These facetted eyes, which are found in both insects and crustacea, have apparently a more complex structure than the ocelli. Externally—in the bee, for example—the surface is seen to be divided up into a great number of hexagonal areas, each of which is called a facet, and forms (in some insects, but not in all) a little lens. Of these the queen bee has on each side nearly five thousand; the worker some six thousand; and the drone upwards of twelve thousand; while a dragon-fly (Æschna) is stated to have twenty thousand. Beneath each facet (in transverse section, Fig. 37) is a crystalline cone, its base applied to the lens, its apex embraced by a group of elongated cells, in the midst of which is a nerve-rod which is stated to be in direct connection with the fibres of the optic nerve. Dark pigment is developed around the crystalline cones. And retinal purple is said to be present in the cells which underlie it.
With regard to these facetted eyes there has been much discussion. The question is—Is each facetted organ an eye, or is it an aggregate of eyes? To this question the older naturalists answered confidently—An aggregate. A simple experiment seems to warrant this conclusion. If the facetted surface be cleared of its internal structures (the crystalline cones, etc.) and placed under the microscope, each lens may, at a suitable distance of the object-glass, be made to give a separate image of such an object as a candle reflected in the mirror of the microscope. If each lens thus gives an image, is not each the focussing apparatus of a single eye? But a somewhat more difficult experiment points in another direction. If the facetted cornea be removed with the crystalline cones still attached (Grenacher was able to do it with a moth's eye), and placed under the microscope, when the instrument is focussed at the point of the cone (where the nerve-rod comes), a spot of light, and not an image, is seen. No image can be seen unless the microscope be focussed for the centre of the cone; and here there are no structures capable of receiving it and transmitting corresponding waves of change to the "brain."
Fig. 37.—Eye of fly.
Transverse section through head. (After Hickson.)
But what, it may be asked, can be the purpose of an eye-structure which gives, not an image, but merely a spot of light? The answer to this question can only be found when it is remembered that there are thousands of these facets and cones giving thousands of spots of light. The somewhat divergent cones and facets of the insect's eye ([Fig. 37]) embrace, as a whole, an extended field of vision; each has its special point in that field; and each conveys to the nerve-rod which lies beneath it a stimulation in accordance with the brightness, or intensity, or quality of that special point of the field to which it is directed. The external field of vision is thus reproduced in miniature mosaic at the points of the crystalline cones—thus there is produced by the juxtaposition of contiguous points a stippled image. And it must be remembered that, even in human vision, the stimulation is not that of a continuum, but is stippled with the fine stippling of the ends of the rods and cones. In insect-vision the stippling is far coarser, and the image is produced on different principles.
Fig. 38.—Diagram of mosaic vision.]
In the vertebrate the image is produced by a lens; in the insect's eye, by the elongated cones. How this is effected will be readily seen with the aid of the diagram. At a b are a number of transparent rods, separated by pigmented material absorbent of light. They represent the crystalline cones. At c d is an arrow placed in front of them; at e f is a screen placed behind them. Rays of light start in all directions from any point, c, of the arrow; but of these only that which passes straight down one of the transparent rods reaches the screen. Those which pass obliquely into other rods are absorbed by the pigmented material. Similarly with rays starting from any other point of the arrow. Only those which, in each case, pass straight down one of the rods reach the screen. Thus there is produced a reduced stippled image, c' d', of the arrow.
There has been a good deal of discussion as to the relative functions of the ocelli and the facetted eyes of insects. The view generally held is that the ocelli are specially useful in dark places and for near vision; while the facetted eyes are for more distant sight and for the ascertainment of space-relations. How the two sets of impressions are correlated and co-ordinated in insect-consciousness, who can say?[FJ]