Crematogaster captiosa

A widespread member of a group of Crematogaster species associated with Macaranga plants in Sumatra, Borneo, and Peninsula Malaysia.

Identification
Feldhaar et al. (2016) - A member of the Crematogaster captiosa subgroup within the Crematogaster borneensis group. Worker: Propodeal spines present, scape index (SI) < 0.65, RLEG 0.61 to 0.69. Queen: EL ~ 0.6mm, REL 0.38 - 0.48, OD1 < OW, scape length usually <0.75 mm.

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

Biology
Crematogaster captiosa is a very widespread species occurring over the whole distributional range of the Crematogaster-Macaranga association (Peninsula Malaysia, Sumatra, and Borneo). The species has a wide host-range but is usually restricted to non-waxy hosts of the section Pachystemon (Macaranga angulata, Macaranga bancana, Macaranga calcicola, Macaranga glandibracteolata, Macaranga hullettii, Macaranga indistincta, Macaranga petanostyla, Macaranga trachyphylla, Macaranga umbrosa, the non-waxy form of Macaranga aetheadenia, and the thinly wax-covered Macaranga glandibracteolata as an exception of a glaucous host species). Queens colonize seedlings as well as the tips of mature hosts that have been abandoned due to the loss of the original colony. On Peninsula Malaysia secondarily polygynous colonies that contained several queens have been found locally in the Gombak area.

Fiala et al. (2017) studied mating flights and reproductive dynamics in C. captiosa living in Macaranga bancana. The authors have acquired a good knowledge of Crematogaster-Macaranga ant-plants over decades: After localization of a host-plant, single queens enter a sapling by chewing an entrance hole into an internode. The hole is sealed from the inside and colony founding is claustral. The first workers open up the internodes and start foraging for food bodies produced by the plant as well as for honeydew from specific coccids (Fiala & Maschwitz 1990; Heckroth et al. 1998, Ueda et al. 2008). Whereas several colonies may be founded simultaneously in different internodes, only a single monogynous colony finally inhabits each host (Fiala & Maschwitz 1990; Feldhaar et al. 2003). The only fertile queen of the colony is physogastric and always situated in a low region of the stem, so all female alates found elsewhere in the tree are newly produced. Alate production of C. captiosa (provisionally named Crematogaster msp. 4 in our earlier publications) begins in colonies containing 5000 or more workers (Feldhaar et al. 2003). Crematogaster captiosa is a very widespread species occurring in Peninsula Malaysia, southern Thailand, Sumatra, and Borneo.

Mating flights were observed at a field station near Kuala Lumpur (peninsular Malaysia), and sexual production monitored through nest censuses. Mating flights occur as swarming events and take place in the evening within the first few hours of darkness. Individual colonies released sexuals on numerous nights. One observed colony released sexuals for a few nights, then did not swarm for a few nights, then swarmed again for a few nights. No obvious weather events appeared to precipitate the flights. Some matings were observed to take place on the plant. These did not appear to be individuals, i.e., male and females, leaving the same nest entrances. Most alates flew from the plant after emerging and without mating on the plant. Males began flying first, then females become more numerous. It was estimated that less than 50 females flew from a colony during each swarming event.

Mating flights apparently occur throughout the year. Alates were continually produced, as noted by finding new sexuals in nests sampled by cutting open plants with ant colonies every month for a year. Newly colonized plant saplings were also readily found throughout the year, which is indicative of a recent swarming flights. Frequent swarming of some sexuals, as opposed to a large scale annual mating period, might be the best strategy for continually providing newly mated queens to colonize saplings that are emerging throughout the year in newly disturbed sites.

Nomenclature

 *  captiosa. Crematogaster (Decacrema) captiosa Forel, 1911a: 37 (w.) BORNEO. Senior synonym of harpyia, insulsa, novem, symbia: Feldhaar, Maschwitz & Fiala, 2016: 668.
 * harpyia. Crematogaster (Decacrema) borneensis var. harpyia Forel, 1911a: 33 (w.q.) BORNEO. Junior synonym of captiosa: Feldhaar, Maschwitz & Fiala, 2016: 668.
 * insulsa. Crematogaster (Decacrema) borneensis var. insulsa Forel, 1911a: 33 (w.q.m.) BORNEO. Junior synonym of captiosa: Feldhaar, Maschwitz & Fiala, 2016: 668.
 * novem. Crematogaster (Decacrema) borneensis subsp. novem Forel, 1911a: 35 (w.q.) BORNEO. Junior synonym of captiosa: Feldhaar, Maschwitz & Fiala, 2016: 668.
 * symbia. Crematogaster (Decacrema) borneensis subsp. symbia Forel, 1911a: 34 (w.q.m.) BORNEO. Junior synonym of captiosa: Feldhaar, Maschwitz & Fiala, 2016: 668.

Worker
Feldhaar et al. (2016) - Holotype. CI 0.97, DPPW 0.19, DPW 0.2, EL 0.1-0.15, HL 0.61, HW 0.6, LHT 0.4, LPS 0.095, MTW 0.35, PI 1.07, REL 0.18, RLEG 0.59, SI 0.55, SL 0.33, (TL 2.5), WL 0.68.

(n=16) CI 0.92-0.98, DPPW 0.15-0.22, DPW 0.16-0.23, EL 0.1- 0.15, HL 0.53-0.73, HW 0.51-0.7, LHT 0.36-0.57, LPS 0.05- 1.08, MTW 0.3-0.45, PI 1.02-1.13, REL 0.17-0.21, RLEG 0.61- 0.69, SI 0.57-0.62, SL 0.31-0.45, (TL 2.3-3.3), WL 0.57-0.82.

Colour light to reddish brown (large workers) with head and gaster being a slightly darker shade than the alitrunk. Workers monomorphic in size. Total body length of workers 2.3 to 3.3 mm. Head and gaster shiny with smooth surface, alitrunk slightly less shiny and faintly shagreened. All body parts bear appressed pubescent hairs. Long flexuous setae present on head, gaster and abdomen: on head especially in frons on gaster more on the posterior margins of tergites and sternites. Only few setae on alitrunk and one pair each on petiole and postpetiole.

Head subquadratic but slightly elongated, always being longer than wide (CI<0.98) and only slightly convex on sides. Anterior clypeal margin slightly convex and with a row of long erect setae projecting anteriorly. Occipital margin slightly concavely rounded, occipital lobes rounded. Mandibles relatively short and with four denticles, capable of closing tightly against the clypeus. Denticles increasing continuously in size from posterior (close to clypeus) to anterior. Surface of mandibles smooth, covered with short pubescent hairs.

Antennae relatively short in comparison to head width (SI 0.55-0.62; mean 0.61) and covered in short pubescent hair. Terminal three funicular segments form a club. Compound eyes elliptically shaped and not protruding over margin of head in full-face view.

Pronotum and mesonotum form a convex dome in profile. Anterodorsal surface of pronotum sloping downwards as steep as posterodorsal surface of mesonotum. Metanotal groove slightly notched and clearly developed, whereas the promesonotal suture visible but not prominent.

Propodeal spines in lateral view usually strong and acute. Tip of the spines always protruding over posterior margin of the propodeal spiracle. Dorsal face of the propodeum confluent with the horizontal spines or spines bent slightly upwards. Slope of the posterior face of the propodeum similar to posterior slope of mesonotum and approximately 45° or slightly less steep.

In dorsal view petiole always wider than postpetiole (PI: 1.02-1.13). Node of petiole in dorsal view longer than wide and considerably rounded, dorsal surface flat. In profile the anterior face of the petiolar node flattened and sloping downwards anteriorly and petiole longer than postpetiole. Dorsal surface of the postpetiolar node in profile rounded, lateral nodes visible. Subpetiolar process usually absent.

Queen
Feldhaar et al. (2016) - CI 0.90-1.02, DPPW 0.44-0.58, DPW 0.41-0.55, EL 0.55-0.65, HL 1.23-1.47, HW 1.21-1.47, LHT 0.82-1.18, MTW 0.86-1.21, OD1 0.11-0.21, OD2 0.03-0.08, OW 0.16- 0.22, PI 0.92-1.10, REL 0.38-0.48, RLEG 0.42-0.49, ROD 0.1-0.17, ROD2 0.024-0.061, SI 0.52-0.59, SL 0.66-0.81, (TL 6.9-8.0), WL 2.17-2.68.

Queens are large, 6.9 to 8.0 mm in total body length and uniformly light to medium brown in colour. Surface of head and gaster smooth and shiny, alitrunk slightly less shiny and faintly shagreened. All body parts bear appressed pubescent hairs. Long flexuous setae present on head gaster and abdomen: on head especially in frons, on gaster more on the posterior margins of tergites and sternites. A row of long erect setae pointing anterior are present on the clypeus. Mandibles relatively short, capable of closing tightly against the clypeus.

Head subquadratic, always slightly longer than wide (CI: 0.90-1.02; mean 0.96). Sides of the head straight and head behind compound eyes as wide as in front of compound eyes. Occipital margin of the head slightly concave. Occipital lobes strongly rounded. Anterior clypeal margin slightly convex. Terminal four segments of funiculus continuously increasing in size forming an indistinct antennal club. Antennal scrobes strongly developed, with an acute and marked dorsal margin; the frontal carinae short.

Compound eyes oval-shaped from lateral view and convex from dorsal view, with the margin of the compound eyes protruding from the sides of the head. Compound eyes large relative to head length spanning more than one third of HL (REL 0.38-0.48). Maximum diameter of compound eyes usually from 0.55 to 0.65 mm (mean EL: 0.6 mm). Maximum width of head including compound eyes usually at least 10% wider than HW (mean 13%). Ocelli relatively large in diameter. Diameter of median ocellus (OW) always larger than distance between lateral ocelli (OD1).

Mesoscutum convexly rounded anterodorsally. Mesoscutellum nearly in horizontal plane in lateral view. Propodeum flattened dorsally and then drops off at an angle of approximately 45° posterior of the propodeal spiracle. Mesoscutum long, stretching out over approximately half of the alitrunk in lateral view. In dorsal view, the posterior margin of the propodeum forms a straight line and the mesonotum broadly triangular. Propodeum not armed with spines.

Node of petiole and postpetiole in dorsal view rounded and approximately the same width (mean PI: 0.98), but variable (PI range: 0.93-1.10). In lateral view the petiole anterodorsally flattened and sloping downwards and slightly longer than the postpetiole. Postpetiole round in dorsal and lateral view without distinct nodes.

Type Material
Feldhaar et al. (2016) - Holotype, 1 worker Sarawak (Borneo), Malaysia (Haviland). [In addition 1 queen; however in the original description of Forel (1911) only 1 worker is mentioned but no queen; collection data given on the same label, examined.]

Determination Clarifications
In former publications by our group this species was referred to as Crematogaster msp. 4 (Fiala et al., 1999; Feldhaar et al., 2003a; Feldhaar et al., 2003b).