Adetomyrma venatrix, Ward, P. S., 1994

Ward, P. S., 1994, Adetomyrma, an enigmatic new ant genus from Madagascar (Hymenoptera: Formicidae), and its implications for ant phylogeny., Systematic Entomology 19, pp. 159-175 : 161-167

Adetomyrma venatrix sp. n. (Figs 1 - 7,12,13, 18, 24, 30,

36, 41)

Holotype worker. MADAGASCAR, Zombitse Forest, along Route Nationale 7. 15 km E Sakaraha , 760 m, 22 ° 54 ' S, 44 ° 41 ' E, 15 February 1993, P. S. Ward no. 11932 , ex rotten log, tropical dry forest ( MCZC) .

Paratypes. Series of twenty workers, same data as holotype (to be deposited in ANIC, BMNH, LACM, MCZC, MNHN, PBZT, PSWC, UCDC) .

Worker measurements (n = 13). HW 0.40 - 0.49, HL 0.48 - 0.56, SL 0.29 - 0.34, PW 0.28 - 0.33, DPW 0.20 - 0.27, LHT 0.32 - 0.37, CI 0.83 - 0.90, SI 0.66 - 0.73.

Description (worker). Small (HW <0.50 mm), pale and blind. Mandibles subfalcate, without distinct basal and masticatory margins (Fig. 12); inner margin with 3 or (more commonly) 4 teeth, equally spaced and lying in the same plane as the front of the head, followed by a gap (0.05 - 0.06 mm) and two longer (subapical and apical) teeth which, as a consequence of the curvature of the mandibles, lie in the dorsoventral plane when the mandibles are closed. Closed mandibles with apices overlapping. Clypeus very short, its principal surface deflected vent rally, anterior margin broadly convex and furnished with a row of about 20 small, specialized, conical setae (Figs 7, 12). Frontal carinae short, low, expanded laterally as small frontal lobes that over no more than about a third of the antennal insertions (dorsal view). Medial portion of the antennal sclerite (torulus) upturned and fusing with the frontal carinae. Scape notably shorter than head length (SL / HL 0.59 - 0.61); first funicular segment c. 2.3 times longer than broad, and approximately equal to the combined length of the next three funicular segments; funicular segments 2 - 8 broader than long, segments 8 - 11 becoming gradually enlarged but not forming a distinct club. Terminal funicular segment c. 2.5 times longer than penultimate segment, and about half the scape length. Head subquadrate (Fig. 6), longer than wide (CI 0.83 - 0.90), widest near the mandibular insertions; sides slightly convex, converging posteriorly and rounding into the concave posterior margin. Mesosoma dorsum somewhat flattened in profile, lateral margins rounded; in dorsal view pronotum longer than broad, with convex sides, mesonotum very short and twice as wide as long (Fig. 4). Basal (= dorsal) face of propodeum narrower than pronotum, about 1.5 times longer than wide, with subparallel sides that converge slightly towards the mesonotum; basal face of propodeum about 2.5 times the length of the declivitous face, and rounding gently into the latter (Figs 1, 24). Metapleuron fully fused with propodeum, the two not distinguishable in lateral view. Metapleural gland bulla conspicuous, manifested as a large circular patch on the lower posterolateral corner of the mesosoma, its dorsoventral height about two-thirds the length of the declivitous face of the propodeum. Inferior propodeal (' metapleural') lobes essentially undeveloped. Abdominal tergum 2 c. 1.4 times broader than long, in dorsal view. Abdominal sternum 2 with a conspicuous subpetiolar process, shaped like an irregular axe blade (Fig. 2). Abdominal sternum 3 with anteroventral surface evenly convex, lacking protuberant ridges near the helcium.

Mandibles smooth with scattered punctures. Most of body smooth and shining; head and mesosoma dorsum with numerous piligerous punctures (c. 0.010 - 0.015 mm diameter) separated by one to several times their diameters, densest on the head (except for a smooth puncture-free median strip). A few scattered punctures on abdominal tergum 2, remainder of metasoma with small, less con- sulcuous punctalae preceded on the exposed portions or the interior margins of each sclerite by fine transverse reticulate-striolate sculpture. Sides of propodeum and metapleuron with weak reticulations. Body with a rather dense cover of pale erect and suberect hairs; more than 30 standing hairs visible in profile on the mesosoma dorsum: anterior margin of clypeus with a row of long (up to 0.12 mm), slender, curved setae (dorsad of the specialized

tooth like setae) that exceed the closed mandibles; erect setae also present on the scapes, funiculi and extension surfaces of the tibiae. Colour: light yellow-brown, with narrow darker bands at the posterior margins of abdominal segments 2 to 4 or 5.

Comments. Features of Adetomyrma venatrix that are likely to be species-specific include the small size, man-dibular dentition, body sculpture, dense standing pilosity, size and density of clypeal setae, and shape of the antero-ventral petiolar tooth.

Larva. A single ant larva, recovered from the vial con-taining the workers, may be that of A. venatrix . It is 2.46 mm long and essentially " leptanilloid " (Wheeler & Wheeler, 1976) in shape, i. e. long, slender, and club-shaped, widest neat the posterior end (at abdominal segments 8 and 9). The thorax is slender and curved ventrally. The body hairs are numerous, short and inconspicuous. No thoracic protuberances or specialized dorsal tubercles were detected.

Biology. The twenty-one workers were collected from the lower surface of a rotten log. at the log / soil interface, in a tract of tropical dry forest in western Madagascar. The workers appeared to be foraging as a group, much in the manner of several small Cerapachys species that are characteristic of the dry forest of western Madagascar, although it is possible that they were recruiting to a prey item (not seen). Unfortunately time did not permit a detailed search for the colony. One of the workers stung my finger and this produced a noticeable stinging sensation (and later a slight swelling that persisted for several days) despite the minute size of the worker. It seems reasonable to surmise thai Adetomyrma venatrix is a specialized

predator or ground-dwelling arthropods. The apparent group foraging behaviour is suggestive of the habits of leptanilline ants (Masuko. 1990) and true army arm (Got-wald. 1982) and leads to the prediction that the queen of Adetomyrma will prove to be a morphologically specialized wingless female.

The collection took place after a period of exceptionally heavy rains on this part of the island that effectively broke a 2 - year drought. It seems likely that this ant is usually subterranean and elusive, and that its discovery was aided by the wet soil conditions. A Winkler litter sample taken at the same site faded to produce additional material of Adetomyrma .

The Zombitse Forest where Adetomyrma was found (see illustration in Tattersall. 1982: 31). although falling within the bounds of what is considered tropical dry forest, is nevertheless more [[ ... ]] than most of the dry forests of western Madagascar. Moreover, the forest is under severe threat from human activities. Large swaths of the forest along Route Nationale 7 east of Sakaraha have been destroyed by slash-and-burn agriculture. After a few cycles of corn and other crops the land becomes a degraded savannah woodland. It seems certain that the collection site for Adetomyrma which is located no more than 100 m from the main road will suffer the same fate unless urgent protective measures are taken.

Relationship to other formicids

Adetomyrma presents something of a puzzle. At first glance it would appear to be unplaceable in any of the existing ant subfamilies since it possesses none of the derived traits that individually characterize them (Baroni Urbani et al.

incompleta .

1992; Bolton, 1994), In Bolton's (1994) subfamily key, for example, it stalks at couplet 11 - a terminal couplet for Apomyrminae and Ponerinae (part) - because it dis-plays a mixture of features from both lugs of the couplet. The lack of tergosternal fusion of abdominal segment 4 would seem to preclude placement of Adetomyrma in the Ponerinae . At the same time Adetomyrma exhibits almost none of the distinctive characteristics of the ' doryline section' of subfamilies (Bolton, 1990 b) such as a horizontal torulus. protruding helcial sternite specialized pygidium. reduction / loss of furcula, metatibial gland, or cuticular flap over the metapleural gland. The exposed spiracle on abdominal segment 5 is reminiscent of the greater exposure that occurs, presumably convergently, in the doryline section. Finally, the unfused condition of abdominal segment 3 in Adetomyrma indicates that it does not even belong to the more inclusive ' poneroid group' (Bolton, 1990 b), i. e. that group of subfamilies, comprising Ponerinae , Leptanillinae , Apomyrminae and the ' doryline section', whose workers show tergosternal fusion of abdominal segment 3 and all castes of which exhibit fusion of the presclerites of the same segment (Bolton, 1990 b; Ward, 1990; Baroni Urbani et al., 1992). Since Adetomyrma has an apparently fused helcium (presclerites 3) this could imply that it is in a basal position, perhaps as a sister of the entire poneroid group.

A survey of additional character systems, beyond those used for subfamily characterization, became necessary for clarifying the phylogenetic affinities of Adetomyrma . Focussing in particular on the morphology of the clypeal setae, metapleural gland, metacoxal cavities and petiolar sclerites, this survey revealed striking similarities (documented below) between Adetomyrma and members of the ponerine tribe Amblyoponini , but not between Adetomyrma and any other ants. The results support placement of Adetomyrma in this tribe, and hence in the subfamily Ponerinae , despite the absence of tergosternal fusion.