Secostruma lethifera

The only species in its genus, this ant is known from a single worker. Based on the collection information (soil sample) and worker morphology, Bolton (1988) inferred: "Secostruma is well adapted for a subterranean and apparently carnivorous lifeway. It has vestigial eyes, large powerful mandibles, and a striking modification of the gaster which, it is postulated, is specialized to bring the sting into play in confined spaces or tunnels in the earth."

Identification
Autapomorphies isolating this genus (see the nomenclature section of the genus page) include the unique structure of the mandibles and the construction of the gaster which are not duplicated elsewhere in the Myrmicinae. The combination of characters given in the diagnosis of the genus immediately isolates Secostruma (and its only species Secostruma lethifera) from all other known myrmicine ants.

Distribution based on Regional Taxon Lists
Indo-Australian Region: Borneo.

Biology
Bolton (1988) inferred the following from the collection information and morphology of the single known worker: Like many deeply subterranean ants S.lethifera has very reduced eyes, but it is not depigmented and its sculpture everywhere is strong. The whole ant has a very armoured appearance and the aspect of a thoroughly predaceous species. The striking modification of the gaster gives added protection to the dorsum, but primarily it brings the sting into a ventral position when the gaster is horizontal. This is most probably an aid to employing the sting in a relatively confined space, where free movement of the entire gaster to bring an apically-placed sting into play would be very difficult. If the long narrow petiole is elevated against the propodeal declivity and the postpetiole and gaster are flexed downwards at the petiole-postpetiole and postpetiole-gaster joints, then the sting would be directed approximately anteriorly, between the legs.

This useful adaptation to life in subterranean confined spaces, coupled with the strong sculpture and armour of this ant, and its powerful specialized mandibles, renders speculation about its prey very interesting. Some ponerine genera which feed on arthropod eggs, such as Proceratium and Discothyrea (Brown, 1980b), also have the gaster downcurved. However, the mandibles in these genera are feeble by comparison and hardly resemble the powerful blades of Secostruma. The most striking feature of the mandibles of S. lethifera is their division into a sharp, blake-like edentate 'incisor' region apically, and a dentate projecting 'molar' region basally. Such a mandible would provide a good combination of penetrating and gripping power, sufficient to hold prey firmly until the sting could be brought into action. I doubt if such specializations would be necessary to deal with eggs or soft-bodied prey such as earthworms or even termites, but, taken in combination with the armoured body and deeply recessed, strongly protected, antennal insertions, they would be very efficient in coping with hard-bodied arthropods struggling in an earth tunnel or confined space in the soil. I am tempted to speculate that the prey may be geophilomorph centipedes, or even millipedes.

A few other genera of Myrmicinae show hypertrophy of the first gastral tergite. In the arboreal genus Cataulacus the first tergite forms the gastral dorsum, but here the gaster is elongate and usually flattened, segments behind the first being apicoventral and reduced. The function here is one of protection (Bolton, 1974). When threatened Cataulacus species either grip firmly onto the bark and present an almost unbroken armoured surface to an aggressor, or roll into a protective ball. The modification of the gaster in this genus has not been for the same reasons as in Secostruma. Ankylomyrma shows a development of the first gastral tergite far beyond that seen in Secostruma. In this genus (Bolton, 1981) the first tergite forms almost the whole of the gaster; it is ball-like with an anteroventral orifice within which the remaining gastral segments are telescoped. The relatively powerful sting projects anteriorly. The modification here appears to have taken place for the same reasons as in Secostruma but in a very different habitat. As far as is known Ankylomyrma occurs only in the topmost branches of high rainforest trees in West and Central Africa; its prey and biology remains unknown.

Castes
Known only from the worker caste.

Nomenclature

 *  lethifera. Secostruma lethifera Bolton, 1988c: 265, figs. 1-4 (w.) BORNEO.

Worker
HOLOTYPE. TL4.5, HL 1.00, HW 0.94, CI 94, SL 0.82, SI 87, PW 0.70, AL 1.20 (measurements in millimetres, as defined in Bolton, 1980).

First and fourth teeth of the mandibular dental row very slightly larger than second and third teeth. Mandibles longitudinally rugose basally, the sculpture fading out apically so that the vicinity of the large apical tooth is smooth. Median indentation of anterior clypeal margin continued on short near-vertical anteriormost section of clypeus as a narrow transverse concavity. Median portion of clypeus, behind the downcurved anteriormost section, with 2-3 longitudinal rugae but the sharply defined frontal triangle unsculptured. Remainder of head capsule, dorsally, laterally and ventrally, strongly reticulate-rugose everywhere. Funicular segments 2-8 of antenna much broader than long, the antennal club sharply differentiated and the segments conspicuously larger than those preceding. Vestigial eye-spots almost invisible on sides of head, at about the midlength, measuring only about 0.02; extremely difficult to see in full-face view. Dorsal, posterior and leading edge of antennal scapes with erect to suberect hairs, and also with finer, more reclinate pilosity present. All surfaces of head with short erect to sub erect hairs, the dorsum also with sparse fine pubescence which is roughly directed towards the midline. Alitrunk dorsally and laterally reticulaterugose, the forecoxae similarly sculptured. Femora and tibiae rugulose to reticulate-rugulose, the basitarsi with fine longtidinal rugular sculpture. Dorsal alitrunk and all surfaces of legs with numerous erect to suberect short hairs. Side of pronotum with a flattened to slightly concave anterolateral area, behind the lower occipital corners of the head. Mesopleuron with a broad cuticular flange anteriorly which overlaps the posterior margin of the front coxa. Promesonotum convex, metanotal groove not impressed. Anterior portion of propodeal dorsum shallowly concave; behind this the propodeal dorsum in profile humped, rising to a blunt peak then sloping posteriorly to the triangular and more or less horizontal short spines. Metapleural lobes broad and deep, rounded, projecting farther posteriorly than the apices of the propodeal spines and linked to the spines by short narrow lamellae down the margins of the declivity. Petiole and postpetiole reticulate-rugose dorsally and laterally, both with numerous short standing hairs. Petiole in profile lacking a strongly differentiated node; with a short stout anterior peduncle which is almost as deep as the remainder of the subcylindrical and weakly curved segment. Petiolar spiracle situated in anterior one-third of length of segment, approximately at the end of the peduncular section. Subpetiolar process very low and rounded, giving rise ventrally to a pair of roughly parallel longitudinal ridges. Petiole subcylindrical in dorsal view, narrowing at the anterior peduncle and broadening posteriorly. Fetiole in dorsal view 0.58 long and 0.26 wide at maximum. Postpetiole fractionally broader (width 0.34) than long (0.30), with more or less straight sides and a convex posterior margin. Sternite of postpetiole reduced, very small in profile in comparison with the tergite. Gaster immediately behind postpetiole with a short flattened surface both dorsally and ventrally. First tergite comprising most of the gaster, as stated in generic diagnosis. Gaster reticulate-rugose to foveate-rugose everywhere, the sculpture on the first sternite coarser and more sharply defined than on most of the first tergite. Erect to suberect short hairs numerous on all surfaces of gaster. Colour a dull red throughout, the legs slightly lighter in shade than the body.

Type Material
Holotype worker, EAST MALAYSIA: Sabah, Gn. Silam, 810 m, soil sample, AI8/9.2, 1983 (R.Leakey).