Dorylus gribodoi Emery 1892

Schöning, Caspar, Gotwald, William H., Kronauer, Daniel J. C. & Vilhelmsen, Lars, 2008, Taxonomy of the African army ant Dorylus gribodoi Emery, 1892 (Hymenoptera, Formicidae) — new insights from DNA sequence data and morphology, Zootaxa 1749, pp. 39-52 : 42-48

Dorylus gribodoi Emery 1892

( Figures 2–5 , 7 , 9 , 11–13 )

Dorylus gribodoi Emery 1892: 570 . Two syntype males, West Africa [ Wheeler (1922) and Bolton (1995) listed Togo as country of the type location]: Amu, collected by M. Gribodo, MCSN (examined). Dorylus (Anomma) gerstäckeri (sic!) Emery 1895: 713 – 714. Holotype worker, Ghana: Accra, MCSN (examined), new synonomy.

Dorylus (Dorylus) gribodoi ; Emery 1895: 723 (combination in subgenus Dorylus ), Emery 1910: 10, Wheeler 1922: 732, Bernard 1953: 217 , Bolton 1995: 179, Kronauer et al. 2007: 56 (examined).

Dorylus (Anomma) gerstaeckeri ; Emery 1901: 415 – 436, Emery 1910: 11, Santschi 1912: 154, Santschi 1914: 332, Wheeler 1922: 736, Raignier and van Boven 1955: 85, Gotwald 1974a: 877 –886, Gotwald 1974b: 705 –713, Gotwald 1978: 161 –169, Bolton 1995: 179, Kronauer et al. 2007: 56 (examined), Schöning et al. 2007: 125–133 (examined), Boesch and Boesch 1990: 86 –99, Humle and Matsuzawa 2002: 133 –148 (examined).

Dorylus gribodoi var. confusa Santschi 1915: 246 – 247. Five syntype males, Ivory Coast: Grand Bassam and Imbokro collected by J.H. Lohier and Posth, MNHN (examined), new synonomy.

Dorylus (Dorylus) gribodoi var. confusus Santschi 1915 ; Wheeler 1922: 732, Bolton 1995: 178.

Dorylus (Anomma) gribodoi var. insularis Santschi 1937: 98 . Two syntype males, Equatorial Guinea: Bioko Island, collected by Conradt and J. Moser, NHMB (examined), new synonymy.

Dorylus (Anomma) lamottei Bernard 1953: 219 . 12 syntype workers, Guinea: Mt Nimba, MNHN (examined), new synonomy.

Dorylus (Anomma) lamottei Bernard 1953 ; van Boven and Levieux 1970: 351 –358. Dorylus (Dorylus) gribodoi var. insularis Santschi 1937 ; Bolton 1995: 179.

Dorylus gerstaeckeri complex sp. 1; Schöning et al. in press (examined).

Other material examined: Ghana: Kade ([N 6º4', W 0º49'], 150 m asl, W.H. Gotwald Jr., WHGC), Osenase ([N 5º57', E 0º45'], 180 m asl, D. Leston, WHGC), Tafo ([N 6º43', W 1º37', 278 m asl], W.H. Gotwald Jr., WHGC). Guinea: Bossou (N 7º38'42'', W 8º30'35'', 572 m asl, T. Humle, K. Koops, Y. Sugiyama, CSAC), Seringbara (N 7º37'37'', W 8º28'2'', 608 m asl, T. Humle, K. Koops, CSAC). Ivory Coast: Taï ([N 6°20' – 5°10', W 4°20' – 6°50'], 150 m asl, Y. Möbius, T. Deschner, C. Schöning, CSAC), Lamto ([N 6º13', W5º2'], 100m asl, queen specimen collected from nest along with workers in 1974, sample label #740701, J.M. Leroux, WHGC), Orstrom Experiment Station, nr. Abidjan ([N 5º20'28'', W 4º1'41''], 70 m asl, W.L. Brown, WHGC). Liberia: Charlesville ([N 6º12'22'', W 10º22'29''], 90 m asl, D. Kistner, RMCA). Nigeria: Gambari ([N 8º12'22'', W 4º19’], 308 m asl, B. Bolton, WHGC), Gashaka (N 7º19'46.1'', E 11º35'1.9'', 560 m asl, D. Ellis, CSAC), Ibadan ([N 7º23'16'', E 3º53'47''], 230 m asl, W.H. Gotwald, WHGC).

Worker measurements: Worker caste highly polymorphic, among samples from Taï ( Ivory Coast) HW varies between 0.78 mm and 2.9 mm. SL, hind leg length and mandible length notably shorter than in workers of the driver ant clade ( Kronauer et al. 2007, Schöning et al. in press).

Male measurements (n = 12, mean ± SD, range): HW 4.38 ± 0.11, 4.19 – 4.55; SL 1.57 ± 0.05, 1.48 – 1.67; HL 2.24 ± 0.07, 2.12 – 2.33; EL 1.78 ± 0.08, 1.61 – 1.88; DPF 2.77 ± 0.12, 2.57 – 2.95; PeW 3.70 ± 0.14, 3.52 – 3.90; HTL 2.29 ± 0.08, 2.07 – 2.42; PL 2.10 ± 0.08, 1.99 – 2.24; PD 1.01 ± 0.05, 0.96 – 1.09; SPWB 1.05 ± 0.07, 0.93 – 1.14.

Queen measurements (n = 1): HW 4.58, SL 1.44, HL 3.32, PeW 4.54, HFL 2.73, HAD 2.53, maximum total length, in lateral view, from anterior surface of head to tips of hypopygial apices 40.45.

Distribution: West Africa (from Guinea and Liberia to Cameroon).

Biology: Restricted to humid habitats such as rainforest and gallery forest in mixed savanna – forest landscapes. Occurs at least up to 1600 m asl. Raids are conducted in the leaf-litter stratum ( Gotwald 1974a, see also Kronauer et al. 2007). Workers retreat in tunnels when disturbed or exposed to sun ( Gotwald 1978). Limited observations indicate that the species seems to prey almost exclusively on earthworms ( Gotwald 1974b, C.S. personal observation), but this information has to be viewed with caution since variation in prey composition can be considerable in other Dorylus species ( Schöning et al. 2008). Analysis of more extensive samples is therefore needed. Chimpanzees are known to feed on D. gribodoi at two sites (Bossou in Guinea and Taï in Ivory Coast); they either use sticks to “dip” for ant workers at trails or nests or open nests and take brood and workers with their hands ( Boesch and Boesch 1990, Humle and Matsuzawa 2002, Schöning et al. in press). Nests are much less conspicuous than those of driver ant species and extremely difficult to find (for humans, chimpanzees seem to do much better). It is interesting to note that D. emeryi is broadly sympatric with D. gribodoi and has nearly identical hunting behavior and habitat requirements, such that the two species may be easily confused in the field. The largest workers of D. emeryi are much larger (HW up to 3.92 mm, Schöning et al. in press).

Comments: The diagnostic features of the D. gribodoi male are its mandible shape (the anterior quarter notably flexed inward, Fig. 3 ) ( Emery 1892) and the nearly flat posterior head margin ( Fig. 3 ). In the original description Emery compared D. gribodoi with D. atratus and stated that D. atratus was darker than D. gribodoi and that its scape was longer than half the length of the funiculus. Both Smith (1859) in the original description and later Santschi (1914) mentioned that the posterior head margin of the D. atratus male was convex. While the colouration of D. gribodoi is variable, it is not uniformly black in any of the specimens examined by us and the head shape is nearly rectangular with a flat posterior margin. Unfortunately, the D. atratus type specimen appears to be lost (George Else pers. comm., Natural History Museum London), so that we could not directly examine the relationship between D. gribodoi and D. atratus . However, the distinctness of the taxa appears beyond doubt. Workers of D. gribodoi are unique (and easily identifiable) among all Dorylus species in having a nearly round petiole in dorsal view ( Fig. 9 ) and a distinct ridge on the posterior head margin that is laterally developed into tiny horns in larger specimens (> 1.5 mm HW). Dorylus gribodoi workers of the same size class can be separated from workers of the D. kohli / D. congolensis complex also by the posterior margin of the head being angular in lateral view and not smoothly rounded ( Fig. 7 ). Larger Dorylus emeryi workers can be separated from Dorylus gribodoi workers of the same size class by the following features: posterior angles of head drawn out backwards and ventrally [“Die ziemlich dick-kegeligen, am Ende stark abgerundeten Hinterecken des Kopfes (den Kopf von der Seite gesehen und nach vorne gestreckt gedacht) ziemlich stark hinabgebogen und zwar schief nach hinten und unten gerichtet.” ( Mayr 1896)] ( Fig. 8 ), petiole in dorsal view angled at widest point ( Fig. 10 ), petiole has several conspicuous erect setae on dorsal surface (which are lacking in D. gribodoi ), largest workers have massive heads with width ≤ 3.92 mm (HWmax of largest D. gribodoi workers 2.9 mm, see above). The unique petiole shape of D. gribodoi was recognized by Emery in his original description of D. gerstaeckeri ( Emery 1895) and also by Bernard (1952) who described D. lamottei based on workers collected at Mount Nimba in Guinea but failed to realize that these specimens were in fact conspecific with Emery’s D. gerstaeckeri . Bernard (1953) also identified males taken from the same location as D. gribodoi , but the association between these males and his D. lamottei workers necessarily remained unknown to him.

A D. gribodoi queen was collected along with workers from a nest at Lamto ( Ivory Coast) by Jean-Marie Leroux and below we provide the queen description. Dorylus gribodoi is thus now one of the few Dorylus species for which workers, the male and the queen are now known.

Queen description. Habitus as in Figure 11 . Head: Antenna 11-segmented ( Fig. 12), funicular segments and scape yellow; head glossy, finely punctate, without conspicuous pilosity, antennal fossae deeply impressed; small (0.18 mm diameter) ocellus-like body located on the midline of head in frontal view 1.98 mm dorsad of the anterior clypeal border ( Fig. 12); head impressed along the midline; mandibles with smooth medial (masticatory) border (i.e., without subapical teeth), sharply pointed apical tooth directed medially; clypeal margin straight with a row of short setae; labrum smoothly emarginate medially and weakly bilobed, reminiscent of the labrum in queens of the New World genus Eciton (see Gotwald 1969), not of other Dorylus queens; head deeply impressed posteriorly at pronotal insertion, posterior angles rounded, directed posteriorly ( Fig. 14). Head and mandibles orange brown. Mesosoma: Similar in colour to head except for dark reddishbrown patches on dorsum of pronotum, mesonotum, and propodeum. Pronotal suture deeply impressed; mesonotum, metanotum and propodeum separated by shallow but well-defined sutures ( Fig. 14); propodeum in dorsal view rectilinear ( Fig. 14); mesosoma finely punctate without conspicuous pilosity. Petiole: Dark reddish-brown, finely punctate, without conspicuous setae or pilosity; posterior lateral lobes conspicuous in dorsal view ( Fig. 14). Gaster: Segments reddish-brown to orange-brown, finely punctate, without conspicuous pilosity; posterior margin of pygidium in dorsal view with deep semicircular notch medially, sharp, downward pointed horns laterally ( Figs. 11 , 13 ). The two lobes of the hypopygium of the D. gribodoi queen terminate in apically pointed processes ( Fig. 13 ) as in D. kohli (currently classified in Anomma ) (van Boven 1968), D. ghanensis and D. brevipennis (both classified in the subgenus Dorylus ) (van Boven 1975), whereas the two lobes are very broad posteriorly in driver ant queen hypopygia (see e.g. Figs. 20a and 20b in Raignier & van Boven 1955).

Figures

FIGURE 2. Lateral overview of a Dorylus gribodoi male from Taï, Ivory Coast.

FIGURE 3. Frontal view of the head of a Dorylus gribodoi male from Taï, Ivory Coast.

FIGURE 4. Dorylus gribodoi male: genital capsule and subgenital plate.

FIGURE 5. Lateral overview of a large Dorylus gribodoi worker from Taï, Ivory Coast.

FIGURE 7. Lateral view of the head of a large D. gribodoi worker (HW 2.79 mm), Taï, Ivory Coast.

FIGURE 8. Lateral view of the head of a large D. emeryi worker (HW 3.62 mm) from Taï, Ivory Coast.

FIGURE 9. Dorsal view of the petiole of a large D. gribodoi worker (HW 2.85 mm) from Taï, Ivory Coast.

FIGURE 10. Dorsal view of the petiole of a large D. emeryi worker (HW 3.74 mm) from Taï, Ivory Coast.

FIGURE 11. Lateral overview of the D. gribodoi queen collected from a nest at Lamto, Ivory Coast.

FIGURE 13. Dorsal view of the hypopygium of the D. gribodoi queen (HAD 2.53 mm).