Polyrhachis laboriosa

A common west-African species that is common in forests and forest edge. P. laboriosa builds its nests on the underside of leaves. Its abundance, coupled with its readily noticeable workers and nests, has meant this species has garnered attention from numerous mymrecologists.

Identification
A member of the Polyrhachis militaris species-group. Bolton (1973) - A member of the militaris-group, laboriosa is easily separated from all other species by the unique form of the petiole.

Distribution based on Regional Taxon Lists
Afrotropical Region: Angola, Cameroun, Congo, Democratic Republic of Congo, Equatorial Guinea, Ghana, Ivory Coast, Nigeria, Sierra Leone, Togo, Uganda.

Biology
P. laboriosa lives in pioneer trees and shrubs on forest edges in west Africa (Lenoir & Dejean 1994). Nests are built at a height of 2-3m in the canopy of small trees, using living leaves and plant debris. There is great diversity in the architecture of these nests, showing high opportunism by the ants. Leaves are often incorporated in the nests, and provide structural strength for the carton. P. laboriosa belongs to the subgenus Myrma, in which there is no weaving using larval silk. The use of spider silk for nest construction has been described for P. laboriosa (Collart 1932). J.-L. Mercier (unpublished) observed a worker collecting silk from a spider web on a leaf surface. The ant pulled on the silk strands one by one, and rolled them into a ball using her gaster. During this time, the spider stood about 5cm away, and once attempted to chase the ant. At the end, the worker carried the silk to her nest that was being constructed by about 10 nestmates.

Founding queens have been observed retrieving various vegetal material which was used to build a small cell under a leaf (Lenoir & Dejean 1994). Two of 18 foundations involved two queens.



Foraging for plant secretions and small prey or insect corpses is done mostly by single workers orientating visually, although chemical recruitment occurs when larger food sources are exploited (Mercier & Lenoir 1999).

61 nests censused near Yaoundé (Cameroon) yielded an average of 400 workers per nest (range 25-1919). Only 21% of nests had a dealated fertile queen, and such nests were more populous (881±505 workers, N=12) than queenless nests (277.5±263 workers, N=44) (J.-L. Mercier, unpublished). Five nests collected in Dja Reserve (Cameroon) contained 63-457 workers and up to 259 worker cocoons; a dealate queen was present in only one nest (K. Yéo & C. Peeters, unpublished). These data support the idea that P. laboriosa colonies are polydomous (i.e. distributed into several nests) and monogynous (single queen).



Bolton (1973) - The nests of this species, common in West Africa, are well known and consist of a mixture of vegetable fragments and small twigs bound together by silk and fungal hyphae and adherent to the undersides of leaves or situated at the fork of small branches. Form and construction of the nest have been discussed many times, for example by Wheeler (1922a:259), Santschi (1909:393) and Collart (1932). P. laboriosa appears to be restricted to forested areas. When disturbed the workers curve the gaster under the alitrunk to eject formic acid, and also tap the gaster upon the substrate, making a rattling noise when performed by a number of workers together. If individual arboreal foragers are disturbed they tend to release their grip on the bark and fall into the undergrowth.

Alate females have been recorded as follows, GHANA : July, September. CONGO (KINSHASA) : March, August.

Castes






Nomenclature

 *  laboriosa. Polyrhachis laboriosus Smith, F. 1858b: 72, pl. 4, figs. 21, 22 (w.) SIERRA LEONE. Mayr, 1896: 250 (q.m.); Wheeler, G.C. & Wheeler, J. 1953e: 207 (l.). Combination in P. (Myrma): Santschi, 1914d: 384. Senior synonym of architecta, hortulana: Bolton, 1973b: 308.
 * architecta. Polyrhachis (Myrma) laboriosa var. architecta Santschi, 1924b: 224 (w.) DEMOCRATIC REPUBLIC OF CONGO. Junior synonym of laboriosa: Bolton, 1973b: 308.
 * hortulana. Polyrhachis (Myrma) hortulana Arnold, 1955: 735, fig. 3 (w.) UGANDA. Junior synonym of laboriosa: Bolton, 1973b: 308.

Worker
Bolton (1973) - TL 10.2-11.6, HL 2.15-2.25, HW 1.56-1.81, CI 71-79, SL 2.85-3.34, SI 182-196, PW 1.19-1.36, MTL 3.26-4.11. (30 measured.)

Anterior clypeal margin arcuate, entire, the clypeus with a distinct median, longitudinal carina. Eyes strongly convex, the sides of the head behind the eyes strongly convergent to a short convex occipital margin. Alitrunk marginate throughout its length, interrupted by impressions at the sutures. Pronotum and propodeum weakly concave transversely, the former armed with a pair of long acute, divergent spines, the latter with a pair of small teeth. Promesonotal suture distinct, metanotal groove impressed. In profile the propodeum rises abruptly from the metanotal groove, behind which it slopes convexly to the propodeal teeth. Petiole armed with a single pair of spines, set at the dorsolateral corners of the scale, weakly divergent, their apices strongly hooked, directed posteriorly and somewhat laterally.

All dorsal surfaces of head, alitrunk and gaster with numerous erect hairs, varying in colour from grey to golden-yellow. Pubescence dense, mostly hiding the sculpturation on the gaster, usually grey on the head and alitrunk, golden or bronzy on the gaster, but sometimes the pubescence of the pronotal dorsum is also yellow or golden.

Head reticulate-punctuate except on the vertex where the sculpturation is of longitudinal rugulation. Pronotal dorsum superficially reticulate-punctate, the mesonotum and propodeum usually more coarsely sculptured, rugulose or more strongly reticulate-punctate; the sculpturation of the pronotum may be concealed by the pubescence. Gaster finely reticulate-punctate, mostly hidden by the dense pubescence.

Queen
Bolton (1973) - As worker, larger, with the marginations of the alitrunk less distinct. Pronotal spines and propodeal teeth reduced, the latter often to a pair of tubercles.

Type Material
Bolton (1973) - Holotype worker, SIERRA LEONE (D. F. Morgan) [examined].