Strongylognathus testaceus and S. huberi live in association with Tetramorium caespitum, and are cared for by these latter ants; it is notable that as in the case of the slave-making Polyergus rufescens the mandibles of the Strongylognathus are cylindrical and pointed, and therefore unsuitable for industrial occupations. S. testaceus is a weak little ant, and lives in small numbers in the nests with T. caespitum, which it is said to greatly resemble in appearance. The proportions of the forms of the two species usually associated is peculiar, there being a great many workers of T. caespitum both in the perfect and pupal states, and also all the sexes of the Strongylognathus, of which, however, only a few are workers. This would seem to suggest that S. testaceus attacks and pillages the nests of T. caespitum in order to carry off worker pupae, just as Polyergus rufescens does. But the facts that S. testaceus is a weaker Insect than the Tetramorium, and that only a few of its worker-caste are present in a community where there are many workers of the Tetramorium, seem to negative the view that the latter were captured by the former; and the mode in which the associated communities of these two species are started and kept up is still therefore in need of explanation.
Strongylognathus huberi is a much stronger Insect than its congener, S. testaceus, and Forel has witnessed its attack on Tetramorium caespitum. Here the raid is made in a similar manner to that of Polyergus rufescens on Formica; the Tetramorium is attacked, and its pupae carried off to the abode of the Strongylognathus to serve in due time as its slaves. The mandibles of S. huberi, being similar in form to those of Polyergus rufescens are used in a similar manner.
Although T. caespitum is common enough in South-East England, it is to be regretted that none of the guests or associates we have mentioned in connection with it occur in this country. It is a most variable species, and is distributed over a large part of the globe.
Our British species of Myrmicides, about ten in number, all belong to the group Myrmicini; none of them are generally common except Myrmica rubra, which is a most abundant Insect, and forms numerous races that have been considered by some entomologists to be distinct species; the two most abundant of these races are M. ruginodis and M. scabrinodis, which sometimes, at the time of the appearance of the winged individuals, form vast swarms.
The tiny Monomorium pharaonis is a species that has been introduced into Britain, but now occurs in houses in certain towns; it sometimes accumulates on provisions in such numbers as to be a serious nuisance. Seventeen thousand individuals weigh 1 gramme, and it is probable that a nest may include millions of specimens.
The genus Aphaenogaster[[69]] and its immediate allies include the harvesting ants of Europe and North America: they form subterranean nests consisting of isolated chambers connected by galleries; some of the chambers are used as store-houses or granaries, considerable quantities of corn, grass, and other seeds being placed in them. A. structor and A. barbarus have been observed to do this in Southern Europe, by Lespès, Moggridge, and others.
Fig. 70—Aphaenogaster (Messor) barbarus. Algeria. A, male; B, winged female; C, large worker or soldier; D, small worker. × 3⁄2.
In the deserts about Algeria and Tunis a harvesting ant, Aphaenogaster (Messor) arenarius, is an important creature: its subterranean dwellings are very extensive, and are placed at a depth of several feet from the surface. Entrance to these dwellings is obtained by small holes, which are the orifices of galleries many feet in length: the holes are surrounded by pellets of sand projecting somewhat above the general surface, and consequently making the places conspicuous. The subterranean works occupy an area of fifty or a hundred square yards excavated at a depth of three to six feet. In these immense nests there exists a form of worker, of very small size, that never comes to the surface.[[70]]
Pogonomyrmex barbatus and other species have been observed to do harvesting in North America. After the workers of P. barbatus have taken the seeds into the nest they separate the husks and carry them out, depositing them on a heap or kitchen-midden, formed near by. M‘Cook has witnessed and described the process of stripping the seeds.