=Mode of vision by single eyes or ocelli.=—In their simplest condition, the eyes of worms and other of the lower invertebrates, probably only enable those animals to distinguish light from darkness. The ocelli of spiders and of many insects, however, probably enable them, as Lubbock remarks, to see as our eyes do. The simple lens throws on the retina an image, which is perceived by the fine terminations of the optic nerve. The ocelli of different arthropods differ, however, very much in degree of complexity. Müller considered that the power of vision of ocelli “is probably confined to the perception of very near objects.” “This may be inferred,” Müller states, “partly from their existing principally in larvæ and apterous insects, and partly from several observations which I have made relative to the position of these simple eyes. In the genus Empusa the head is so prolonged over the middle inferior eye that, in the locomotion of the animal, the nearest objects can only come within the range. In _Locusta cornuta_, also, the same eye lies beneath the prolongation of the head.... In the Orthoptera generally, also, the simple eyes are, in consequence of the depressed position of the head, directed downwards towards the surface upon which the insects are moving.”[46] Lowne considers that in the ocellus of Eristalis, the great convexity of the lens must give it a very short focus, and the comparatively small number of rods render the picture of even very near objects quite imperfect and practically useless for purposes of vision, and that the function of the ocelli is “the perception of the intensity and the direction of light, rather than of vision, in the ordinary acceptation of the term.” Réaumur, Marcel de Serres, Dugès, and Forel have shown by experiment, that in insects which possess both ocelli and compound eyes, the former may be covered over without materially affecting the movements of the animals, while if the facetted eyes are covered, they act as if in the dark (Lubbock). While Plateau regards the ocelli as of scarcely any use to the insect, and Forel claims that wasps, humble bees, ants, etc., walk or fly almost equally well without as with the aid of their ocelli, Lubbock demurs to this view, and says the same experiments of Forel’s might almost be quoted to prove the same with reference to the compound eyes. Indeed, the writer has observed that in caves, eyeless beetles apparently run about as freely and with as much purpose, as their eyed relatives in the open air. Plateau has recently shown that caterpillars which have ocelli alone are very short-sighted, not seeing objects at a distance beyond one or two centimetres, and it has been fully proved by Plateau and others, that spiders, with their well-formed ocelli, are myopic, and have little power of making out distinctly the shape of the objects they see.