Dorylus (Anomma) arcens Westwood

Type location Liberia (Anomma arcens, Westwood, 1847b: 16, illustrated, worker; describing the ants in the paper by T.S. Savage, 1847, from Cape Palmas; also illustrated by Santschi, 1912b: 155); junior synonym pubescens (Anomma pubescens, Roger, 1861a: 47, worker) from Liberia .

Westwood's (1849) description, as published in Savage (1849), is at . Santschi (1912b) had notes, these are at . Haldeman (1849a) paper on Savage's collection of rubella "red drivers" from Gabon, noted that "the abdominal peduncle [petiole] in arcens is slightly smaller than in rubella. ... arcens [from Liberia] being black"; this is at . Roger's (1861a) description of pubescens minor workers is at .

Wheeler (1922) remarked upon specimens that he thought were of the subspecies arcens, as being very dark almost black, the specimens were no more than 10.5 mm long, whereas "the largest workers, according to Emery and Santschi, measure 13 mm". The surface of the body is very shining the head more opaque in front. He described subspecies sjoestedti as like burmeisteri in having the inferoposterior angles of the petiole prolonged outwards as distinct tubercles but readily distinguished by having the heads of the larger workers (7.5 to 12 mm) opaque instead of shining and that of the smaller workers elongate.

Raignier & van Boven (1955) remarked that the original descriptions by Westwood (1847-1849) did not give them any unequivocal characteristics. Westwood laid an emphasis on the petiole having small posterior ventral and laterally flared lobes. This they found, from many specimens in Yangambi, to be very variable among the several subspecies and varieties of nigricans. Roger (1861) found specimens from Angola to have characters of both burmeisteri and arcens. Emery (1891) noted, however, that arcens had the characteristic form with the head strongly narrowed behind and with pointed angles; with a stronger sculpture giving the head a matt aspect. On the other hand burmeisteri had the sides of the head more arcuate and the posterior angles less acute, more rounded; the fine reticulate punctuation more superficial and leaving the head quite shiny in the posterior part. Also, arcens was more robust, with the mandibles more strongly arcuate. Raignier & van Boven commented that the variability they had observed rendered this separation on levels of strength of sculpturation unsound. In their examination of specimens they collected at Yangambi, they did not encounter any they denoted as arcens, although Collection 21b, of "burmeisteri var. terrificus" had the form of the head - with the posterior angles rounded and the cheeks convex, CI about 106 - as typical of either arcens or terrificus but the colour was too bright for arcens and they plumped for terrificus, with the cheeks rounded and the rear of the head shiny. I found their comments confusing as they cited the description of terrificus by Santschi (1923) - differing from burmeisteri in being more robust; the head of the major is large and trapezoidal; the sides are more convex, as with arcens, although the funiculus segments are more clearly longer as in burmeisteri; the head is glabrous and shiny, except the anterior which is semi-matt; the petiole has the same form as burmeisteri. Raignier & van Boven commented, however, that two specimens in the Royal Congo Museum, apparently determined by Santschi, had heads which are entirely matt.

Thus, here, I have opted for arcens as being the species with a totally matt head. The terrificus specimens (see below) have the much shinier appearance and the long funiculus segments.

Wheeler (1922) listed findings from Sierra Leone (Kortright Hill, by W.G. Clements); Ivory Coast (by Lohier, and at Assinie by C. Alluaud); Ghana (Gold Coast, Aburi by F. Silvestri); Congo (French Congo, Fort Crampel by Schubotz; Brazzaville, by A. Weiss); Gabon; Zaïre (Medje, Lang & Chapin).

Three forms were found in Guinea (Bernard, 1952). Typical form, dark red, head often black, abundant at N'Zo, 500 m, several workers at Yanlé (station F19). ssp. arcens, black and more matt, head more convex, large worker from Yalanzou, purely a western form. ssp. burmeisteri red and matt, more widespread, reaching 2400 m, at the peak of Fernando Po - Lamotte took several minor workers in his transects but never found a colony; common at Yanlé, Nion, N'Zo (rare), Camp I of Mount Tô (1600 m, abundant).

Polymorphism

The photomontages are of specimens collected at Bossou, south eastern Guinea by Tatyana Humle (Humle 7, 11.viii.2001). The ant colony was observed as eaten by chimpanzees using the "ant-dipping" (using tools to gather the ants). The heavy bulbous head and mandible with no more than a small basal tooth and no other teeth or denticles are characteristic. The various morphs are shown in detail on the Dorylus (Anomma) arcens morphs page.

In Nigeria, collected at CRIN (B. Bolton). My earlier description of species A was - TL 12.88-6.50 mm. Five morphs; largest HL 3.61, HW 3.80, SL 1.90, PW 1.52, petiole length 1.12
Colour red-brown, head darker, anteriorly near black; all morphs similarly coloured. Erect hairs only on sternites and subpetiole. Head massive, greatest width just before anterior margin, and a deeply convex posterior margin; in profile with a very convex dorsum. Anterior clypeal margin shallowly concave, only sparse short erect setae. Mandibles with a very long apical tooth and a smooth inner margin to a much reduced basal tooth, only much reduced setae on inner margin. Antennal scapes narrow, parallel sided. Promesonotum with a near flat dorsum. Propodeum near flat; spiracle ovoid vertically. Subpetiolar process very broad based, a small anterior extension and a slightly longer, triangular posterior extension.

Full new description - Humle 7
Overall - shiny matt very dark chestnut
TL 10.5 HW 3.5 HL 3.25 HD 2.45 CI 108 SL 1.75 SI 54 AL 3.5 PW 1.2 PetL 1.0 GL 4.25 MFL 2.75
Head - trapezoid, widest pre-anterior, posterior no more than 2/3 as wide near straight, rear third angled inward; posterior margin more shallow scallop but slight median outward bump, posterolateral angles sharp; sculpturation of extremely fine spiculation, almost effaced on posterior half of face, which has minute sparse hair pits; median line visible only in some angles of light as a faint impression; clypeal margin very shallow concave arc, no median hair, very fine pair away from midpoint; mandible long and slender, with small basal tooth, minute hair pits; scape moderately thick, broadening from base to apex, near smooth; funiculus segments progressively lengthening and widening; apex no more than 1/3 longer than 9; head in profile very bulbous, posterior corners fairly acute angles; hairs barely visible @ 32x.
Alitrunk - in profile a shallow dome, near flat dorsally distinct "saddle"; spiracle a vertical oval rearward facing; declivity higher than others with incurve at bottom; metapleural gland with upper margin distinctly longer than lower; all over fine spiculation, coarsening rearward and downwards; hairs none.
Petiole - moderately long, flat dorsum, posterior face vertical and in turned; dorsum basally widening front to back; spiracle small but distinctly raised; subpetiolar process a moderately large rearward facing triangle; sculpturation as alitrunk; hairs several below.
Gaster - all segments very finely spiculated like the head; waisted but less so than others; hairs sparse on posterior margins, fine pilosity on apical segments.
Legs - coxae moderately long but more rounded outer surface; femora broad, very fine spiculation; tibiae moderately wide, coarse pilosity; tarsae quite long, bristly, large claws.