INSECTS. 279 



Of the organs of sense of Insects the eyes are best known. 

 We have already spoken above (p. 249) of the distinction between 

 simple and compound eyes. The simple eyes have a crystalline 

 lens and a vitreous humour. The cornea, on which the crystalline 

 lens lies, without being separated from it by an aqueous humour, is 

 formed by the common horny integument of the body, which at 

 that part is raised convexly and is more transparent. The vitreous 

 humour is surrounded by a black pigment of the choroid. The 

 compound eyes, always two in number, present a cornea which is 

 divided into many fa9ettes, ordinarily hexangular. Each of these 

 divisions has the form of a small, usually biconvex lens. Behind 

 them lie an equal number of transparent pyramids or conical 

 bodies which are turned by their base to the cornea and by their 

 apices approach each other inwards 1 . Lastly, there is a nerve at 

 the apex of every cone; the optic nerve in fact divides into as 

 large a number of branches as there are divisions of the cornea. 

 A dark-coloured pigment, often violet or blackish brown, separates 

 the nervous fibres and the transparent cones, especially at their 

 pointed extremities, from each other. At the base of the cones, 

 beneath the cornea, there is frequently a pigment of a different and 

 more lively colour ; hence arises the metallic splendour of the eyes 

 in some Insects, as in Hemerobius and Chrysops^ which however 

 disappears after death. No eyelids are present in Insects, but 

 between the fa9ettes of the cornea there are found in certain 

 Hymenoptera and Lepidoptera, here and there, some hairs, which 

 ward off substances from the eyes and defend them. Surrounding 



Nervensystem der Einyeweide bci den Insecten, Nov. Act. A cad. Cces. Leop. Car. Tom. 

 xiv. P. T, 1828, pp. 71108, Tab. vn. ix., and J. F. BRANDT, BemerTcungen ueber die 

 Mundmagen- oder Eingeweidnerven der Evertebraten, Mem. de I'Acad. des Sc. de 

 St. Petersb. (vi. Serie, Tom. in. 2, Sciences not.} published separately, Leipzig, 1835, 

 4to, with in. plates ; also in French in the Ann. des Sc. nat. 2e SeYie, Tom. v. 1836, 

 Zool. pp. 8 1, &c. and 138. 



1 WILL considers these cones, which MUELLER compares to the vitreous humour, 

 for the most part as crystalline lenses, and supposes that behind them there is still a 

 vitreous body with concave anterior surface to be found. In Sphinx Atropos, where 

 these cones are very large, (I found them one-seventeenth Par. lin. long,) I have 

 several tunes observed the separation pointed out by WILL at the posterior extremity 

 of the cone. In other insects the cones are so short, that the separation, even if it be 

 present, cannot well be perceived, whilst even on that account TREVIRANUS thought 

 there was reason to suppose that in some insects the cones in question were absent in 

 their compound eyes. Erscheinungen u. Gesetze des organ. Lebens II. i, Bremen, 1832, s. 77. 



