As in all these cases the ants live on amicable terms with their guests, and in some cases even bestow labour upon them (as in carrying them from one nest to another during migration), it is evident that these insects are not only tolerated, but fostered by the ants. Moreover, as it seems absurd to credit ants with any mere fancy or caprice such as that of keeping pets, we can only conclude that these insects, like the aphides, are of some use to their hosts, although we are not yet in a position to surmise what this use can be.
Habits of Sleep and Cleanliness.—It is probable that all species of ants enjoy periods of true sleep alternating with those of activity; but actual observations on this subject have only been made on two or three species. The following is MacCook's account of these habits in the harvesting ant of Texas:—
The observation upon the ants now before me began at 8 o'clock; at 11 P.M. the cluster had nearly dissolved, only a few being asleep. To illustrate the soundness of this sleep I take the quill pen with which I write, and apply the feather end of it to an ant who is sleeping upon the soil. She has chosen a little oval depression in the surface, and lies with abdomen upon the raised edge, and face toward the lamp. Her legs are drawn up close to the body. She is perfectly still. I gently draw the feather tip along the body, stroking 'with the fur,' if I may so say. There is no motion. Again and again this action is repeated, the stroke gradually being made heavier, although always quite gentle. Still there is no change. The strokes are now directed upon the head, with the same result. Now the tip is applied to the neck, the point at which the head is united to the pro-thorax, with a waving motion intended to produce a sensation of tickling. The ant remains motionless. After continuing these experiments for several minutes, I arouse the sleeper by a sharp touch of the quill. She stretches out her head, then her legs, which she also shakes, steps nearer to the light, and begins to cleanse herself in the manner already described. This act invariably follows the waking of ants from sleep. The above description applies to the general habit of somnolence as observed upon the two named species of harvesting ants for nearly four months. I have often applied the quill, and even the point of a lead pencil, to the sleeping Floridians without breaking their slumber. There are some other details which have not appeared in the behaviour of the individual just put under observation.
Thus, I have several times seen the ants (Crudelis) yawning after awaking. I use this word for lack of one which more accurately expresses the behaviour. The action is very like that of the human animal; the mandibles are thrown open with the peculiar muscular strain which is familiar to all readers; the tongue also is sometimes thrust out, and the limbs stretched with the appearance, at least, of that tension which accompanies the yawn in the genus homo. During sleep the antennæ have a gentle, quivering, apparently involuntary motion, which seemed to me, at times, to have the regularity of breathing. I also often noted an occasional regular lifting up and setting down of the fore-feet, one leg after another, with almost a rhythmic motion.
The length of time during which sleep is prolonged appears to vary according to circumstances and, perhaps, organism. The large head-soldiers of the Floridian harvesters appear to have a more sluggish nature than the smaller workers. Their sleep is longer and heavier. The former fact the watch readily determined. The latter appeared from the greater stolidity of the creatures under disturbance. While the ants of one group are taking sleep others may be busy at work, and these stalk among and over the sleepers, jostling them quite vigorously at times. Again, new members occasionally join the group, and, in their desire to get close up to the heat and light, crowd their drowsy comrades aside. I have seen ants who had been at work in the galleries drop their pellets, push thus into the cluster, and presently be apparently sound asleep. This rough treatment is invariably received with perfect good humour, as are like jostlings when the ants are awake. I have never seen the slightest display of anger or attempt to resent disturbance even under these circumstances, so peculiarly calculated to excite the utmost irritation in men. But of course some of the sleepers are aroused. They change position a little, or give themselves a brief combing, and then resume their nap, unless, indeed, they are satisfied. In watching these movements it was quite evident that the Florida soldiers were far less easily disturbed than their smaller fellows. They slept on stolidly while all the others were in agitation around them. Moreover, their very appearance, particularly when awaking out of sleep, indicated the greater sluggishness of their temperament in this respect.
The ordinary duration of sleep MacCook takes to be about three hours.
Ants, like many other insects, are in the habit of cleaning themselves, being, like them, provided by nature with combs and brushes, &c., for the purpose. But, unlike other insects, several species of ants are also in the habit of assisting each other in the performance of their toilet. The author last quoted gives the following account of this process in the genus Atta:—
We take a couple; the cleanser has begun at the face, which is licked thoroughly, even the mandibles being cared for, they being held apart for convenient manipulation. From the face the cleanser passes to the thorax, thence to the haunch, and so along the first leg, along the second and third in the same manner, around to the abdomen, and thence up the other side of the ant to the head. A third ant approaches and joins in the friendly task, but soon abandons the field to the original cleanser. The attitude of the cleansed all this while is one of intense satisfaction, quite resembling that of a family dog when one is scratching the back of his neck. The insect stretches out her limbs, and, as her friend takes them successively into hand, yields them limp and supple to her manipulation; she rolls gently over upon her side, even quite over upon her back, and with all her limbs relaxed presents a perfect picture of muscular surrender and ease. The pleasure which the creatures take in being thus 'combed' and 'sponged' is really enjoyable to the observer. I have seen an ant kneel down before another and thrust forward the head, drooping, quite under the face, and lie there motionless, thus expressing, as plainly as sign-language could, her desire to be cleansed. I at once understood the gesture, and so did the supplicated ant, for she at once went to work. If analogies in nature-studies were not so apt to be misleading, one might venture to suggest that our insect friends are thus in possession of a modified sort of Emmetonian Turkish bath.
The acrobatic skill of these ants, which has often furnished me amusement, and which I shall yet further illustrate, was fully shown one morning in these offices of ablution. The formicary was taken from the study, where the air had become chilled, and placed in an adjoining chamber upon the hearth, before an open-grate fire. The genial warmth was soon diffused throughout the nest, and aroused its occupants to unusual activity. A tuft of grass in the centre of the box was presently covered with them. They climbed to the very top of the spires, turned round and round, hanging by their paws, not unlike gymnasts performing upon a turning-bar. They hung or clung in various positions, grasping the grass blade with the third and fourth pairs of legs, which were spread out at length, cleansing their heads with the fore-legs or bending underneath to comb and lick the abdomen. Among these ants were several pairs, in one case a triplet, engaged in the cleansing operation just described. The cleanser clung to the grass, having a fore-leg on one side and a hind leg on the other side of the stem, stretched out at full length, while the cleansed hung in a like position below, and reached over and up, submitting herself to the pleasant process. As the progress of the act required a change of posture on the part of both insects, it was made with the utmost agility.
Similarly, Bates thus describes the cleansing process in another genus of ants (Ecitons):—