The eyes of insects are of two kinds. There are compound eyes, or eyes composed of many lenses, united by their margins and forming hexagonal facettes; and there are also simple eyes, or ocelli.
The exterior of the eye is called the cornea ([Fig. 2]), each facette being a cornea; and the facettes, which vary in size even in the same eye, unite and form a common cornea, which is represented by the entire figure.
In order to show the immense number of the facettes possessed by many insects, we give the following list:—
| In the genus Mordella (a genus of beetles) the eye has | 25,008 | facettes. |
| In the Libellula (dragon-fly) | 12,544 | " |
| In the genus Papilio (a genus of butterflies) | 17,355 | " |
| In Sphinx convolvuli (the convolvulus hawk-moth) | 1,300 | " |
| In Bombyx mori (the common silkworm moth) | 6,236 | " |
| In the house-fly | 4,000 | " |
| In the ant | 50 | " |
| In the cockchafer | 8,820 | " |
The facettes appear to be most numerous in insects of the genus Scarabæus (a genus of beetles). They are so minute, that they can only be detected with a magnifying glass.
Looked at in front, a compound eye may be considered an agglomeration of simple eyes; but internally this is hardly correct.
On the under side of each facette we find a body of a gelatinous appearance, transparent, and usually conical; the base of this occupies the centre of the facette in such a manner as to leave around it a ring to receive some colouring matter. This body diminishes in thickness towards its other extremity, and terminates in a point where it joins a nervous filament proceeding from the optic nerve. These cones, agreeing in number with the facettes, play the part of the crystalline lens in the eyes of animals. They are straight and parallel with each other. A pigment fills all the spaces between the cones, and between the nervous filaments, and covers the under side of each cornea, except at the centre. This pigment varies much in colour. There are almost always two layers, of which the exterior one is the more brilliant. In fact, these eyes often sparkle with fire, like precious stones.
M. Lacordaire, in his "Introduction à l'Entomologie," from which we borrow the greater part of this information, has summed up as follows, the manner in which, according to M. Müller, the visual organs of insects operate:—
"Each facette, with its lens and nervous filament, separated from those surrounding them by the pigment in which they are enclosed, form an isolated apparatus, impenetrable to all rays of light, except those which fall perpendicularly on the centre of the facette, which alone is devoid of pigment. All rays falling obliquely are absorbed by that pigment which surrounds the gelatinous cone. It results partly from this, and partly from the immobility of the eye, that the field of vision of each facette is very limited, and that there are as many objects reflected on the optic filaments as there are corneæ. The extent, then, of the field of vision will be determined, not by the diameter of these last, but by the diameter of the entire eye, and will be in proportion to its size and convexity. But whatever may be the size of the eyes, like their fields of vision, they are independent of each other; there is always a space, greater or less, between them; and the insect cannot see objects in front of this space without turning its head. What a peculiar sensation must result from the multiplicity of images on the optic filaments! This is not more easily explained than that which happens with animals which, having two eyes, see only one image; and probably the same is the case with insects. But these eyes usually look in opposite directions, and should see two images, as in the chameleon, whose eyes move independently of each other. The clearness and length of vision will depend, continues M. Müller, on the diameter of the sphere of which the entire eye forms a segment, on the number and size of the facettes, and the length of the cones or lenses. The larger each facette, taken separately, and the more brilliant the pigment placed between the lenses, the more distinct will be the image of objects at a distance, and the less distinct that of objects near. With the latter the luminous rays diverge considerably; while those from the former are more parallel. In the first case, in traversing the pigment, they impinge obliquely on the crystalline, and consequently confuse the vision; in the second, they fall more perpendicularly on each facette.