In 1899 at Faisons, North Carolina, I was fortunate enough to find a temporary nest of the totally blind little Eciton carolinense in a rotten log. I placed the ants in a bag and made them the subject of some observations. The Eciton workers carry their elongate larvæ in their jaws and extending back between their legs in such a position that the antennæ have full play in front.
Their ability to follow one another and to find their way about rapidly and unanimously in new territory without a single ant going astray, is incredible. I threw a handful of Ecitons with their young into a strange garden in Washington, i. e., after a long railway journey and far away from their nest. Without losing a moment’s time, the little animals began to form in files which were fully organised in five minutes. Tapping the ground continually with their antennæ, they took up their larvæ and moved away in order, reconnoitering the territory in all directions. Not a pebble, not a crevice, not a plant was left unnoticed or overlooked. The place best suited for concealing their young was very soon found, whereas most of our European ants under such conditions, i. e., in a completely unknown locality, would probably have consumed at least an hour in accomplishing the same result. The order and dispatch with which such a procession is formed in the midst of a totally strange locality is almost fabulous. I repeated the experiment in two localities, both times with the same result. The antennæ of the Ecitons are highly developed, and it is obvious that their brain is instinctively adapted to such rapid orientation in strange places.
In Colombia, to be sure, I had had opportunities of observing, not the temporary nests, but the predatory expeditions of larger Ecitons (E. Burchelli and hamatum) possessing eyes. But these in no respect surpassed the completely blind E. carolinense in their power of orientation and of keeping together in files. As soon as an ant perceives that she is not being followed, she turns back and follows the others. But the marvellous fact is the certainty of this recognition, the quickness and readiness with which the animals recognise their topochemical trail without hesitation. There is none of the groping about and wandering to and fro exhibited by most of our ants. Our species of Tapinoma and Polyergus alone exhibit a similar but less perfect condition. It is especially interesting, however, to watch the perpetuum mobile of the antennæ of the Ecitons, the lively manner in which these are kept titillating the earth, all objects, and their companions.
All this could never be accomplished by a tactile sense alone. Nor could it be brought about by an olfactory sense which furnished no spatial associations. As soon as an Eciton is deprived of its two antennæ it is utterly lost, like any other ant under the same circumstances. It is absolutely unable to orient itself further or to recognise its companions.
In combination with the powerful development of the cerebrum (corpora pedunculata) the topochemical olfactory sense of the antennæ constitutes the key to ant psychology. Feeling obliged to treat of the latter in the preceding lecture, I found it necessary here to discuss in detail this particular matter which is so often misunderstood.
[In his latest Souvenirs entomologiques (Seventh Series) J. H. Fabre has recorded a number of ingenious experiments showing the ability of the males of Saturnia and Bombyx to find their females at great distances and in concealment. He tried in vain (which was to have been foreseen) to conceal the female by odors which are strong even to our olfactories. The males came notwithstanding. He established the following facts: (1) Even an adverse wind does not prevent the males from finding their way; (2) if the box containing the female is loosely closed, the males come nevertheless; (3) if it is hermetically closed (e. g., with wadding or soldered) they no longer come; (4) the female must have settled for some time on a particular spot before the males come; (5) if the female is then suddenly placed under a wire netting or a bell-jar, though still clearly visible, the males nevertheless do not fly to her, but pass on to the spot where she had previously rested and left her odor; (6) the experiment of cutting off the antennæ proves very little. The males without antennæ do not, of course, come again; but even the other males usually come only once: their lives are too short and too soon exhausted.
At first Fabre did not wish to believe in smell, but he was compelled finally, as a result of his own experiments, to eliminate sight and hearing. Now he makes a bold hypothesis: the olfactory sense of insects has two energies, one (ours), which reacts to dissolved chemical particles, and another which receives “physical odor-waves,” similar to the waves of light and sound. He already foresees how science will provide us with a “radiography of odors” (after the pattern of the Roentgen rays). But his own results, enumerated above under (4) and (5) contradict this view. The great distances from which the Bombyx males can discern their females is a proof to him that this cannot be due to dissolved chemical particles. And these same animals smell the female only after a certain time and smell the spot where she had rested, instead of the female when she is taken away! This, however, would be inconceivable on the theory of a physical wave-sense, while it agrees very well with that of an extremely delicate, chemical olfactory sense.
It is a fact that insects very frequently fail to notice odors which we perceive as intense, and even while these are present, detect odors which are imperceptible to our olfactories. We must explain this as due to the fact that the olfactory papillæ of different species of animals are especially adapted to perceiving very different substances. All biological observations favor this view, and our psycho-chemical theories will have to make due allowance for the fact.]