Fig. 385.—A tangential or side section of Eye of Fly, with palp or pads protruded.
We will take for examination a typical member of Muscidæ, a family embracing a large and varied assortment of species, among which the house-fly and the blow-fly are the best known forms. Musca domestica needs no description. An interesting part of the house-fly to the microscopist is the wonderful component parts of the head. On examination we find a couple of protuberances, more or less prominent, and situated symmetrically one on each side. Their outline at the base is for the most part oval, elliptical, circular, or truncated; while their curved surfaces are spherical, spheroidal, or pyriform. These horny, round, and naked parts are the corneæ of the compound eye of the fly, and they are appropriately so termed, from the analogy they bear to the larger transparent tunics in the higher classes of animals. They differ, however, from the latter, as when viewed by the microscope they display a large number of hexagonal facets, which constitute the medium for the admission of light to several hundred simple eyes. Under an ordinary lens, and by reflected light, the entire surface of one cornea presents a beautiful reticulation, like very fine wire gauze, with minute papilla, or at least a slight elevation, in the centre of each mesh. These are resolved, however, by the aid of a compound microscope, and with a power of from 80 to 100 diameters, into an almost incredible number (when compared with the space they occupy) of minute, regular, geometrical hexagons, well defined, and capable of being computed with tolerable ease, their exceeding minuteness being taken into consideration.
[Fig. 386] represents a vertical section of the eye, showing the hexagonal faceted arrangement of cylindrical tubes.
Fig. 386.—Section of Eye of Fly.
l. Lenses; co. Cones; pl. Pigment layer, consisting of rings round the rods; r.r. Rods; a.v1. Air vessels between the rods; m1. Membrane on which the rods and air vessels rest; a.v2. Shorter lengths of air vessels which form a layer above the first nerve junction; n.j1. First nerve junction; m2. Membrane on which it stands; A. V., A. V. Large air vessel surrounding the eye; n.j2. Second nerve junction; a.v3. Air vessels; op. n. Optic nerve; b.n. Brain substance. (Magnified × 160.)
In this section it appears to be questionable whether the normal shape of the lenses is not round, assuming the hexagonal shape during the process of growth in consequence of their agglomeration. The corneal surface can be peeled off, and if carefully flattened out and mounted it will be seen that each lens is not a simple lens, but a double-convex compound one, composed of two plano-convex lenses of different densities or refracting power joined together.
Experiments made on the eyes of insects, and also of crustaceæ, show that in the insect a real and reversed image of external bodies is formed in each ommatidium; it coincides with the internal face of the crystalline cone in immediate contact with the retina. Although small, the retinal image is distinct and subtends an angle of nearly forty-five. In the same way in the crustacean, the crystalline lens forms on the retinula a reversed image, but the refractive media have a longer focus, and the retinal membrane is not connected with the lens, the interval being filled up by a substance analogous to the vitreous of vertebrates. In both cases it would appear that light does not act directly on the rods; these latter can only receive impressions through the intermediary retinal cells. The retinal images of arthropods, as might have been surmised, are much less perfect than those of the higher orders; on the other hand, their eyes seem to be better adapted for seeing objects in relief and the movements of bodies. The shyness of butterflies and moths is certainly an inherited instinct as a protection against danger from their many enemies.
