CHAPTER VI
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.
Fig. III. Repletes of a common honey-ant. (From a drawing by Wheeler.)
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.
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....
Fig. IV. Repletes of a honey-ant (Myrmecocystus hortideorum) hanging from the roof of a honey chamber. (After McCook.)
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.”
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.”