Ch. XII.]

ROCK-CARVING.

217

room, found it consisted only of coffee and two small cakes called "noskears" for each of us; and we were told they had nothing else to offer us. So, munching our dry noskears, we mumbled over them as long as we could, and did not waste a crumb, wondering how our host got so fat on such fare. We were as hungry when we finished as when we began, and soon laid down on our hard couches to forget our hunger in sleep:

We started off early the next morning, as we were within a few leagues of the town of Matagalpa, and knew when we got there we should obtain plenty of provisions. About a league before arriving at Matagalpa there is a high range, with perpendicular cliffs near the summit. Rito told us that near the base of these cliffs there was a carving of a bull, and that the place was enchanted. I had heard in other parts stories of bulls being engraved or painted on rocks, but was very doubtful about their being true, as, up to the advent of the Spaniards, the Indians of Central America had never seen any cattle; and since the conquest they appear to have entirely given up their ancient practice of carving on stone, whilst the Spaniards and half-breeds have never learnt the art; so that I have never seen a single carving in the central departments that could be ascribed to a later period than the Spanish conquest.

Tired and hungry though we were, I was determined to put this story to the test; so Velasquez and I climbed up to the cliffs, and searched all round them, but could find no carving. At one place there was a large black stain on the cliff, produced by the trickling down of water from above; and I afterwards learnt that this stain at a distance somewhat resembled a bull, and a

little imagination completed the likeness. The lady of the house where we stayed at Matagalpa assured us she had seen it, and that everything appertaining to a bull was there. This she insisted on with a minuteness of detail rather embarrassing to a fastidious auditor.

Clambering down the rocks, we reached our horse and mule, and started off again, passing over dry weedy hills. One low tree, very characteristic of the dry savannahs, I have only incidentally mentioned before. It is a species of acacia, belonging to the section Gummiferæ, with bi-pinnate leaves, growing to a height of fifteen or twenty feet. The branches and trunk are covered with strong curved spines, set in pairs, from which it receives the name of the bull's-horn thorn, they having a very strong

These

resemblance to the horns of that quadruped.
thorns are hollow, and are tenanted by ants, that make a
small hole for their entrance and exit near one end of the
thorn, and also burrow through the partition that separates

Ch. XII.] ADAPTATION OF A TREE TO ITS ANTS.

219

the two horns; so that the one entrance serves for both. Here they rear their young, and in the wet season every one of the thorns is tenanted; and hundreds of ants are to be seen running about, especially over the young leaves. If one of these be touched, or a branch shaken, the little ants (Pseudomyrma bicolor, Guer.) swarm out from the hollow thorns, and attack the aggressor with jaws and sting. They sting severely, raising a little white lump that does not disappear in less than twentyfour hours.

These ants form a most efficient standing army for the plant, which prevents not only the mammalia from browsing on the leaves, but delivers it from the attacks of a much more dangerous enemy-the leaf-cutting ants. For these services the ants are not only securely housed by the plant, but are provided with a bountiful supply of food; and to secure their attendance at the right time and place, this food is so arranged and distributed as to effect that object with wonderful perfection. The leaves are bi-pinnate. At the base of each pair of leaflets, on the mid-rib, is a crater-formed gland, which, when the leaves are young, secretes a honey-like liquid. Of this the ants are very fond; and they are constantly running about from one gland to another to sip up the honey as it is secreted. But this is not all; there is a still more wonderful provision of more solid food. At the end of each of the small divisions of the compound leaflet there is, when the leaf first unfolds, a little yellow fruit-like body united by a point at its base to the end of the pinnule. Examined through a microscope, this little appendage looks like a golden pear. When the leaf first unfolds, the little pears are not quite ripe, and the ants are con

tinually employed going from one to another, examining them. When an ant finds one sufficiently advanced, it bites the small point of attachment; then, bending down the fruit-like body, it breaks it off and bears it away in triumph to the nest. All the fruit-like bodies do not ripen at once, but successively, so that the ants are kept about the young leaf for some time after it unfolds. Thus the young leaf is always guarded by the ants; and no caterpillar or larger animal could attempt to injure them without being attacked by the little warriors. The fruit-like bodies are about one-twelfth of an inch long, and are about one-third of the size of the ants; so that the ant bearing one away is as heavily laden as a man bearing a large bunch of plantains. I think these facts show that the ants are really kept by the acacia as a standing army, to protect its leaves from the attacks of herbivorous mammals and insects.

The bull's-horn thorn does not grow at the mines in the forest, nor are the small ants attending on them found there. They seem specially adapted for the tree, and I have seen them nowhere else. Besides the Pseudomyrma, I found another ant that lives on these acacias; it is a small black species of Crematogaster, whose habits appear to be rather different from those of Pseudomyrma. It makes the holes of entrance to the thorns near the centre of one of each pair, and not near the end, like the Pseudomyrma; and it is not so active as that species. It is also rather scarce; but when it does occur, it occupies the whole tree, to the exclusion of the other. The glands on the acacia are also frequented by a small species of wasp (Polybia occidentalis). I sowed the seeds of the acacia in my garden, and reared some young

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Ch. XII.]

THE BULL'S-HORN THORN ANT.

221

plants. Ants of many kinds were numerous; but none of them took to the thorns for shelter, nor the glands and fruit-like bodies for food; for, as I have already mentioned, the species that attend on the thorns are not found in the forest. The leaf-cutting ants attacked the young plants, and defoliated them; but I have never seen any of the trees out on the savannahs that are guarded by the Pseudomyrma touched by them, and have no doubt the acacia is protected from them by its little warriors. The thorns, when they are first developed, are soft, and filled with a sweetish, pulpy substance; so that the ant, when it makes an entrance into them, finds its new house full of food. It hollows this out, leaving only the hardened shell of the thorn. Strange to say, this treatment seems to favour the development of the thorn, as it increases in size, bulging out towards the base; whilst in my plants that were not touched by the ants, the thorns turned yellow and dried up into dead but persistent prickles. I am not sure, however, that this may not have been due to the habitat of the plant not suiting it.

These ants seem to lead the happiest of existences. Protected by their stings, they fear no foe. Habitations full of food are provided for them to commence housekeeping with; and cups of nectar and luscious fruits await them every day. But there is a reverse to the picture. In the dry season on the plains, the acacias cease to grow. No young leaves are produced, and the old glands do not secrete honey. Then want and hunger overtake the ants that have revelled in luxury all the wet season; many of the thorns are depopulated, and only a few ants live through the season of scarcity. As