Leptogenys chalybaea

A member of a small complex of SE Asian Leptogenys species (L. chalybaea species group, Arimoto & Yamane 2018). In Cambodia, Thailand and Vietnam workers of species in this group have been observed linking their bodies in chains during the retrieval of freshly immobilized large millipedes (see below, the Leptogenys cyanicatena species page and Peeters & De Greef, 2015). Chains are a striking example of cooperation in ants, and this behavior is a novel method of prey transport.

Identification
Arimoto and Yamane (2018) - A member of the Leptogenys chalybaea species group. Worker: Mandible with basal angle bluntly angular; basal margin slightly concave near basal angle; masticatory margin straight, but curved ventrad near apex. Eye length in full-face view less than one-fourth of head lateral margin length; eye outer margin close to lateral outline of head. Apex of clypeal median lobe with two peg-like setae. Anterior lobe of subpetiolar process posteroventrally with acute tooth. Body blue and with metallic luster. Head nearly mostly smooth, but anterior part of frons and gena weakly longitudinally striate. Mesosternum with few transverse striae. Petiole smooth, sometimes with few longitudinal to oblique striae. Gaster smooth.

Distribution
The most recent examination of the distribution of this species (Arimoto and Yamane 2018) determined this species is known from East Malaysia (Borneo): Sarawak, Sabah. Additional occurrence data from other locations are not believed to be this same species.

Distribution based on Regional Taxon Lists
Indo-Australian Region: Borneo, Indonesia, Malaysia. Oriental Region: Cambodia, Thailand.

Biology
Arimoto and Yamane (2018) - Leptogenys chalybaea was observed nesting in soil under a rock and foraging on the forest floor in Borneo, Malaysia. The hunting behavior of this species has not been observed. The ecology of this species is thought to be similar to that of Leptogenys cyanicatena (as described on the L. cyanicatena species page, in Peeters & De Greef, 2015 and below) due to their similar morphology.

Peeters & De Greef (2015) - Chain behaviour in L. cyanicatena functions for the collective transport of large millipedes. This bluish species is a swarm raider with a small range of prey: millipedes belonging to four orders, and occasional earthworms. Small prey were carried individually or dragged by a few ants, while chains made it possible to move millipedes weighing up to 16.4 g. Chains (either linear or branched) changed according to obstacles along the way to the nests. Between 2 and 52 workers were observed to drag single prey items, with only a few ants directly grasping the prey. One 15-cm-long millipede was captured rodeo-style after being encircled by 25–30 ants. As it uncurled from a defensive coil, the ants held back except one that tried to sting between its legs. The millipede started thrashing about which caused many ants to attempt stinging.



Many species of Leptogenys with large workers occur in Africa and the Neotropics (Bolton 1975; Lattke 2011), while large millipedes are also distributed on these continents. Yet chain behavior has never been reported outside SE Asia, suggesting that its evolution requires a complex combination of traits (Peeters & De Greef 2015). Four other ponerine genera hunt millipedes solitarily, but chains are not used.

Nomenclature

 *  chalybaea. Lobopelta chalybaea Emery, 1887b: 432 (w.) BORNEO. Combination in Leptogenys (Lobopelta): Emery, 1911d: 102.

Worker
Arimoto and Yamane (2018) - (n=5, lectotype in parentheses). HL: 2.22–2.47 (2.44), HLL: 1.82–2.04 (2.04), HLA: 0.75–0.89 (0.89), HW: 1.68–1.94 (1.94), CML: 0.44–0.55 (0.55), CI: 75–80 (79), CLI: 20–23 (23), SL: 2.52–2.82 (2.72), SI: 140–152 (140), EL: 0.41–0.47 (0.47), OI: 22–24 (23), PrL: 1.39–1.53 (1.46), PrH: 0.92–1.10 (0.93), PrW: 1.33–1.51 (1.48), WL: 3.74–4.03 (4.00), PeL: 0.99–1.12 (1.12), PeH: 1.27–1.41 (1.36), PeW: 0.86–1.02 (1.02), LPI: 121–129 (121), DPI: 87– 95 (95).

Head width in full-face view less than 0.8 times as long as its length. Mandible with basal angle bluntly angular, sometimes rounded; basal margin slightly concave near basal angle; masticatory margin straight, but curved ventrad near apex. Eye length in full-face view less than one-fourth of head lateral margin length; outer margin close to lateral outline of head. Antennal scape surpassing posterior head margin by more than two-fifths its length; antennomere XII approximately 2.7 times as long as wide. Clypeus without longitudinal median carina; apex of clypeal median lobe with two peg-like setae. Pronotum in dorsal view shorter to longer than wide. Propodeal dorsum in profile almost straight; declivity separated from dorsum by blunt angle, concave and approximately half of length of dorsum; propodeal spiracle elliptical. Petiole in dorsal view distinctly longer than wide. Anterior lobe of subpetiolar process posteroventrally with acute tooth. Head nearly mostly smooth, but anterior part of frons, and gena weakly longitudinally striate. Head posterior to eye with scalloped depressions, which shallowly impressed posterolaterally and sloping anteromedially. Mandible with longitudinal to oblique striae. Antennal scape smooth basally and becoming weakly imbricate apicad. Clypeus smooth, but with few longitudinal to oblique striae near anterior margin. Mesosoma mostly with weak oblique to longitudinal striae; anteroventral margin of mesopleuron with distinct transverse coarse costulae; prododeal dorsum with scalloped depressions, which shallowly impressed anteriorly and sloping posteriorly; propodeal side medially smooth; propodeal declivity transversely striate. Meso-metapleural suture well impressed, broad and scrobiculate. Metapleuro-propodeal suture impressed and narrow. Mesosternum with few transverse striae. Coxa smooth. Petiole smooth, but node sometimes with few longitudinal to oblique striae; subpetiolar process with slight alveolate. Gaster smooth; anterior margin of gastral segments II with a brief scrobiculate band. Body dark-blue to light-blue and with metallic luster. Antennal scape dark-blue; antennomeres II–XII red-brown, sometimes antennomeres II–IV blackish. Mandible and clypeus dark-red; median lobe of clypeus tinged with blue and with metallic luster. Coxa, femur and tibia dark-blue, tinged with red; tarsi red-brown. Ventral half of petiole and gastral sternites blackish. Hairs yellow-brown. Antennal scape with dense short pubescence and with sparse erect long hairs. Erect hairs moderate in density on dorsum of body.

Queen
Arimoto and Yamane (2018) - (n=2). HL: 2.11–2.19, HLL: 1.74-1.80, HLA: 0.76, HW: 1.69–1.74, CML: 0.47-0.49, CI: 79–80, CLI: 22, SL: 2.24–2.33, SI: 133–134, EL: 0.37-0.38, OI: 20-21, PrL: 0.85-0.96, PrH: 0.85-0.96, PrW: 1.22-1.23, WL: 3.35–3.38, PeL: 0.90–1.00, PeH: 1.44–1.48, PeW: 0.76–0.78, LPI: 144-164, DPI: 76–86. Propodeal declivity separated from dorsum by blunt angle. Petiolar node high rectangular in profile; anterior margin parallel in posterior margin; dorsal margin truncate. Anterior lobe of the petiolar process with rounded posteroventral angle. Petiole and gaster smooth. Subpetiolar process with slight alveolate.

Determination Clarifications
Arimoto and Yamane (2018) - Mayr (1872) recorded Lobopelta iridescens (Smith, 1858) from Borneo; however, the specimen had been misidentified. This species was described as Lobopelta chalybaea Emery 1887 from Sarawak, Malaysia. Subsequently, it has been treated as Leptogenys chalybaea, after which Lobopelta was synonymized with Leptogenys (Bolton, 1995). Leptogenys chalybaea has also been recorded from Thailand (Jaitrong and Nabhitabhata, 2005), but that specimen was also misidentified. The Thailand species closely resembles Leptogenys chalybaea; however, they can be distinguished by the configuration of the mandible and eye, clypeus, and body sculpture (see notes for Leptogenys cyanicatena). Therefore, we determined that the Thailand species remains undescribed