Page:Popular Science Monthly Volume 71.djvu/559

Rh by workers of an alien species. This is the case with many queen ants that have lost the power of establishing colonies unaided. Third, the queen must always be adopted by an alien species. This is the case in certain ants, especially in the highly parasitic forms that have lost their worker caste. The three conditions here enumerated clearly represent the transition from parasitism of the queen on the same, to parasitism on an alien species. The latter alone is commonly regarded as true parasitism, but the former, which, of course, can occur only among social organisms or during social stages in the lives of solitary organisms, is parasitism in every essential particular. It is not confined to ants and other social insects, but has analogies also in human societies (trusts, "grafters," criminal and ecclesiastical organizations) and in human families (when the parents become senile).

Ant colonies are such closed and exclusive societies that the adoption of strange queens, even of the same species but from alien colonies, usually meets with insuperable opposition on the part of the workers, and, as a rule, female ants have to overcome even greater hostility when they seek adoption in colonies of alien species. There are, nevertheless, at least three different methods of overcoming this hostility and of effecting an adoption. These may be taken to characterize three different forms of social parasitism, as follows:

1. Temporary Social Parasitism.—I have given this name to a form of parasitism which I first observed in our American Formica difficilis var. consocians. The fertilized female of this ant, quite unable to found a colony unaided, enters a colony of F. schaufussi var. incerta and is adopted with surprising facility. The queen of the latter species disappears, in some manner hitherto unknown, and the consocians brood is reared by the incerta workers, which, after functioning as nurses, gradually die off and leave a pure consocians colony thenceforth able to wax large and lead an independent and aggressive existence. This interesting type of parasitism occurs in most, if not all, Formicæe of the exsecta and rufa groups, both in America and in Europe, in a Myrmicine ant, Aphœnogaster tennesseensis (parasitic on A. fulva) and in a Dolichoderine ant, Bothriomyrmex meridionalis (parasitic on Tapinoma erraticum). The females of these parasitic species tend to become greatly reduced in size (F. difficilis and several allied species: F. dakotensis, microgyna, impexa, nepticula, suecica, etc.) or at any rate to become smaller than the queens of their host species (F. truncicola, wasmanni, oreas, ciliata, crinita, pressilabris, etc.). This is clearly an adaptation to a mode of life for which an endowment of fat and vigorous muscle is not needed, since these various queens do not have to starve for weeks or even months while bringing up a brood