THE HARVESTING ANTS
The works of Pliny and other ancient writers contain references to ants
which collected great stores of seeds, and these accounts were quoted
by numerous mediaeval authors. Modern students of ants, however, worked
mostly in northern and central Europe, and as they did not find any of
these harvesting ants they were rather inclined to dismiss the classic
stories as fiction pure and simple, and class the seed-gathering ants
with the unicorn and the mermaid.
In 1829, however, one W. H. Sykes, an Englishman located in India,
reported that certain ants near his station not only collected great
quantities of grass seed, but after a heavy rain could always be
seen bringing their cereal out of the underground granaries to dry
it in the sun. These observations went far to vindicate the ancient
naturalists, and the work of J. T. Moggridge, in 1873, completed the
vindication. Moggridge watched the workers bring in the seeds, bite
off the germinating part to prevent the seeds from sprouting, and
store them in the nests, which often contain a pint or so of grain. By
examination of these hoards he identified as many as eighteen different
families of plants represented in a single nest. Despite the efforts to
prevent germination by biting off the radicles (a fact noted by Pliny
some sixteen hundred years before) many of the seeds do sprout, and
thus the harvesting ants play a part in the distribution of plants. Of
this subject Moggridge says: “As the ants often travel some distance
from their nest in search of food, they may certainly be said to be,
in a limited sense, agents in the dispersal of seeds, for they not
infrequently drop seeds by the way, which they fail to find again,
and often also among the refuse matter which forms the kitchen hidden
in front of their entrances, a few sound seeds are often present, and
these in many instances grow up and form a little colony of strange
plants. This presence of seedlings foreign to the wild grounds in which
the nest is usually placed, is quite a feature where there are old
established colonies of _Atta barbara_, where young plants of fumitory,
chickweed, cranesbill, Arabis thaleana, etc., may be seen on or near
the rubbish heap.... One can imagine cases in which the ants during
the lapse of long periods of time might pass the seeds of plants from
colony to colony, until after a journey of many stages, the descendants
of the ant-borne seedlings might find themselves transported to places
far removed from the original home of their immediate ancestors.”
There are many species of harvester ants in America; one of the most
interesting is _Solenopsis geminata_, popularly known as the fire-ant
because of its readiness to use its painful sting. Although the
fire-ant certainly stores up seeds, often to the extent of damaging
crops of soft fruits like strawberries, it will also eat insects, or
almost anything else that it can get. The nests are usually found
beneath flat stones, and in some localities are so common and so
populous that Wheeler refers to the fire-ant as being “in possession
of a large portion of the soil of the American tropics.” In Louisiana
and other southern states these ants nest along the shores of lagoons
and bayous; when the floods come and the nest is submerged the workers
cling together in a ball as much as eight inches in diameter, with the
brood in the center. This ball floats in the water, the ants constantly
shifting about so that very few are drowned, and very little brood
lost, until they are able to effect a landing.
The so-called Texas harvester (_Pogonomyrmex molefaciens_) has become
famous because a man named Lincecum, about 1862, published a paper in
which he claimed that this ant actually _plants_ seeds in the ground,
weeds and cultivates its fields all summer, gathers the crop, dries
it in the sun, and finally stores it away in subterranian granaries.
This story was accepted and promulgated by Charles Darwin, and so was
believed in many quarters. It seems to rest solely upon the fact that
ant-rice (_Aristida_) is usually found growing about the nest, although
it may occur nowhere else in the immediate vicinity. “Four years of
nearly continuous observation,” writes Wheeler, “enable me to suggest
the probable source of Lincecum’s misconception. If the nests of this
ant can be studied during the cool winter months--and this is the only
time to study them leisurely, as the cold subdues the fiery stings of
their inhabitants--the seeds, which the ants have garnered in many
of their chambers will often be found to have sprouted. Sometimes, in
fact, the chambers are literally stuffed with dense wads of seedling
grasses and other plants. On sunny days the ants may often be seen
removing these seeds when they have sprouted too far to be fit for food
and carrying them to the refuse heap, which is always at the periphery
of the crater or cleared earthen disk. Here the seeds, thus rejected as
inedible, often take root and in the spring form an arc or a complete
circle of growing plants around the nest. Since the _Pogonomyrmex_
feeds largely, though by no means exclusively, on grass seeds, and
since, moreover, the seeds of Aristida are a very common and favorite
article of food, it is easy to see why this grass should predominate in
the circle. In reality however, only a small percentage of the nests,
and only those situated in grassy localities, present such circles.
Now to state that _molefaciens_, like a provident farmer, sows this
cereal and guards and weeds it for the sake of garnering its grain, is
as absurd as to say that the family cook is planting and maintaining an
orchard when some of the peach stones, which she has carelessly thrown
into the backyard with the other kitchen refuse, chance to grow into
peach trees.”
Wheeler has also observed the mating flight of the Texas harvester,
and his graphic description is worth setting down in its entirety:
“During three successive years (1901-1903) at Austin, Texas, the
nuptial flight of _molefaciens_ took place on one of the last days
of June (28 and 29) or the first in July. On one of these occasions
(July 4, 1903) the flight was of exceptional magnitude and beauty. A
few days previous the country had been deluged with heavy rains, but
Independence Day was clear and sunny, the mesquite trees were in full
bloom and the air resounded with the hum of insects. For several days
I had seen a few males and winged females stealthily creep out of the
nest entrance as if for an airing, but hurry back at the slightest
alarm. From 1:30 to 3 o’clock, however, on the afternoon of July 4,
all the numerous colonies I could visit during a long walk west of the
town, gave forth their males and females as by a common impulse. The
number issuing from a single large nest was often sufficient to have
filled a half liter measure. Soon every mound and disk was covered with
the bright red females and darker males, intermingled with workers,
many of whom kept on bringing seeds and dead insects into the nest
as unconcernedly as if nothing unusual were happening. The males and
females, quivering with excitement, mounted the stones or pebbles of
the nest or hurriedly climbed onto the surrounding leaves and grass
and rocked to and fro in the breeze. Then, raising themselves on their
feet and spreading their opalescent wings, they mounted obliquely one
by one into the air. I could follow them only for a distance of ten or
twenty meters when their rapidly diminishing bodies melted away against
the brilliant cloudless sky. Many pairs, hesitating to take flight,
chased one another about on the surface of the nest. The amorous males
seized many of the females before they could leave the ground. Lizards
crept forth in great numbers and gulped down quantities of the fat
females, while others were borne off into the air by large robber flies
(_Asilidae_). By a little after three o’clock the males and females had
left the nest and only the workers were seen pursuing the quiet routine
business of bringing in seeds.”