Holotype worker: Total length (TL) 2.9, head length (HL) 0.68, head width (HW) 0.60, mandibles extend beyond median clypeal free margin (ML) 0.11 mm; when head is tilted back a little from full-face view, mandibles may extend beyond clypeus as much as 0.18 mm; trunk length (WL) 0.64, scape L (excluding radicle) 0.32, greatest length of eye 0.07 mm. Petiole length (chord of arc) 0.45, hind femur length 0.43, hind tibia length 0.34, hind tarsus length 0.60 mm, of which metatarsus is half.
Details of form and sculpture are well shown in the figures. As seen in dorsal view, eyes hidden in full-face view, barely visible when head is tilted back slightly, as in fig. 1. Pronotum slightly broader than long (width 0.38 mm), rounded in front, excised behind; mesonotum subquadratic, about 0.20 mm wide, with feebly convex borders on all four sides. Petiolar node 0.17 mm wide, postpetiolar node with rounded sides, slightly wider behind (width 0.21 mm, length 0.20 mm).
Pronotum smooth and shining, with about 10 costulae running longitudinally at different lengths; mesonotum with 5 longitudinal costulae on rugulae. About 5 longitudinal rugules each on petiolar and postpetiolar nodes, but weaker on the postpetiole, so that its disc is primarily smooth and shining like the gaster.
Fig 1. Pilotrochus besmerusHNS,
and species, holotype worker, head in dorsal view, tilted back slightly from the full-face position so as to show the mandibular dentition better, X 67 Drawing by Susan Poulakis
Hairs mostly a little less than 0.1 mm long on anterior head and scapes, a little more than 0.1 mm on posterior vertex; 0.15 - 0.25 on trunk, petiole, postpetiole and gaster, becoming shorter again at gastric apex; about 0.1 to about 0.3 mm on legs; flagelliform, many with tips reflexed or even looped back.
Holotype (Museum of Comparative Zoology at Harvard University) a unique worker taken in a Berlese sample of forest humus and litter from along the road to Anosibe, 33 km south of Mora-manga, in east central Madagascar, 4 - 12 April 1975 (A. Peyrieras).
The relationships of PilotrochusHNS are obscure. The shape of the head, with its broad and deep scrobes and small ventrolateral eyes, recalls that of DacetinopsHNS, or the CodiomyrmexHNS group of DacetiniHNS, or even TatuidrisHNS, but PilotrochusHNS differs strongly from all of these in its 8 - segmented antennae, in the form of its mandibles and their teeth, and in the form of its trunk and petiole. The lateral " hairwheel " organs are also striking, and so far as I am aware are unmatched among the ants, although the region of the ventral furrow between pro- and mesothorax is sometimes modified and apparently glandular in some dacetines and a few other myrmicines.
Fig. 2. Pilotrochus besmerusHNS,
and species, holotype worker in side view, X 67. Drawing by Susan Poulakis.
The 2 - segmented antennal club, together with some points of habitus (especially coarse sculpture and long, flexuous pilosity, plus the antennal scrobes), recalls the neotropical genus LachnomyrmexHNS, but the shape of the trunk in PilotrochusHNS is completely different, the propodeal teeth so prominent in LachnomyrmexHNS are completely absent, and the mandibles in the two genera are very different.
At the moment, all one can say without seeing the winged forms and larvae of PilotrochusHNS, and without knowing something of its lifeway, is that it is a member of subfamily MyrmicinaeHNS, but one not belonging to any of the well-circumscribed " higher " tribes ( CrematogastriniHNS, DacetiniHNS, BasicerotiniHNS, CataulaciniHNS, AttiniHNS, etc.). Thus, it falls among the mass of generic complexes related to MyrmicaHNS, PheidoleHNS, MyrmecinaHNS, RogeriaHNS, etc., among which tribal boundaries are impossible to define for the present, or at least are in dispute.
Considering the revisionary work that must be done before we have a rational tribal classification of MyrmicinaeHNS, it seems to me that nothing would be gained by erecting a new tribe for PilotrochusHNS, even though for the time being I am able to fit it comfortably into any existing myrmicine tribe.
The generic name is derived from the Greek ' pilos' (hair) + ' tro-chos' (wheel), while the specific name besmerusHNS combines the Latin ' bes' (eight of twelve) with the Greek ' meros' (part), in reference to the 8 - segmented antennae as compared to the primitive myrmicine (and formicid) 12 - merous condition.