44:6 Dynamic Theory, 



oil globules and it is interesting to note that they are of a light yellow 

 color. ' ' Since yellow disappears in the twilight last of all colors it is 

 probable that these globules exist for the purpose of collecting or 

 intensifying the last traces of this color for the benefit of the owl on its 

 nocturnal expeditions." (Finck.) The existence of these yellow 

 globules in the owl after those of all other colors have disappeared 

 through disuse, is strongly confirmatory of the theory advanced in the 

 last chapter regarding color differentiation that it begins with yellow. 

 One of the most remarkable and highly developed eyes as to color is 

 that of the reptile Chameleon. Instead of having a spot like our yel- 

 low spot covered with color seeing cones, the entire retina of the 

 Chameleon is covered with such cones, and as they are extremely slender 

 they are far more numerous than ours in proportion to the space oc- 

 cupied. Add to this that they are much longer, and we have all the 

 conditions that are required for excellence. This remarkable e} 7 e is 

 intimately connected with that peculiar characteristic of the Chameleon 

 by which its color undergoes rapid changes. 



FIG. 178. Section of the eye or 

 Ocellus of a Beetle Larva. 



CL Corneal Lens. 



Gk. Hypodermis Cells ; the equiv- 

 alent f the Vitreous humor. 



P. Pigment in the peripheral 

 cells of the latter 



Rz. Retinal cells. 



St. Cuticularrods of the latter. 



Opt. Optic Nerve. 



(Grenacher & Glaus.) 



The eyes of insects are of 

 two kinds. .The first is the 

 Ocellus or Stemma. It con- 

 sists of a group of pigment 

 cells under the skin, and con- 

 nected inside with a nerve- 

 filament. This simple form 

 is found in the larva of the 

 Bot-fly and others. The outside skin over this spot is transparent and 

 convex, and calls to mind the cornea of mammal eyes. The ocellus 

 exists in a higher degree of development in many insect tribes. The 

 cornea becomes very convex and smooth, and under it is a spherical 

 crystalline lens and beneath that, a body of vitreous humor. This ad- 

 vanced ocellus is the only sort of eye possessed by the spider and Myria- 

 pod tribes, with a small exception. And they are the only eyes of the 

 larvae of many of the six-footed insects. The other sort of e}^e is the 

 compound eye. These consist of a great number of simple eyes set 

 side and side ; and by their crowding together their corneas or facets 

 take the form of a hexagon generally, but sometimes a square. These 

 facets are equivalent to lenses having convex faces, especially in the rear. 



178 



