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Ant colonies > Ant nests
Other mound ant nests, differing from the foregoing in their smaller size and compact earthen structure, have been designated by Forel as masonry domes. He was surprised to find that many circumpolar ants (Lasius niger and flavus, Formica fusca and sanguinea), which construct masonry domes in Europe, fail to exhibit this peculiarity in the United States. He concluded that these structures, which, like the large ant mounds, serve as incubators, must be unnecessary in this country on account of its great annual extremes of climate. This inference is certainly premature, for although it is true that many of the circumpolar species do not make domes in the Atlantic States, they have this habit in the Mississippi Valley and Rocky Mountains, where the annual extremes of temperature are even greater.
Formica subsericea and many species of Lasius and Acanthomyops become dome builders in Illinois and Wisconsin, although it must be admitted that the term "masonry domes " is not always strictly applicable to their nests, since the earth of which they consist is not firmly compacted but carried up rather loosely around grass and plant stems. I have frequently seen such mounds of Lasius aphidicola, Acanthomyops interjectus and claviger and F. subsericea fully 30 cm. in height and 60 cm. to 1 m. in diameter. In the Rocky Mountain region large mound nests of Pogonomyrmex occidentalis, Formica obscuripes, opaciventris and argentata abound, and in these regions they are much needed for maturing the brood, as the nights are cold in the summer and the heat of the daylight hours must be utilized.
Formica glacialis, one of the varieties of F. fusca, in Maine, makes true masonry domes like the European ants, and in this region such nests must be very useful as incubators since the summers are short and comparatively cool.
I have already called attention to the constant position of the nest opening at the base of the southern or eastern slope of the mounds of Pogonomyrmex occidentalis. Huber says that the yellow ants (Lasius flavus) of Switzerland "serve as compasses to the mountaineers when they are enveloped in dense fogs or have lost their way at night; for the reason that the ant nests, which in the mountains are much more numerous and higher than elsewhere, take on an elongated, almost regular form. Their direction is constantly from east to west. Their summits and more precipitous slopes are turned towards the winter sunrise, their longer slopes in the opposite direction." These remarks of Huber have been confirmed by Tissot and Linder. The latter has shown that the elongate shape of the mounds is due to the fact that the ants keep extending them in an easterly direction in such a manner that only the extreme easterly, highest and most precipitous portions are inhabited by the insects. I have observed a similar and equally striking orientation of the ant mounds of Formica argentata in the subalpine meadows of Colorado.
By far the greatest number of ant-nests, at least in many parts of the world are excavated in the soil under stones, logs, boards, etc. Most of our ants, including even those that construct large mounds, are very fond of nesting in such places during the younger colonial stages. In fact only two of our terricolous species - Dorymyrmex pyramicus in the Southern, and Prenolepis imparis in the Northern States - are so rarely found under stones as to indicate that they have a pronounced aversion for such sites. The advantage of nesting under stones is considerable, for these not only protect the entrances, galleries and chambers from rain and wind and enable the ants to dispense with the labor of roofing over their surface excavations, but they are of even greater service in conserving the moisture in the underlying soil while rapidly taking up the sun's heat and thus accelerating the incubation of the brood. Nearly all ants prefer flat stones of moderate dimensions and not too deeply buried in the soil.
Many Formicae of the rufa and sanguinea groups (nepticula, difficilis, consocians, microgyna, obscuriventriS, oreas, ciliata, dakotensis, integra, rubicunda, etc.) bank the edges of the stones with earth or plant detritus into which they often extend their galleries and chambers for the aeration and incubation of the brood. Even the mound-building forms frequently start their nests in this way and gradually heap the detritus up over the stones till they are concealed under typical domes and the original lapidicolous habit of the ant colony is no longer recognizable.
In the North American forests the logs and branches strewn about on the ground afford a substitute for stones and cover the nests of many glade ants, such as F. rubicunda, integra, haemorrhoidalis, Aphaenogaster fulva, etc., F. integra in the Eastern and haemorrhoidalis in the Western States fill out the spaces in and under logs with vegetable detritus.
Ants are not fond of nesting under cow dung, but in Texas I have found Solenopsis geminata and Camponotus fumidus resorting to such nesting sites in regions where stones were scarce and moisture and protection from the intense heat of the sun nowhere else to be found.