104 Insect Vision and Insect Sleep. 



the outer covering of the eye, is a layer of colour. It covers the 

 whole of the inner surface of the corneules, excepting only in 

 the centre of each where a minute aperture is seen, admitting 

 light by the iris. Between the iris and the end of the cornea 

 is a space, flattened and convex, filled with an aqueous humour. 

 Each convex lens corresponds with each facet. The rays of 

 light passing through them fall upon a transparent space occu- 

 pied by a vitreous humour. The choroid in the eyes of insects, 

 like the choroid in the vertebrata, is the proper vascular struc- ' 

 ture of the organ of vision. The pigment of the choroid is 

 subject to much variety of colour in different insects. In some 

 it is nearly black, in others dark blue, violet, green, purple, 

 brown, and yellow, and in some, two or three layers of pigment 

 are of different colours. The usual arrangement of these varie- 

 gated pigments is, first, a dark-coloured portion near the bulb 

 of the optic nerve, then a lighter colour, and lastly, again, a 

 darker near the cornea. 



Puget adjusted the eye of a flea (Pulex irritans) in such a 

 way as to see objects through it. On applying the microscope 

 to the multitude of mirrors, nothing could exceed the singularity 

 of what was seen. " A soldier appeared like an army of pig- 

 mies ; for what it multiplied it diminished ; the arch of a bridge 

 exhibited a spectacle more magnificent than an edifice erected 

 by human skill ; and the flame of a candle seemed the illumina- 

 tion of a thousand lamps." The minute regularity of the 

 objects in each of the facets, so disposed as to converge to a 

 central ganglion, make but a single picture in perception. The 

 great optic nerve uniting into a focal point the coincidence of 

 what Dr. Wells designates " the visual direction," impresses an 

 image intensely concentrated. The perception of each impres- 

 sion being confined to that of the object immediately in a line 

 with the axis of vision, the impacted lights and shadows of a 

 thousand representations of one and the same form — the visual 

 product of a thousand facets — give a stereoscopic representa- 

 tion under a thousand adjustments, and render the small organ 

 of the small animal, in power and concentration, a microscope. 



The successive zones in the insect eye modify the rays that 

 penetrate the sight, passing by each facet, and by the centre of 

 each converging cylinder radiating to the optic ganglion. The 

 layer of pigment does nothing but diminish the quantity of 

 light, and adjust it. It is found in most if not all diurnal 

 insects, and the iris being perforated with as many holes as 

 there are facets in the cornea, it is subjected to multiplied 

 modifications. As might be expected, this pigment is not met 

 with in any of the nocturnal insects. 



Insects that fly require an ample field of vision. The com- 

 bined corneules become one large pupil. The multiplied facets 



