Ocymyrmex

Bolton, B., 1981, A revision of six minor genera of Myrmicinae (Hymenoptera: Formicidae) in the Ethiopian zoogeographical region., Bulletin of the British Museum (Natural History) Entomology 43, pp. 245-307 : 260-262

OCYMYRMEX Emery

(Figs 18 - 32)

Ocymyrmex Emery, 1886: 364.

Type-species: Ocymyrmex barbiger Emery , loc. cit.; by monotypy.

Diagnosis of worker. Monomorphic myrmicine ants. Mandibles short and powerful, armed with five sharp teeth which decrease in size from apex to base. The third and fourth teeth, counting from the apical, are paired, having flanking teeth internally on the masticatory margin which are only visible when the mandibles are open. Palp formula 4, 3 in barbiger , but 3, 3 is the predominant count (thus in ankhu , celer , foreli , fortior , micans , monardi , nitidulus , phraxus , picardi , shushan , sobek , sphinx , velox ). Ventral surface of head with a strongly developed psammophore, the ammochaete hairs arising on the gular surface, base of the ventral borders of the mandibles and bases of the mouthparts. Clypeus large, projecting over the basal borders of the mandibles; posteriorly the clypeus broadly inserted between the frontal lobes. Frontal lobes well developed but short, mostly or wholly covering the antennal insertions, ending at the same level as do the antennal fossae; frontal carinae and antennal scrobes absent. Antennae with 12 segments, filiform, without an apical club. Eyes well developed, situated slightly behind the midlength of the sides of the head and usually failing to break the outline of the sides in full-face view. Mesothoracic spiracles opening high on the sides, clearly visible in dorsal view, with a slit-like or crescent-shaped orifice. Propodeal spiracle extremely elongate, slit-shaped and very conspicuous. Propodeum unarmed, rounded in all known species. Legs extremely long and slender, their coxae large and powerful. Petiole with a long narrow anterior peduncle and with a rounded node which is usually low and small. Behind the node a short posterior peduncle is present which runs to the articulation with the postpetiole. Postpetiole low and generally shallowly curved, often voluminous but not usually forming a prominent node. Sting small, perhaps not functional. First segment of gaster often with a narrow neck-like constriction basally (not in barbiger and allies), the sides of the tergite usually with a series of roughly transverse, parallel indentations or grooves.

Diagnosis of females (queens). Extremely ergatoid, answering to all the characters stated above and differing from the workers only slightly, having thicker scapes, broader and more parallel frontal lobes, and usually possessing conspicuous transverse sculpture on the head. Characters normally associated with female ants, such as larger eyes, presence of ocelli, swollen alitrunk with flight sclerites and wings etc., are never developed. Females are discussed in more detail below.

This easily defined and spectacular genus, which has not been revised previously, is confined to the Ethiopian zoogeographical region where its 23 species inhabit dry to semi-desert conditions in the eastern and southern parts of the continent. All the species nest directly into the ground, either in the open or at the bases of plants. In the former case the nest is usually in sandy soil and a crater is formed around the entrance hole. Arnold (1916), who was acquainted with and reviewed the South African species, pointed out the remarkable swiftness of these ants, saying that for speed they far outstrip ' all other ants with which I am acquainted, so much so that they appear almost to fly over the surface of the ground'. Prins (1965) has recorded that Ocymyrmex species are granivorous but will also attack and destroy other insects.

In the classification of both Emery (1922) and Wheeler (1922) the genus Ocymyrmex is the sole constituent of its own tribe, the Ocymyrmecini , and Kugler's (1978) study ofthe sting structure does nothing to undermine this view. In general the construction of the head in Ocymyrmex suggests affinities with the pheidoline genera, but so many specialized characters are present that this cannot be certain. What does seem certain is that Ocymyrmecini is best retained as a separate tribe. It is easily isolated by the form of the alitrunk spiracles, which are unique amongst the Myrmicinae .

Arnold (1916) and Emery (1922) both recorded that no females of Ocymyrmex had ever been found, but they were both aware of the presence of strange variants in a number of nest-series which had transverse sculpture on the head instead of the usual longitudinal form. Several of these variants were described from isolated examples as separate species or subspecies, despite the fact that long ago Arnold (1916) had recorded that they occurred in the same nests as the more normally sculptured form.

It is now apparent that these forms are in fact the ergatoid females of the species; their resemblance to the workers is truly remarkable. Most characters regarded as normal for female ants are absent or have been suppressed, the body is extremely worker-like, without trace of flight sclerites, and the head lacks ocelli or enlarged eyes. The head, however, has three specializations which serve to distinguish the females from their workers; compare Figs 22 and 23.

Firstly, the outer margins of the frontal lobes are more widely separated in their posterior halves in females, and the margins of the frontal lobes behind the level of the antennal insertions are parallel or nearly so, whereas in workers they are obviously convergent behind.

Secondly, the antennal scapes are broader and frequently slightly shorter in females than in workers.

To illustrate these two points all available females were measured for width across the margins of the frontal lobes at their posteriormost point (FW), and the maximum width of the shaft of the scape (SW) discounting the apical swelling when present. The same measurements were taken for an equal number of workers chosen at random from the series in which the respective females originated. These measurements were compared with the standard measures of HW and SL, as follows (where n = number of females measured).

Finally, the dorsum of the head behind the level of the eyes usually has strong regular transverse sculpture in females, whereas such sculpture is generally longitudinal in workers. Exceptions to this occur in robecchii where both known specimens have transverse sculpture, implying that they are female, but lack the specialized characters of scapes and frontal lobes noted above. They are treated as workers in this paper although it is realized that they may turn out to be females. In hirsutus most workers have arched-transverse sculpture on the head, but in this case it is coarse, sharply developed, irregular and vermiculate. In some species ( velox ) the cephalic sculpture is reduced in both castes and may not be apparent.

A few individual workers in any species may show some transverse costulae or rugulae close to the occipital margin, especially in species where the main longitudinal cephalic sculpture arches outwards towards the occipital corners, but none have the extensive transverse sculpture developed by their respective females and their frontal lobes and antennal scapes are of the worker form.

As the females are apterous the founding of new nests must be by colony fission or by single newly mated females setting out on their own. Quite probably a relatively large number of females are retained in the nest at all times as captures are frequent. Whether all females in a colony lay eggs or whether this function is dominated by a single laying female who suppresses the rest is not known, but I suspect the latter as stray females appear to be fairly common in worker samples collected outside the nest and which are, apparently, behaving like workers.

Males, not dealt with here, are very poorly represented in collections, being known only for the species fortior , barbiger and weitzeckeri .

Synonymic list of species

ankhu sp. n.

barbiger Emery

barbatus Emery

barbiger var. robustior Stitz syn. n.

barbiger var. flavescens Stitz syn. n.

cavatodorsatus Prins

celer Weber

cursor sp. n.

flaviventris Santschi stat. n.

foreli Arnold stat. n.

fortior Santschi stat. n.

weitzeckeri st. transversus Santschi syn. n.

arnoldi Forel syn. n.

weitzekeri [sic] st. abdominalis Santschi

weitzaeckeri [sic] var. usakosensis Stitz syn. n.

hirsutus Forel

laticeps Forel

micans Forel stat. n.

monardi Santschi stat. n.

nitidulus Emery stat. n.

phraxus sp. n.

picardi Forel

carpenteri Donisthorpe syn. n.

robecchii Emery

shushan sp. n.

sobek sp. n.

sphinx sp. n.

turneri Donisthorpe

velox Santschi

weitzeckeri Emery

weitzeckeri subsp. wroughtoni Forel syn. n.

zekhem sp. n.