Visual Organs of Insects and Crustacea. 229 



Cones deprived of the cornea, and separated from the filaments 

 of the optic nerve. The pigment is in part removed, but frag- 

 ments are still seen in the spaces between the pointed extremi- 

 ties, b shows some single cones, either with some pigment 

 still adhering, or altogether deprived of it. In this latter 

 state they are nearly as transparent as glass, c represents 

 some cones deprived of their pigment, but with portions of 

 nervous filaments still attached to them, d \s o. group of 

 cones slightly connected together by means of the pigment. 



When the cornea is carefully removed from the surface of 

 the cones, this latter exhibits the appearance of a network 

 of rounded and whitish spaces, the boundaries being formed 

 l)y the pigment penetrating between the sides of the cones. 

 . The Filaments of the Optic Nerve. If, together with the 

 cornea, we remove all the crystalline or vitreous cones, by 

 detaching them at their junction with the nervous filaments, 

 the extremities of these filaments will become visible in the 

 mass of investing pigment; the whole appearing under the 

 form of a convex surface, generally concentric with the con- 

 vexity of the eye. On account of the very small size of these 

 filaments, they will appear like minute white points regularly 

 distributed through a dark-colom-ed velvety mass. 



The filaments extend from the bulb of the nerve in a 

 radiating direction, preserving throughout their whole length 

 the same thickness. In their course they traverse the deeper 

 pigment, and are united to the corresponding points of the 

 transparent cones ; so that the number of filaments, of cones, 

 and of facets, is always the same. The length of the fila- 

 ments, in relation to that of the cones, varies considerably. 

 In the ^Sphinges, the filaments are nearly four times as long 

 as the cones. The soft texture of these prolongations of the 

 optic nerve prevents our being able to divest them completely 

 of their investing pigment. 



The Pigment of the Eyes. The interstices of the cones 

 and of the nervous filaments are filled with pigment and 

 extremely minute tracheal branches, which latter in general 

 communicate with a larger trachea forming a circle round the 

 eye. This tissue of tracheae and pigment may be designated 

 as the choroid membrane of the eyes of insects. 



In most insects, an external or superficial and an internal 

 pigment are usually described ; they are, however, in all 

 cases, continuous with each other. The superficial layer or 

 portion gives to the eye its proper colour ; it is often distin- 

 guishable from the deeper portion by a clearer, more vivid, 

 and varying hue. It is red in the species of O'scinis ; clear 

 yellow in the ^pis retiisa ; bluish orange, brown, red brown, 



