THE MUSHROOM GROWERS
In tropical and subtropical America there are about one hundred species
and varieties of ants which have most extraordinary habits, and are
grouped together in the Myrmicine tribe _Attii_. These ants are usually
rather small and dull colored, and, while they are powerful and
industrious diggers, are not given to rapid movements as most ants are,
but walk slowly and sedately about. When picked up they do not struggle
as many other ants do, but feign death after the manner of certain well
known beetles.
It was long noted that the _Attii_ carried great quantities of leaves
into their nests, and there was considerable doubt as to the use to
which these were put, some observers believing that they were used
immediately as food, and others contending that they served as roofing
and carpets in the underground passageways. Belt, a naturalist who
lived in Nicaragua, was probably the first to discover the secret
of the leaves. Digging into one of the nests in his garden, he was
surprised to find no great quantity of leaves in any of the passages,
although ants were continually bringing them in at the entrance. The
chambers were always partly filled with “a speckled, brown, flocculent,
spongy-looking mass of a light and loosely connected substance.... This
mass, which I have called the ant-food, proved on examination to be
composed of minutely subdivided pieces of leaves, weathered to a brown
color, and overgrown and lightly connected together by a minute white
fungus that ramified in every direction throughout it.... When a nest
is disturbed and the masses of ant-food spread about, the ants are in
great concern to carry away every morsel of it under shelter again; and
sometimes, when I dug into the nest, I found the next day all the earth
thrown out filled with little pits that the ants had dug into it to get
out the covered up food.”
Further investigation brought Belt to the conclusion that the _Attii_
do not eat leaves at all, but use them as manure to grow fungus on; and
further, that they feed upon this fungus, and will eat nothing else.
The _Attii_ are, in Belt’s own phrase, “mushroom growers and eaters.”
While leaves are the chief fertilizer, other substances are often
found suitable for growing fungus on; flowers are sometimes used, and
some species are particularly partial to pieces of orange peel. The
temperature and ventilation of the subterranean gardens are matters
of great importance, and there are many small holes which connect
the larger chambers with the surface. These air-shafts are plugged
and reopened at intervals, and by this means the temperature and
ventilation are regulated.
Alfred Moeller was a naturalist who studied the _Attii_ in Brazil, and
published the results of his labors in 1893. He found that the gardens
contain only one kind of fungus, all alien spores being carefully
weeded out. The ants do not allow the fruits to develop, and this has
made the classification of the fungi a very difficult matter. The fungi
found in the _Attii_ nests are different from any others known, but
no one can tell whether they are really distinct species or merely
modified forms of certain common moulds or mushrooms.
Von Ihering, in 1898, discovered that the virgin queen, when leaving
the nest on her nuptial flight, always carries a little pellet of
fungus in her mouth. After being fertilized by the male the queen
shuts herself up in a little burrow and sets about the founding of a
new colony. There are in this case no leaves available, and she starts
the fungus growing upon some of her new-laid eggs, which she crushes
for the purpose, and which seem to work quite as well as the usual
vegetable fertilizer.
J. Huber, in 1905, studied the same problems which interested Von
Ihering, and concluded that the fungus is not grown upon crushed eggs,
but is nourished by the liquid excrement of the queen. He describes
his observations as follows: “After watching the ant for hours she will
be seen suddenly to tear a little piece of the fungus from the garden
with her mandibles and hold it against the tip of her abdomen, which is
bent forward for this purpose. At the same time she emits from her vent
a clear yellowish or brownish droplet which is at once absorbed by the
tuft of hyphae. Hereupon the tuft is again inserted, amid much feeling
about with the antennae, in the garden, but usually not in the same
spot from which it was taken, and is then patted into place by means
of the fore feet.... According to my observations, this performance
is repeated usually once or twice an hour, and sometimes, to be sure,
even more frequently.” Although, according to Huber, the eggs are not
used directly as fertilizer for the fungus, the same result is brought
about indirectly, as the female is accustomed to feed upon her own
new-laid eggs. Huber estimates that nine out of every ten eggs laid are
eaten at once by the mother. The young larvae, too, are fed with eggs
thrust directly into their mouths by the queen. When the adult workers
appear, however, they live exclusively on the fungus which has been
growing during their larval life, and feed the queen upon fungus also,
while continuing to supply the larvae with their mother’s eggs. After
a week or so the workers dig their way out of the chamber, bring in
leaf-manure for the garden, and the fungus is no longer cared for by
the queen, who now gives all her attention to the serious business of
egg-laying. As the fungus becomes more abundant under this cultivation
it is fed to the larvae also, and eggs are no longer used as food by
any of the individuals in the hive.
The extraordinary habits of the Attine ants have fascinated many
students, and a number of theories about their development have
been advanced. Forel suggested that the ancestors of the present
mushroom-growers must have lived in rotten wood, and fed upon the
fungus which grew upon the moist walls of their nests, or upon insect
excrement. Von Ihering thinks that they may have developed from the
harvesting ants, which gradually acquired such an appetite for the
fungus which happened to grow in their granaries that the original
stores came to be used only as fertilizer. Wheeler points out that,
besides the Attine ants, there are several kinds of beetles and
termites which cultivate fungus upon their own excrement, and suggests
that originally this was the method employed by the ants. Later on they
came to use the excrement of other insects, and finally passed to the
addition of leaves and other non-fecal vegetable matter.
As has been said above, the _Attii_ are primarily tropical and
subtropical insects, but a few species have come north into the United
States. They are found chiefly in peninsular Florida, in southern
Texas, and in Arizona, although one species has been reported as far
north as southern New Jersey.