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CHAPTER VIII.

ON THE SENSES OF ANTS.

The Sense of Vision.

It is, I think, generally assumed not only that the world really exists as we see it, but that it appears to other animals pretty much as it does to us. A little consideration, however, is sufficient to show that this is very far from being certain, or even probable.

In the case of insects, moreover, the mode of vision is still an enigma. They have, at least many of them have, a large compound eye on each side; and ocelli, generally three in number, situated on the summit of the head. The compound eyes consist of a number of facets, each situated at the summit of a tube, to the base of which runs a fibre of the optic nerve.

The structure of the ocellus and that of the compound eye are essentially different, and it does not seem possible that either the ocellus should be derived from the compound eye, or the compound eye from the ocellus. On the contrary, both seem to point back to a less developed ancestral type. Starting from such an origin, an increase of the separate elements and an improvement of the lens would lead to the ocellus, while

an increase of the number of eyes would bring us to

the compound eye.

On the other hand, it must be admitted that there are reasons for considering the different kinds of eyes to be of perfectly distinct origin. The eye of Limulus, according to Grenacher, is formed on a plan quite unlike that of other Crustacea. Again, the development of the eye in Musca, to judge from Weismann's observations, is very dissimiliar from that of other insects. The varied position of the eye in different groups, as, for instance, in Pecten, Spondylus, Euphausia, Onchidium, &c., point to the same conclusion.

It seems clear that the image produced by the ocelli must be altogether different from the picture given by the compound eyes; and we may therefore reasonably conclude that the two organs have distinct functions. It used formerly to be supposed that the compound eyes were intended for distant, the ocelli for near vision. Claparède, however, has maintained the opposite theory, while Mr. Lowne regards the ocelli as incapable of producing anything worthy the name of an image,' and suspects that their function is the perception of the intensity in the direction of light, rather than vision.'

The ocelli, or simple eyes, probably see in the same manner as ours do. That is to say, the lens throws an image on the back of the eye, which we call the retina. In that case they would see everything really reversed,

as we do; though long practice has given us the right impression. The simple eye of insects thus resembles ours in this respect.

As regards the mode of vision of the compound eyes, there are two distinct theories. According to one-the mosaic theory of Müller-each facet takes in only a small portion of the field; while according to the other, each facet acts as a separate eye.

This latter view has been maintained by many high authorities, but it is difficult to understand how so many images could be combined into one picture. Some insects have more than 20,000 facets on each side of their head. No ants, indeed, have so many, but in some-as, for instance, in the males of Formica pratensis—there are not less than 1,000. The theory, moreover, presents some great anatomical difficulties. Thus, in certain cases there is no lens, and consequently there can be no image; in some it would seem that the image would be formed completely behind the eye, while in others again it would be in front of the receptive surface. Another difficulty is that any true projection of an image would in certain species be precluded by the presence of impenetrable pigment, which only leaves a minute central passage for the light-rays. Again, it is urged that even the sharpest image would be useless, from the absence of a suitably receptive surface; since the structure of the receptive surface corresponding to each facet seems to preclude it from receiving more than a single impression.

The prevailing opinion of entomologists now is that each facet receives the impression of one pencil of rays; so that, in fact, the image formed in a compound eye is a sort of mosaic.

On the other hand, this theory itself presents great difficulties. Those ants which have very few facets must have an extremely imperfect vision. Again, while the image produced on the retina of the ocellus must of course be reversed as in our own eyes; in the compound eyes, on the contrary, the vision would, on this theory, be direct. That the same animal should see some things directly, and others reversed; and yet obtain definite conceptions of the outer world, would certainly be very remarkable.

In fact, these, so far fortunate, insects realise the epigram of Plato

Thou lookest on the stars, my love,

Ah, would that I could be

Yon starry skies, with thousand eyes

That I might look on thee!

But if the male of F. pratensis sees 1,000 queens at once, when only one is really present, this would seem to be a bewildering privilege, and the prevailing opinion among entomologists is, as already mentioned, that each facet only takes in a portion of the object.

But while it is difficult to understand how ants see, it is clear that they do see.

From the observations of Sprengel there could of

course be little, if any, doubt, that bees are capable of distinguishing colours; and I have proved experimentally that this is the case. Under these circumstances, I have been naturally anxious to ascertain, if possible, whether the same holds good with ants. I have, however, found more difficulty in doing so because, as shown in the observations just recorded, ants find their food so much more by smell than by sight.

This being so, I could not apply to ants those tests which had been used in the case of bees. At length, however, it occurred to me that I might utilize the dislike which ants, when in their nests, have to light. Of course they have no such feeling when they are out in search of food; but if light is let in upon their nests, they at once hurry about in search of the darkest corners, and there they all congregate. If, for instance, I uncovered one of my nests and then placed an opaque substance over one portion, the ants invariably collected in the shaded part.

I procured, therefore, four similar strips of glass, coloured respectively green, yellow, red, and blue, or, rather, violet. The yellow was rather paler in shade, and that glass consequently rather more transparent than the green, which, again, was rather more transparent than the red or violet. I also procured some coloured solutions.

Prof. Dewar was kind enough to test my glasses and solutions with reference to their power of trans