Onychomyrmex

These uncommon ants have an army-ant life style. They alternate between temporary bivouacs in protected sites on the ground and semi-permanent nests in soil or in rotten logs. Groups of workers forage by forming distinct raiding columns across the surface of the ground. Some species are known to be specialist predators on centipedes. Queens have greatly extended gasters (they are dichthadiiform), and new colonies are formed by the division of established colonies.

Identification
The mandibles are long and slender, with numerous (always more than 3) teeth which vary greatly in size and are scattered along the inner margins, and with a sharp, pointed tooth at their tips which is at least 4 times longer than the next longest tooth. The frontal lobes extend only slightly forward of the antennal sockets and do not cover the clypeus when viewed from the front. The petiole has distinct front and upper faces but lacks a rear face, and its attachment to the gaster is broad and approximately the same height as the petiole so that the upper surfaces of the petiole and gaster are separated by at most a shallow impression. The tibiae of the hind legs usually lack spurs at their tips, but when present they are small, straight and not comb-like (pectinate) (best viewed from the front).

Onychomyrmex workers are most often confused with workers of Amblyopone. However, they differ in having the tooth at the tip of the mandible much longer and in lacking a comb-like spur on the hind tibia.

Species richness
Species richness by country based on regional taxon lists (countries with darker colours are more species-rich). View Data



Biology
Wheeler (1916) - The species of Onychomyrmex are far from common even in Queensland, and the few colonies I secured were the reward of many hours of search and of the destruction of many old logs in places where I was frequently attacked by land-leeches and saw quite a number of the deadly black snakes (Pseudechis porphyriacus). Perhaps it would be possible for the collector to attract colonies by placing large beetle or cossid larvae in holes in the rotten logs usually found along the paths through the “scrub.”

A study of the worker Onychomyrmex certainly reveals a number of highly specialized characters. Such are particularly the shape of the mandibles, the vestigial condition of the palpi, the small size of the eyes, and the enlargement of the terminal joint, claws, and pulvilli of the middle and hind tarsi. The degenerate visual organs show that these ants belong to the hypogaeic series and that they pass their lives concealed in the logs which gradually decompose in the moist shade of the dense tropical jungle. The powerful, toothed mandibles, long sting and great hooked claws indicate that their possessors do not feed habitually on small feeble insects like termites, but on much larger creatures such as the larvae of passalids and scarabaeids and possibly on adult myriopods and scorpions. This I found to be the case in a colony of O. mjobergi, for when the log containing it was broken open, many of the workers were detected in the act of biting and stinging to death a huge lamellicorn beetle larva more than two inches in length, which they had just found in a cavity in the wood. It is not improbable that the colonies move from place to place in search of their prey, like the colonies of the subterranean Dorylinae (Eciton coecum and Dorylus), which they very closely resemble in behavior, color, sculpture, and pilosity.

Nomenclature

 *  ONYCHOMYRMEX [Amblyoponinae]
 * Onychomyrmex Emery, 1895g: 349. Type-species: Onychomyrmex hedleyi, by monotypy.
 * LITHOMYRMEX [junior synonym of Stigmatomma]
 * Lithomyrmex Clark, 1928a: 30. Type-species: Lithomyrmex glauerti, by original designation.
 * Lithomyrmex junior synonym of Amblyopone: Brown, 1960a: 156.
 * Lithomyrmex junior synonym of Stigmatomma: Yoshimura & Fisher, 2012:17.
 * Lithomyrmex junior synonym of Onychomyrmex: Ward & Fisher, 2016: 691.

Wheeler (1916):

Worker
Small, slender, monomorphic. Mandibles rather long, narrow at the base, broadest in the middle, with long, curved, acute tips, their inner borders armed with a number of unequal teeth, some of which, near the middle of the series, are directed backward. Both the maxillary and labial palpi very short, 2-jointed. Clypeus very short, abrupt, with rounded, entire anterior border beset with a regular row of minute teeth. Frontal carinae small, prominent, closely approximated, enlarged and dilated anteriorly, separated by a very narrow groove. Frontal groove lacking. Eyes very small, consisting of about 6 or 8 ommatidia, situated behind the middle of the head. Antennae 12-jointed, funiculus filiform, not clavate or conspicuously enlarged at the tip. Thorax slender, with very distinct promesonotal and mesoepinotal sutures; mesonotum smull, digeoidal, with distinet sutures on all sides. Petiole with a short peduncle in front and a large, prominent compressed ventral projection, the node rounded, scarcely narrowed behind where it articulates by means of its whole posterior surface with the postpetiole. Postpetiole large, convex below, separated by a pronounced constriction from the gaster, which is rather short. Sting very long and well-developed. Legs long; middle and hind tibiae without spurs; terminal joints of the middle and hind tarsi conspicuously elongated and incrassated, with very large, strongly curved, simple claws and large pulvilli.

Queen
Apterous and ergatoid. Head broadened in front and more depressed at the anterior corners than in the worker. Eyes very small; ocelli absent. Mandibles more falcate, not abruptly curved at the tips, with only a few short, blunt teeth. Mesonotum somewhat longer than in the worker. Petiole differing from that of the worker in being much broader, with a very short and narrow peduncle and lacking the ventral projection. Constriction between the postpetiole and gaster much less distinct than in the worker. Gaster much larger, elongate elliptical, sting somewhat smaller. In other respects like the worker.

Larva
Slender, smooth and nontuberculate, with twelve very distinct postcephalic segments, the constrictions between which are everywhere deep and conspicuous, even at the posterior end of the body. Head short, rounded, with well-developed, slender, acute, falcate mandibles, destitute of teeth. Clypeus rather long, projecting. Antennae very small. Maxillary sensillae long and prominent. Head sparsely, remainder of body more densely and uniformly covered with short, straight, stiff hairs or bristles.