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 {138}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. [Illustration: Fig. 59—Ant-plant, _Hydnophytum montanum_. Java. (After Forel.)]