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 facettes, ordinarily hexangular. ach 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’. 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 facettes 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 Eingeweide bei den Insecten, Nov. Act. Acad. Ces. Leop. Car. Tom. 
xiv. P. 1, 1828, pp. 71—108, Tab. vil. 1x., and J. F,. Branpt, Bemerkungen ueber die 
Mundmagen- oder Eingeweidnerven der LEvertebraten, Mém. de VAcad. des Se. de 
St. Pétersb. (v1. Série, Tom. m1. 2, Sctences nat.) published separately, Leipzig, 1835, 
4to, with 11. plates; also in French in the Ann. des Sc. nat. 2e Série, Tom. v. 1836, 
Zool. pp. 81, &e. and 138. 
1 Wit 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 times 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. Lrscheinungen u. Gesetze des organ. Lebens 11. 1, Bremen, 1832, 8. 77. 
