 

 hemispheres of solid glass that are set in a ship's deck to 

 enlighten the side-cabins. On the front side of the cres- 

 centic cushion there is a third similar glassy sphere, but 

 much larger than the two lateral ones. What are these 

 three spherules? 



They are eyes, in no important respect differing from 

 the individuals which compose the compound masses, ex- 

 cept that they are isolated. The shining glassy hemisphere 

 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 colored pigment-layer. 



You may study these simple eyes, or stemmata, as they 

 are called, in many other insects, though they are not so 



