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Ants of the subfamily Pseudomyrmecinae:
the Pseudomyrmex viduus group

The Pseudomyrmex viduus group contains about 13 species. All but one of these appear to be obligate occupants of specialized ant-plants, primarily plants of the genera Triplaris (Polygonaceae) and Tachigali (Fabaceae). One species, P. viduus, inhabits a more diverse assemblage of myrmecophytes. All members of this group have very aggressive workers and they are notorious for their painful stings. In many parts of Amazonia these ants have the vernacular name “tachi”.

Key to workers and queens of the P. viduus group, and other Tachigali– and Triplaris-associated Pseudomyrmex

Taken from Ward (1999b). Figure numbers refer to those in that article.

To improve its utility this key covers all species of Pseudomyrmex that have been found inhabiting Triplaris or Tachigali plants. The Pseudomyrmex viduus group is treated in couplets 2-13. The remaining couplets of the key are concerned with other Triplaris and Tachigali associates, both obligate and facultative. Information on the host plant association(s) of each species is given in parentheses.

1. Eyes relatively short, half the head length (HL) or less (worker REL 0.32-0.48, queen REL 0.33-0.50); frontal carinae usually not closely contiguous and usually separated by an amount greater than basal scape width (worker MFC > 0.079 in all species except P. tachigaliae and P. viduus) …. 2
Eyes longer than half the head length (worker REL 0.53-0.68, queen REL 0.51-0.63); frontal carinae more closely contiguous (worker MFC < 0.080) …. 14

2. Head notably broader than long (worker CI 1.08-1.16, queen CI 1.06-1.08); compound eye located on the posterior third of head; body pilosity long and abundant (Fig. 36), worker MSC approximately 95-150, worker HTC 25-50; Mexico to Argentina, Paraguay and Brazil (not a specialized inhabitant of plant domatia) …. kuenckeli (Emery)
Head usually longer than broad, less commonly as broad as long (worker CI 0.70-1.04, queen CI 0.56-0.98); compound eye positioned about the middle of the side of the head; body pilosity less abundant (Figs 12-22), worker MSC < 90, worker HTC < 22 …. 3

3. Head, eyes and propodeum elongate (worker CI < 0.80, queen CI 0.56-0.67; worker REL2 > 0.49, queen REL2 0.56-0.68; worker DPL/HW > 0.96) (Figs 4, 15, 37, 38) …. 4
Head, eyes and propodeum less elongate (worker CI 0.81-1.04, queen CI 0.72-0.98; worker REL2 0.35-0.48, queen REL2 0.40-0.52; worker DPL/HW 0.76-0.97) (Figs 1-3, 5-14, 16-22) …. 5

4. Head extremely elongate, queen CI 0.56 (Fig. 38), worker unknown but expected to have CI < 0.70; body size smaller, queen HW 0.71, queen LHT 0.76, worker expected to have HW and LHT < 0.80; Colombia (known only from a single queen; biology unknown) …. vitabilis Ward
Head less elongate, worker CI 0.70-0.79, queen CI 0.60-0.67 (Fig. 37); larger body size, worker HW 0.74-0.97, queen HW 0.86-0.96, worker LHT 0.63-0.88, queen LHT 0.81-0.94; Mexico to Bolivia, Brazil (recorded from live cavities of various plants including Cordia, Coussapoa, Macrolobium, Ocotea, Pterocarpus, Pseudobombax, Sapium, Triplaris) …. viduus (F. Smith)

5. Frontal carinae closely contiguous and median lobes of the antennal sclerites correspondingly well exposed (Fig. 7) (worker MFC 0.023-0.065, worker PFC/ASD 0.27-0.48; queen FCI2 0.34-0.51); standing pilosity relatively scarce on mesosoma dorsum (worker MSC 6-26) and absent from external faces of tibiae (worker HTC = 0); median clypeal lobe of worker laterally angulate (Fig. 7), that of queen in the form of a tongue-like anterodorsal protrusion; head subopaque to sublucid, densely punctulate; Peru, western Brazil (Tachigali inhabitant) …. tachigaliae (Forel)
Frontal carinae better separated and median lobes of the antennal sclerites less exposed (Figs 1-3, 5, 6, 8-11) (worker MFC 0.080-0.196, worker PFC/ASD 0.54-0.91, queen FCI2 0.45-0.95); standing pilosity usually more common on mesosoma dorsum (worker MSC 6-84) and present on external faces of tibiae (worker HTC 1-21); median clypeal lobe of both worker and queen usually laterally rounded, but if angulate (P. malignus) then upper third of head with sparse punctures and extensive shiny interspaces …. 6

6. Scape and funicular segments shorter; worker FLI 0.97-1.44, queen FLI 1.11-1.54; see also plot of worker SL on HW (Fig. 108) and (LF1 + LF2 + LF3) on HW (Fig. 111); punctures on upper third of head sparse, of uneven density, mostly small and separated by many diameters, leaving extensive smooth interspaces; palp formula 4,3 (Tachigali inhabitants) …. 7
Scape and funicular segments longer; worker FLI 1.36-2.24, queen FLI 1.48-2.22; see also Figs 108, 111; punctures on upper third of head moderately dense, uniformly dispersed, most punctures separated by one to several diameters; palp formula 6,4 or 5,3 (Triplaris inhabitants) …. 10

7. Erect pilosity lacking on sides of head above eyes and on posterior margin of head (frontal view), and scarce on the external face of the metatibia (worker HTC 1-6); median clypeal lobe laterally angulate (Fig. 8); queen mandible with a marked basal incision on its outer face; Venezuela, Trinidad, Guianas, Brazil …. malignus (Wheeler)
Erect pilosity present (usually common) on the sides and posterior margin of the head, and conspicuous on the external face of the metatibia (worker HTC 6-16); median clypeal lobe laterally rounded (Figs 9-11); queen mandible lacking a basal incision on its outer face …. 8

8. Petiolar node expanded posterolaterally (Fig. 32), such that worker PWI 0.92-1.04 and queen PWI approx. 0.79; pubescence on worker abdominal tergite IV sometimes sparse, the tergite correspondingly lucid; Colombia, Ecuador(?) …. insuavis Ward
Petiolar narrower (Figs 31, 33), worker PWI 0.74-0.90, queen PWI 0.63-0.73; pubescence on worker abdominal tergite IV usually rather dense …. 9

9. Worker and queen concolorous orange-brown; Venezuela, Guyana, Brazil …. concolor (F. Smith)
Worker and queen with gaster and (usually) most of mesosoma dark brown; remainder of mesosoma and head variable in color, often a contrasting bright orange-brown; possibly conspecific with P. concolor, see discussion in text; Venezuela, Guyana, Surinam, Brazil …. penetrator (F. Smith)

10. Petiole very short and broad (Figs 17, 28); worker PLI 0.96-1.05, queen PLI 0.76-0.81; worker PWI 1.04-1.14, queen PWI 0.92-0.99; junction of basal and declivitous faces of worker propodeum marked by slight angular protuberance, laterally on either side (Fig. 17); standing pilosity conspicuous on the posterior margin and upper sides of the head (Fig. 6); uniform dark brown; Ecuador …. ultrix Ward
Petiole longer and narrower (Figs 12-14, 16, 23-25, 27); worker PLI 0.70-0.94, queen PLI 0.61-0.74; worker PWI 0.72-0.98, queen PWI 0.63-0.86; junction of basal and declivitous faces of worker propodeum without angular protuberance (Figs 12-14, 16); head pilosity and color variable …. 11

11. Posterior margin and upper sides of head with conspicuous standing and decumbent pilosity (Fig. 5); dorsum of head with coarse, dense punctures, 0.010-0.020 mm in diameter, subcontiguous on anterior half of head and usually separated by about their diameters on upper half; subpetiolar process with a recurved tooth, directed posteroventrally (Fig. 16); smaller species, with more elongate head (worker HW 0.89-1.14, worker CI 0.85-0.93, queen HW 1.07-1.25, queen CI 0.83-0.89) (Figs 5, 132); palp formula 5,3; Colombia to Peru, Brazil …. triplaridis (Forel)
Standing and decumbent pilosity lacking on sides of head and generally sparse or absent on the posterior margin (Figs 1-3); punctures on head usually finer, less dense, separated by one to several diameters on upper half of head; subpetiolar process variable but if furnished with a posteroventral tooth then worker HW > 1.06, worker CI > 0.93, queen HW > 1.70, and queen CI > 0.89; palp formula 6,4 …. 12

12. Worker body light- to medium-brown, nearly always with a contrasting dark brown head; standing pilosity less dense on mesosoma dorsum and tibiae (worker MSC 7-53, worker HTC 1-11), especially in relation to body size (see Fig. 131); subpetiolar process usually conspicuously recurved and directed posteroventrally (Fig. 12); queen large, HW > 1.70; Colombia, Venezuela to Bolivia …. dendroicus (Forel)
Worker body varying in color (light or dark) but head concolorous with or only slightly darker than mesosoma; standing pilosity denser on mesosoma dorsum and tibiae (worker MSC 25-84, worker HTC 5-21) in relation to body size (Fig. 131); subpetiolar process usually not conspicuously recurved and directed posteroventrally (Figs 13, 14); queen smaller (HW < 1.62) …. 13

13. Legs short in relation to other measures of body size (worker LHT/HL 0.70-0.74, worker EL/LHT 0.57-0.60) (see also Fig. 133); smaller average size, worker HW 0.99-1.19, queen 1.20-1.41; Panama, Colombia, Venezuela …. mordax (Warming)
Legs longer (worker LHT/HL 0.78-0.93, EL/LHT 0.48-0.55); larger species, worker HW 1.00-1.41, queen HW 1.46-1.60; Colombia to Peru, Bolivia and Brazil …. triplarinus (Weddell)

14. Standing pilosity abundant on body, including outer surfaces of the tibiae (worker MSC > 90, worker and queen HTC >8) (Fig. 40); petiole much longer than high (worker PLI 0.46-0.70, queen PLI 0.48-0.71) and with a well differentiated anterior peduncle in lateral view (Figs 40, 41) …. 15
Standing pilosity less common (worker MSC < 40), and absent from the outer surfaces of the tibiae (worker and queen HTC = 0); petiole relatively shorter and higher (worker PLI 0.79-1.26, queen PLI 0.68-1.10) and without a distinctly differentiated anterior peduncle in lateral view (Figs 51-59) …. 16

15. Petiole relatively short and broad (Fig. 40) (worker PLI 0.67-0.70, queen PLI approx. 0.70, worker PWI 0.55-0.59, queen PWI approx. 0.59); standing pilosity on propodeum and petiole consisting of some thick, long, black setae intermingled with finer, pale-silvery hairs; Peru (Tachigali inhabitant) …. hospitalis Ward
Petiole more slender (Fig. 41) (worker PLI 0.46-0.57, queen PLI 0.48-0.57, worker PWI 0.38-0.54, queen PWI 0.42-0.56); standing pilosity on propodeum and petiole uniformly fine, pale-silvery; widespread and common throughout the Neotropics (generalist inhabitant of dead twigs and branches, recorded occasionally from Triplaris) …. gracilis (Fabricius)

16. Very small species, with elongate head (worker HW 0.56-0.68, queen HW 0.56-0.61, worker CI 0.66-0.79, queen CI 0.59-0.66); standing pilosity common on body dorsum (worker MSC > 15); found throughout the Neotropics (generalist inhabitant of dead twigs, recorded occasionally from Triplaris) …. elongatus (Mayr)
Larger species, with broader head (worker HW > 0.70, queen HW > 0.75, worker CI usually > 0.80, queen CI > 0.64); standing pilosity relatively uncommon on body dorsum (worker MSC 2-12) (sericeus group) …. 17

17. Worker head broad (CI 0.91-1.03), with strongly convex sides (Figs 42-44, 156); worker petiole short and subtriangular in lateral view (Figs 51-53); worker PL/(HW x LHT) 0.30-0.40; worker legs relatively long (LHT/HL 0.72-0.91); queen very large in size (queen HW 1.34-1.62) (Tachigali inhabitants) …. 18
Worker head more elongate (CI 0.81-0.91), with less strongly convex sides (Figs 45-49, 156); worker petiole typically longer and often subtrapezoidal or subglobose in lateral view (Figs 54-58); worker PL/(HW x LHT) 0.44-0.64; worker legs shorter (LHT/HL 0.62-0.72); queen smaller in size (queen HW 0.78-1.22) …. 20

18. Legs shorter in relation to head length (worker LHT/HL 0.72-0.77, queen LHT/HL 0.63-0.71); worker profemur broader (worker FI 0.43-0.47); Colombia, Peru, Brazil …. ferox Ward
Legs longer (worker LHT/HL 0.82-0.91, queen LHT/HL 0.73-0.76); worker profemur more slender (worker FI 0.36-0.42) …. 19

19. Body, including appendages, predominantly orange-brown in color, although parts (especially the mesosoma and petiole) may have darker infuscated patches; dorsal face of worker propodeum convex in profile, with the propodeal spiracle located some distance from it (Fig. 53); larger in size (worker HW 1.19-1.38, queen HW approx. 1.62); Colombia, Peru, Brazil, Bolivia …. pictus (Stitz)
Body uniformly dark brown to black, with all or parts of the appendages a contrastingly light orange-brown; dorsal face of worker propodeum flattened, submarginate laterally; in profile, propodeal spiracle located just below the level of the dorsal face of propodeum (Fig. 51); smaller in size (worker HW 1.13-1.25, queen HW 1.34-1.42); Ecuador, Peru …. eculeus Ward

20. Small (worker HW 0.81-1.03, queen HW 0.78-1.06) with disproportionately short and narrow petiole (worker PL 0.28-0.44, queen PL 0.45-0.52, worker DPW 0.23-0.34, queen DPW 0.32-0.40, worker DPW/HW 0.26-0.35) (see also Figs 66-68, 157-160); worker eye relatively elongate (worker REL 0.63-0.68) …. 21
Averaging larger in size (worker HW 0.96-1.19, queen HW 0.96-1.22), with longer and broader petiole (worker PL 0.40-0.55, queen PL 0.59-0.72, worker DPW 0.37-0.46, queen DPW 0.46-0.61, worker DPW/HW 0.34-0.42) (Figs 63-65, 157-160); worker eye shorter (worker REL 0.57-0.62) …. 23

21. Smaller in size, with very elongate head; queen HW approx. 0.78, queen CI 0.65-0.66 (Fig. 50); worker unknown but by extrapolation expected to have HW < 0.85 and CI < 0.80; Brazil (known from only one collection, from Tachigali) …. deminutus Ward
Larger (worker HW 0.81-1.03, queen HW 0.85-1.06) with broader head (worker CI 0.83-0.91, queen CI 0.73-0.81) …. 22

22. Head broader (worker CI 0.87-0.91, queen CI 0.77-0.81, queen HW 0.96-1.06); petiole shorter (worker PL/LHT 0.44-0.50), subtriangular or subglobose in lateral view, with a convex anterodorsal face that rounds gently into the posterior face (Fig. 58); Colombia, Venezuela, Brazil (Tachigali inhabitant) …. vinneni (Forel)
Head more elongate (worker CI 0.83-0.86, queen CI 0.73-0.77, queen HW 0.85-0.90); petiole longer (worker PL/LHT 0.51-0.61), subtrapezoidal in profile, with differentiated anterior and dorsal faces, the latter meeting the vertical face at a sharply rounded angle (Fig. 57); Mexico to Colombia (generalist inhabitant of dead twigs, occasional in Triplaris) …. ita (Forel)

23. Worker bicolored, with head mostly dark grey-brown, gaster medium-brown, and remainder of body including legs dusky yellow- or orange-brown, with variable amounts of infuscation on the mesosoma dorsum; queen darker overall, but with contrastingly lighter propleuron and protibia; head relatively broad (worker CI 0.84-0.91, queen CI approx. 0.78); petiole relatively long (worker PLI 0.86-1.02, queen PLI approx. 0.80); Brazil, Peru (recorded as an inhabitant of Gustavia, Ocotea, Pleurothyrium, Tachigali and Triplaris) …. rubiginosus (Stitz)
Most of body, including legs (except sometimes protibia), uniformly dark grey-brown or blackish-brown to black; either head tending to be more elongate (worker CI 0.81-0.86, queen CI 0.72-0.73, in P. fortis) or petiole shorter (worker PLI 1.01-1.13, queen PLI 0.91-1.01, in P. sp. PSW-39) …. 24

24. Petiole somewhat angular in lateral profile, with more or less differentiated anterior and dorsal faces (Fig. 54); worker profemur relatively slender (FI 0.43-0.47); queen large (HW 1.13-1.22), with broad petiole (DPW 0.58-0.61, PWI 0.90-0.95) and long eyes (REL 0.60-0.63); Peru (Tachigali inhabitant) …. crudelis Ward
Petiole rounded in profile, with a single convex anterodorsal face (Fig. 55); worker profemur broad (FI 0.48-0.50); queen smaller (HW 0.96-1.07), with more slender petiole (DPW 0.46-0.50, PWI 0.73-0.79) and relatively short eyes (REL 0.53-0.56); Mexico to Colombia (recorded from Triplaris but not restricted to this plant) …. fortis (Forel)

Literature Cited

Ward, P. S. 1999b. Systematics, biogeography and host plant associations of the Pseudomyrmex viduus group (Hymenoptera: Formicidae), Triplaris– and Tachigali-inhabiting ants. Zoological Journal of the Linnean Society 126:451-540.

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