s
the young leaves are always guarded by the ants; and no caterpillar or
large 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
little ants, I found another ant that lives on these acacias, whose
habits appear to be rather different. It makes the holes of entrance to
the thorns near the centre of one of each pair, and not near the end,
and it is not so active as the other 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. I sowed the seeds of the acacia in my garden, and
reared some young 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 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 favor the development of the thorn, as it increases
in size, bulging out toward 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 a