Fig. 58—Portion of combined nest of Formica fusca and Solenopsis fugax. (After Forel.) × ⅔. f, f′, Chambers of Formica, recognisable by the coarser shading; s, s′, chambers of the Solenopsis (with finer shading); s″, opening in one of the chambers, the entrance to one of the galleries that connects the chambers of the Solenopsis; w, walls forming the foundations of the nest and the limits of the chambers.
In South America Camponotus rufipes and other species that habitually dwell in stumps, in certain districts where they are liable to inundations, build also nests of a different nature on trees for refuge during the floods. In Europe, a little robber-ant, Solenopsis fugax, constructs its dwelling in combination with that of Formica fusca (Fig. 58), in such a manner that its chambers cannot, on account of the small size of the orifices, be entered by the much larger Formica. Hence the robber obtains an easy living at the expense of the larger species. The Sauba or Sauva ants of South America (the genus Atta of some, Oecodoma of other authors) appear to be most proficient in the art of subterranean mining. Their systems of tunnels and nests are known to extend through many square yards of earth, and it is said on the authority of Hamlet Clark that one species tunnelled under the bed of the river Parahyba at a spot where it was as broad as the Thames at London Bridge.
A considerable number of ants, instead of mining in the ground, form chambers in wood; these are usually very close to one another, because, the space being limited, galleries cannot be indulged in. Camponotus ligniperdus in Europe, and C. pennsylvanicus in North America, work in this way.
Our British Lasius fuliginosus lives in decayed wood. Its chambers are said by Forel to consist of a paper-like substance made from small fragments of wood. Cryptocerus burrows in branches. Colobopsis lives in a similar manner, and Forel informs us that a worker with a large head is kept stationed within the entrance, its great head acting as a stopper; when it sees a nest-fellow desirous of entering the nest, this animated and intelligent front-door then retreats a little so as to make room for ingress of the friend. Forel has observed that in the tropics of America a large number of species of ants live in the stems of grass. There is also quite a fauna of ants dwelling in hollow thorns, in spines, on trees or bushes, or in dried parts of pithy plants; and the tropics also furnish a number of species that make nests of delicate paper, or that spin together by means of silk the leaves of trees. One eastern species—Polyrhachis spinigera—fabricates a gauze-like web of silk, with which it lines a subterranean chamber after the manner of a trap-door spider.
Fig. 59—Ant-plant, Hydnophytum montanum. Java. (After Forel.)
Some species of ants appear to find both food and shelter entirely on the tree they inhabit, the food being usually sweet stuff secreted by glands of the plant. It is thought that the ants in return are of considerable benefit to the plant by defending it from various small enemies, and this kind of symbiosis has received much attention from naturalists. A very curious condition exists in the epiphytic plants of the genera Myrmecodia and Hydnophytum; these plants form large bulb-like (Fig. 59) excrescences which, when cut into, are found to be divided into chambers quite similar to those frequently made by ants. Though these structures are usually actually inhabited by ants, it appears that they are really produced by the plant independent of the Insects.
Variability and Polymorphism of Ants.—Throughout the Hymenoptera there are scattered cases in which one of the sexes appears in dimorphic form. In the social kinds of bees and wasps the female sex exists in two conditions, a reproductive one called queen, and an infertile one called worker, the limits between the two forms seeming in some cases (honey-bee) to be absolute as regards certain structures. This sharp distinction in structure is rare; while as regards fertility intermediate conditions are numerous, and may indeed be induced by changing the social state of a community.[[55]] In ants the phenomena of the kind we are alluding to are very much more complex. There are no solitary ants; associations are the rule (we shall see there are one or two cases in which the association is with individuals of other species). In correlation with great proclivity to socialism we find an extraordinary increase in the variety of the forms of which species are made up. In addition to the male and female individuals of which the species of Insects usually consist, there are in ants workers of various kinds, and soldiers, all of which are modified infertile females. But in addition to the existence of these castes of infertile females, we find also numerous cases of variability or of dimorphism of the sexual individuals; and this in both sexes, though more usually in the female. Thus there exists in ants an extraordinary variety in the polymorphism of forms, as shown by the table on p. 141, where several very peculiar conditions are recorded.
The complex nature of these phenomena has only recently become known, and as yet has been but little inquired into. The difference between the thoracic structure in the case of the winged and wingless females of certain species (Fig. 55, and in vol. v. fig. 339) is enormous, but in other species this difference appears to be much less. The ordinary distinctions between the queen-female and worker-females appear to be of two kinds; firstly, that the former is winged, the latter wingless;[[56]] and secondly, that the former possesses a receptaculum seminis, the latter does not. In a few cases it would seem that the dimorphism of winged and wingless forms is not complete, but that variability exists. Intermediate conditions between the winged and wingless forms are necessarily rare; nevertheless a certain number have already been detected, and specimens of Lasius alienus have been found with short wings. In rather numerous species some or all of the fertile females depart from the usual state and have no wings; (a similar condition is seen, it will be recollected, in Mutillides and Thynnides of the neighbouring family Scoliidae). A dimorphism as regards wings also exists in the male sex, though it is only extremely rarely in ants that the males are wingless. Neverless a few species exist of which only wingless males have been found, and a few others in which both winged and wingless individuals of this sex are known to occur. The wingless males of course approach the ordinary workers (= infertile wingless females) in appearance, but there is not at present any reason for supposing that they show any diminution in their male sexual characters. The distinction between workers and females as based on the existence or non-existence of a receptaculum seminis has only recently become known, and its importance cannot yet be estimated. The adult, sexually capable, though wingless forms, are called ergatoid, because they are similar to workers (Ἐργατης, a worker).[[57]]
Table of the Chief Forms of Polymorphism in Ants.