490 THE SENSORIAL FUNCTIONS. 



It thus appears that each of the constituent 

 eyes, which compose this vast aggregate, con- 

 sists of a simple tube, furnished with all the ele- 

 ments requisite for distinct vision, and capable 

 of receiving impressions from objects situated in 

 the direction of the axis of the tube. The rays 

 traversing adjacent corneules are prevented from 

 mixing themselves with those which are proper 

 to each tube by the interposition of the black 

 pigment, which completely surrounds the trans- 

 parent cylinders, and intercepts all lateral or 

 scattered light. Thus has nature supplied the 

 want of mobility in the eyes of insects, by the 



quently the vitreous humour (v), instead of forming an elon- 

 gated cylinder, has the shape of a short cone, terminating in a 

 fine point, as shown in Fig. 426. Straus Durckheim appears to 

 have mistaken this part for an enlarged termination of the optic 

 nerve ; believing it to be opaque, and to form a retina applied to 

 the back of the corneule, vphich latter part he considered as pro- 

 perly the crystalline lens. In his elaborate vpork on the ana- 

 tomy of the Melolontha, he describes the filaments (r) of the op^ 

 tic nerve, in their progress inwards, as passing through a second 

 membrane (k, Fig. 428), which he denominates the common 

 choroid, and afterwards uniting to form an expanded layer, or 

 more general retina (r), whence proceed a small number of 

 short but thick nervous columns (n), still converging towards the 

 large central ganglion (g), in which they terminate. The use he 

 ascribes to this second choroid is to intercept the light, which, 

 in so diminutive an organ, might otherwise penetrate to the gene- 

 ral retina and produce confusion, or injurious irritation. The 

 colour of the pigment is not always black, but often has a bluish 

 tint : in the common fly, it is of a bright scarlet hue, resembling 

 blood. In nocturnal insects, the transverse layer of pigment 

 between the corneule and the vitreous humour is absent. 



