mid smooth or simple IHyes of Insects. 2S8 



Thus, as we have already observed that this genus presented 

 neither choroid nor circular trachea, might we not presume that 

 the excess of the rays of light which arrive on the optic nerves, 

 not beina; absorbed Ijy an opaque membrane like the choroid, 

 tnav injure vision considerably, by increasing too forcibly the 

 excitability of the optic nerves ? These insects are as it were 

 rendered stupid by the excess of light : for a contrary reason, a 

 less brisk light not increasing their excitability too much, ought 

 it not to be more favourable for enabling them to perceive ob- 

 jects in a distinct manner ? We might also add, that the nervous 

 filaments which attach below the facets of the cornea, are here, 

 very short, so that thev are nearer the optic CMpansion to 

 which sensation is transmitted: having less intermediate between 

 them and the general retina, this impression may be greater. 

 Their sensibility being besides acuter, they require less light to 

 paint in a clear manner the images on the retina, images which 

 are soon afterwards percei\ed by the brain, on account of the 

 proximity of this organ to the expansion of the optic nerve. 

 We must also remark, that it is in the eyes of the lucifugce alone 

 that the image can be produced, since the impression made by 

 the rays of light on the optic filaments has not an opaque 

 membrane to pass through to be transmitted to the brain. 



As to the black point whicii is most frequently observed at the 

 exterior of the compound eyes, a point which seems moveable, 

 and situated in the interior of the eye, so that it should have 

 some relation with the bull of the eve, it seems to lje produced 

 by the point of the choroid corresponding to the small circle 

 which gives a passage to the optic nerve, since in this part, the 

 furthest removed from the eve, the deeper violet-coloured tunic 

 ought to produce this point iicarly of the breadth of the circle^ 

 It is clear that this point is in proportion to the size of th& 

 circular aperture througli which the optic nerve passes. \Ve 

 see it larger in all the sjiecies in which this aperture is formed 

 by the adductor muscle of the maudibule, as in the Ivciista and 

 llbellula. It is, on the contrary, less considerable in those which 

 have formed it by a circular trachea, like the gryllus, the truxuViSy 

 and the f^-eater number of the diplerce, in which this aperture i% 

 still smaller. The species which have their tunic entirely black, 

 like moat of the coleoptcrce and the hymenopterce, as well as 

 the ac/ttia, &c. do not appear to have any ; for it cannot be seen, 

 on account of the colour of the tmiic. It is, on the contrary, 

 more visible in the species in which this tunic is of a clear colour. 

 In short, the optic nerve does not oppose this arrangement ; 

 for the choroid covers all the internal parts of the eye, except m 

 the points where it is traverved by the nervous filaments. As 

 to the movement v.hich we observe in it, it iv ep.sy to convince 



ouf'^elves 



