THE HONEY ANTS
Many species of ants are in the habit of collecting nectar from
flowers, and the sweet juices excreted by plant-lice, until the crop
is greatly swollen. When they arrive at the nest, however, the sweets
are soon regurgitated and fed to the larvae. Any worker ant is able to
expand its crop to a certain extent, but in some species this power is
developed to an enormous degree. In still other tribes this peculiar
capacity seems to be limited to certain individuals. In the true honey
ants only a comparatively small number of workers are capable of this
honey-carrying, and these individuals are known as honey-bearing or
_repletes_. The repletes never accompany the other workers on their
foraging expeditions, but remain always in the nest, and are used as
living bottles in which to store the nectar brought in from the fields.
In some North American species of _Myrmecocystus_ the abdomen is
distended to such an extent that the repletes are unable to move about
without serious danger of bursting open, and spend their lives hanging
in clusters from the ceilings of certain chambers in the nest. These
honey ants are found in desert regions from central Mexico as far
north as Denver, Colorado, and have since ancient times been highly
prized as sweetmeats by the aborigenes of this region. Honey ants were
described in Mexican publications as long ago as 1832, but the first
important study was made by McCook, whose investigations were carried
out in the so-called Garden of the Gods, near Manitou, Colorado, about
1882. He found several very large nests, covering an area of more than
six feet in diameter, and extending three feet below the surface of
the ground. One of these nests contained some three hundred replete
honey-vessels hanging by their claws from the ceiling, and so distended
with honey that, once fallen from their positions, they were quite
unable to get back up again. McCook saw the ordinary workers bringing
in great quantities of nectar and honeydew, which was immediately
regurgitated and fed to the repletes or _rotunds_, as he called them,
and thus stored up in a living reservoir until needed.
[Illustration: Fig. III. Repletes of a common honey-ant. (From a
drawing by Wheeler.)]
It was formerly supposed that the sweet liquid was kept in the stomach
of the replete, but Forel, in 1880, showed that it is in reality
the enormously distended crop which functions. McCook made careful
dissections which bore out Forel’s views, and demonstrated that the
replete has all the abdominal organs of the ordinary worker, although
these are flattened against the body wall and rendered inconspicuous by
the distension of the crop.
McCook rejected the view that the replete belongs to a separate
caste, saying that “a comparison of the workers with the honey-bearer
shows that there is absolutely no difference between them except in
the distended condition of the abdomen.... The process by which the
rotundity of the honey-bearer has probably been produced, has its
exact counterpart in the ordinary distension of the crop in overfed
ants; the condition of the alimentary canal, in all the castes, is
the same, differing only in degree, and therefore the probability is
very great that _the honey-bearer is simply a worker with an overgrown
abdomen_.... Thus workers are transformed by the gradual distension
of the crop and expansion of the abdomen into honey-bearers, and the
latter do not compose a distinct caste.”
[Illustration: Fig. IV. Repletes of a honey-ant (_Myrmecocystus
hortideorum_) hanging from the roof of a honey chamber. (After McCook.)]
Just why these repletes should be developed in some species and not
in others is a mooted question. The fact that they are found only
in desert regions in North America, Australia, and South Africa may
mean that a dry climate is one of the important conditions of the
phenomena. Forel said: “The extraordinary distension of the crop seems
to be frequent in the Australian species of the general Melophorus,
Gamponotus and Leptomyrmex. I suppose that this is due to the extremely
dry climate of the country, which must compel the ants to remain,
often for long periods, in their subterranean abodes. At such times
a store of provisions in living bags must be very useful to them.”
Wheeler, in commenting on the above statement by Forel, writes: “There
can be little doubt of the truth of this statement, but I believe that
it should be expressed in a different manner. The impulse to develop
repletes is probably due to the brief and temporary abundance of liquid
food (honeydew, gall secretions, etc.) in arid regions and the long
period during which not only these substances, but also insect food
are unobtainable. The honey is stored in the living reservoirs for
the purpose of tiding over such periods of scarcity, and the ants
remain in their nests because they do not need to forage. Hence the
confinement mentioned by Forel is not the immediate but one of the
ulterior effects of drought. I am convinced from my observations on
desert ants that no amount of drought will keep these insects in their
nest when they are in need of food.
“While excavating the nests of _M. hortideorum_ I was impressed with
certain peculiarities in their structure and situation, which seem to
be explainable only as adaptations to the development of repletes.
One of these peculiarities is the great hardness of the soil that is
preferred by the ants. This is the more astonishing because the workers
are very slender and delicate organisms. It is evident that such soil
is well adapted to the construction of vaulted chambers like those in
which the repletes hang, whereas soft or friable soil would be most
unsuitable. The development of repletes also makes it necessary for the
ants to seek very dry situations for their nests. Hence we always find
them, in the environs of Manitou at least, on the summits of ridges
which shed the rain very rapidly. The honey chambers must be kept dry,
both to prevent the disastrous results of crumbling and slipping walls
and to obviate the growth of mould on the repletes, which are, of
course, imprisoned for life in dark cavities and filled with substances
that are favorable to the development of fungi. I believe also that
the size of the nest openings and galleries, which are so much larger
than would seem to be required by such small, slender ants, may be
an adaptation to securing plenty of fresh air in the honey chambers.
If these suppositions are correct, there is obviously a reciprocal
relation between the replete habit and an arid environment: the ants
store honey because they are living in an arid region where moisture
and food are precious, and the storing of honey in replete workers, in
turn, is possible only in very dry soil.”