334 INSECTA. 



a very large scale, in order to show the construction of the hexagonal 

 ocelli that enter into its composition. Each cornea (?') is a double con- 

 vex lens, adapted by its shape to bring to a focus the rays passing 

 through it. Behind every lens so constituted is placed a hexahedral 

 transparent prism (Ji), which from its office may be compared to the 

 vitreous humour of the human eye ; and it is upon the posterior extre- 

 mity of these prisms that the proper optic nerves (fig. 166, A, e) spread 

 themselves out, so as to form so many distinct retinae. When we 

 reflect upon the extreme minuteness of the parts above alluded to, we 

 may well expect slight discrepancies to occur between the accounts 

 given of them by different anatomists. Straus-Durckheim represents 

 every optic nerve as terminating in a minute pyriform bulb (fig. 166, 

 B, /), and points out a dark layer of pigment (g), which forms a choroid 

 tunic proper to each ocellus ; while, according to Miiller and Duges, the 

 vitreous humours (h) are conical, and terminate posteriorly in a sharp 

 point, upon which the terminal expansion of the optic nerve spreads out, 

 without any pyriform enlargement ; they likewise deny the existence of 

 the proper choroid (</) in the situation indicated by Straus-Durckheim, 

 but find a black pigment situated immediately behind the cornea, that at 

 first sight would appear to be continuous over the whole surface of the 

 eye. Even Cuvier seems at one time to have adopted this opinion. Miiller, 

 however, found that, upon carefully removing the internal structures of 

 the organ, leaving the pigment untouched, the dark varnish in question, 

 although very thick at the lines of union of the different facets, where 

 it is continuous with a choroid that separates the individual ocelli, yet 

 towards the centre of each facet becomes exceedingly thin, and at the 

 very centre is quite wanting, so that a minute perforation or pupil is 

 thus left, through which the rays of light enter. The existence of the 

 secondary optic nerves (b) and common retina (c) is likewise disputed 

 by Miiller and Duges, who consider the proper optic nerves to arise 

 immediately from the surface of the brain. 



(874.) With regard to the wonderfully complex structure of these 

 organs, Straus-Durckheim suggests that, the eyes of insects being 

 fixed, nature has made up for their want of mobility by their number, 

 and by turning them in all directions ; so that it might be said that these 

 little animals have a distinct eye for every object. But here we are 

 naturally tempted to inquire whether insects see at the same time 

 distinctly with every one of these eyes, or if they distinguish with one 

 eye only. Upon this point Straus-Durckheim observes that, if they 

 saw clearly with all, the great number of images would necessarily pro- 

 duce confusion, and would prevent creatures so organized from paying 

 special attention to any determinate point. It is probable, therefore, 

 that one ocellus only is at any given time placed in circumstances pre- 

 cisely adapted to the complete examination of an object, the animal 

 seeing things imperfectly with the rest, in the same manner as we see 



