208

EXPERIMENTS WITH HOODWINKED ANTS.

Forel, in some recent experiments, varnished over the eyes of fifteen ants (Camponotus ligniperdus) and put them with fifteen others, which were left in their normal condition, in a flat box with a glass top and divided in the middle into two halves by a cardboard division, which, however, left room enough underneath for the ants to pass freely from one half to the other. After some other experiments, in the course of which one of the varnished ants was accidentally killed, at 1 p.m. all the varnished ants and thirteen of the unvarnished were in the right half of the box, and two unvarnished in the left. He then placed over the whole box two flat bottles containing water to intercept heat-rays-over the right half a piece of cobalt (violet) glass; and over the left, a flat bottle containing a solution of esculine, which is quite transparent, but cuts off the ultra-violet rays. At 1.55 the result was as follows:

Under the esculine.

5 varnished.

13 normal.

Under the cobalt.

9 varnished.
2 normal.

The esculine and cobalt were then transposed. At 2.3 the position was—

Under the cobalt.

4 varnished.

Under the esculine.

3 normal.

13 varnished.

12 normal.

The esculine and cobalt were again transposed, and one normal ant was accidentally wounded and removed. At 3.8

Under the esculine

3 varnished.

11 normal.

Under the cobalt.

12 varnished.

3 normal.

EXPERIMENTS WITH HOODWINKED ANTS. 209

The esculine and cobalt were once more transposed, and at 3.13 there were

Under the cobalt.

3 varnished.

1 normal.

Under the esculine.

11 varnished.

13 normal.

Thus the number of ants which followed the esculine and moved from one half of the box to the other at each transposition of the esculine and cobalt, was as follows:

And the number remaining under the cobalt and esculine respectively was

These experiments clearly showed that, while the normal ants moved from side to side so as to be under the esculine and consequently protected from the ultraviolet rays, those in which the eyes had been varnished remained unaffected by the transposition of the esculine and the cobalt, showing that the difference was perceived, not by the general surface of the skin, but by the eyes, and that when these were covered the ants were unaffected by the change.

P

210 CONFIRMATION OF MY EXPERIMENTS ON ANTS.

It might be suggested that possibly the ants had been injured or stupefied by the varnishing. M. Forel accordingly, on the following day at 8 a.m., placed over one half of the box a layer of water six centimetres deep, and on the other a piece of red glass, which, while intercepting some of the light, allows almost all the heat to pass through. At 9.25 there were—

Under the red glass.

3 varnished.

12 normal.

Under the layer of water.

11 varnished.
2 normal.

Here, it seems that the ants which could see preferred the shade, even though they were rather too warm; while the hoodwinked ants went under the cool water.

This indicated that the varnished ants remained sensitive to heat, though not to light. Indeed, Forel states that they were just as lively, just as sensitive to currents of air, as the normal ants.*

These experiments, then, entirely confirm those I had made. "C'est une confirmation entière," says Forel, "des resultats de Lubbock †" and he sums up as follows:-The ants "paraissent percevoir l'ultra-violet principalement avec leurs yeux, c'est-à-dire qu'elles le voient, car lorsque leurs yeux sont vernis elles s'y montrent presque indifférentes; elles ne réagissent alors nettement qu'à une lumière solaire directe ou moins forte. Les expériences ci-dessus semblent indiquer que les sensations dermatoptiques sont plus faibles chez les fourmis que chez les animaux étudiés par Graber."

From these and other experiments M. Forel comes † Ibid., p. 174.

*Loc. cit., p. 167.

EXPERIMENTS WITH DAPHNIAS.

211

to the same conclusion as I did, that the ants perceive the ultra-violet rays with their eyes, and not as suggested by Graber, by the skin generally. It is very gratifying that my experiments and conclusions should thus be entirely confirmed by an observer so careful and so experienced as M. Forel.

Fig. 118.-Daphnia pulex. a, Antennæ; b, brain; e, eye; h, heart; m, muscle of eye; n, nerve of eye; o, ovary; ol, olfactory organ; s, stomach; y, three eggs deposited in the space between the back and the shell.

212

DAPHNIAS AND COLORS.

ing to the genus Daphnia (Fig. 118), from which he concludes that they perceive all the colors known to us, being, however, especially sensitive to the yellow and green, and that their limits of vision are the same as ours. Nay, he even goes further than this, and feels justified in concluding, from the experience of two species -Man and Daphnia-that the limits of vision would be the same in all cases.

His words are

1. "Tous les animaux voient les rayons spectraux que nous voyons."

2. "Ils ne voient aucun de ceux que nous ne voyons pas."

3. "Dans l'étendue de la région visible, les différences entre les pouvoirs éclairants des différents rayons colorés sont les mêmes pour eux et pour nous."

He also adds, "Puisque les limites de visibilité semblent être les mêmes pour les animaux et pour nous, ne trouvons-nous pas là une raison de plus pour supposer que le rôle des milieux de l'œil est tout à fait secondaire, et que la visibilité tient à l'impressionnabilité de l'appareil nerveux lui-même ?"

These generalizations would seem to rest on a very narrow foundation. I have already attempted to show that the conclusion does not appear to hold good in the case of ants; and I determined, therefore, to make some experiments myself on Daphnias, the results of which are here embodied.*

Professor Dewar was kind enough to arrange for me, at the Royal Institution, a spectrum, which, by means of a mirror, was thrown on to the floor. I then placed some

* These observations were published in the Journal of the Linnean Society for 1881.