Large or medium-sized ants closely allied to Camponotus .
Worker monomorphic. Head orbicular, oval or rounded subrectangular, very convex above, with very prominent, long and sinuate frontal carinae. Palpi long, the maxillary pair 6-jointed, with the basal about half as long as the second joint, the labial pair 4-jointed. Clypeus well developed, usually convex or more or less carinate. Antennae long, 12-jointed, the scapes inserted some distance behind the posterior border of the clypeus, as in Camponotus ; funicular joints considerably longer than broad. Thorax more or less arcuate above, often more or less carinate on the sides, and more or less dentate of spinose, but exhibiting great differences in conformation in different species. Usually either the pronotum or the epinotum or both are armed with teeth or spines, rarely the mesonotum. The petiole has a large scale, the superior border of which is nearly always armed with pairs of spines or teeth, more rarely also with a median, unpaired spine or tooth. Gaster large, broadly elliptical or subglobular, very convex above, the first segment forming more than half of its surface and often more or less truncated or concave in front. Legs long and well developed, the tibiae often constricted at the base. Gizzard much as in Camponotus .
Female decidedly larger than the worker, with massive thorax. Spines and teeth on the thorax and petiole smaller. Wings long, the anterior pair with a radial and a single cubital cell; discoidal cell lacking and cubital vein usually reaching the outer margin of the wing. Gaster massive, its first segment often proportionally shorter than in the worker.
Male closely resembling the male of Camponotus , small and slender; the thorax and petiole quite unarmed, the latter with a low, thick scale. Frontal carinae more approximated, front more convex, pronotum overarched by the mesonotum. External genital valves small and slender. Cerci distinct.
Pupae enclosed in cocoons.
A large genus comprising several hundred species, many of which are among the most beautiful of ants, confined to the tropics of the Old World, though, like Oecophylla , absent from Madagascar (Map 44). The species of Polyrhachis , however, have a wider range, since a small number of forms occur as far north as Syria in Asia and as far south as the eastern Cape Colony and Tasmania. The majority of the species are aggregated in the Indomalayan, Papuan, and Australian Regions. Forel and I have divided the genus into subgenera, eleven of which, based on peculiarities in the structure of the thorax and petiole, have been recognized up to the present time, namely, Polyrhachis , sensu stricto, Campomyrma Wheeler, Hagiomyrma Wheeler, Myrma Billberg, Hedomyrma Forel, Myrmhopla Forel, Chariomyrma Forel, Myrmatopa Forel, Cyrtomyrma Forel, Myrmothrinax Forel, and Dolichorhachis Mann. In the Ethiopian Region only two of these, Cyrtomyrma and Myrma , are known to occur, the former represented by a very few aberrant species, the latter by a number of forms which show much greater diversity of structure than do the species of the same subgenus in the Indomalayan and Papuan Regions. This fact, together with that of the wide distribution of Myrma , would seem to indicate that it is the most archaic of all the subgenera of Polyrhachis .
The species of Polyrhachis form only moderately large colonies and none of them is sufficiently common to be of economic importance. Many of them are, in fact, rare and sporadic. They are very timid or pacific insects and are most frequently found singly walking up or down tree-trunks or on the foliage of trees or bushes. Their nesting habits are very diverse. According to my observations in Australia, the species of Campomyrma nest in the ground, under stones, or more rarely in crater nests. The same is true of the species of Hagiomyrma and Chariomyrma , though I have always found P. (Hagiomyrma) semiaurata Mayr in large logs and certain species of Chariomyrma in earthen termitaria. So far as known, none of the species of these three subgenera employs silk in the construction of the nest. The species of Hedomyrma , as Mann and I have observed, live in high trees, but we have been unable to find the nests. Several of the larger species of Myrma nest in the ground or in logs and some of them line their nests with silk spun by the larvae. Many of the smaller species of this subgenus make carton and silken nests on or between the living leaves of trees, and this is the general habit also of many species of the subgenera Myrmhopla , Myrmothrinax , Myrmatopa , and Cyrtomyrma . A few species of Myrma and Myrmhopla live in hollow stems or in old galls. Jacobson and Mann have described the beautiful carton and silk nests built by various Myrmatopa species on the under sides of leaves in Java and the Solomon Islands. P. (Myrmhopla) armata of the Indomalayan Region sometimes builds its nest in houses. P. (M.) dives and some of the allied species construct small globular nests of nearly pure silk, somewhat like those of tent-caterpillars, on low bushes. The nest of one of the few species of the subgenus Polyrhachis , sensu stricto, the East Indian P. bihamata , was found by Bingham. "It was of silky, yellowish brown material, placed close to the ground in the center of a clump of bamboos, and measured about a foot in diameter." Some species of Polyrhachis , when irritated, emit a strong, pleasant smell. According to Bingham, the odor of P. (Myrmhopla) venus Forel is like that of the tuberose.