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Ants of the subfamily Pseudomyrmecinae:
the Pseudomyrmex ferrugineus group

This group comprises the obligate acacia-ants. These Pseudomyrmex live only in the swollen-thorns of several Mesoamerican acacia species. There are 10 species of ants in the Pseudomyrmex ferrugineus group (Ward, 1993) and they have been recorded, collectively, from 12 species of Acacia. Janzen’s (1966, 1967) classic experiments with Pseudomyrmex ferrugineus and Acacia cornigera demonstrated the mutually beneficial nature of the ant/acacia interaction.

A georeferenced database of specimen records, on which the revision of the Pseudomyrmex ferrugineus group was based (Ward, 1993), has been posted on AntBase.

Key to Pseudomyrmex species associated with swollen-thorn acacias (based on workers and queens)

The following key is modified from Ward (1993). It includes all species of Pseudomyrmex which have been found inhabiting swollen-thorn acacias in Mexico, Central America, or Colombia. Couplets 11-19 cover the P. ferrugineus group. If you are certain that you have one of the obligate, aggressive acacia-ants belonging to the P. ferrugineus group, then proceed directly to couplet 11. If you have a male you can try the separate key to males.

The remainder of the worker/queen key deals with 10 other species of Pseudomyrmex. These fall into two categories: four are obligate inhabitants of the acacias (they appear to behave as “parasites” or commensals, providing little or no protection to the host plant), while six species are generalist Pseudomyrmex that occupy swollen-thorn acacias opportunistically and sporadically. The obligate species are P. nigropilosus, P. reconditus, P. simulans, and P. subtilissimus.

Throughout the key, worker sizes exclude those of nanitic workers, i.e. the first-emerging miniature workers associated with colony-founding queens. Figure numbers match those used in Ward (1993).

  1. Standing pilosity very sparse on the head, including the gula (underside), and on the mesosoma; 1,0, and 0-1 pairs of erect setae on the pronotum, mesonotum, and propodeum, respectively, of the worker …. 2
    Standing pilosity common to abundant on most parts of the body, including the gula and mesosoma; worker usually with more than 10 standing hairs visible in outline on the mesosoma dorsum, not arranged in isolated pairs …. 4
  2. Very small, light brown species (worker and queen HW < 0.60), with elongate head (CI < 0.66) and low, dorsally flattened petiole (PLI < 0.76) (Nicaragua, Costa Rica) …. subtilissimus (Emery)
    Larger species (worker and queen HW > 0.70), with broader head (CI > 0.70) and higher petiole (PLI > 0.80) (e.g., Fig. 1)  …. 3
  3. Smaller species (worker and queen HW < 1.00), with posterodorsally angulate petiole (Fig. 2) (Mexico to Colombia) …. ita (Forel)
    Larger species (worker and queen HW > 1.15), with posterodorsally rounded petiole (Fig. 1) (Mexico to Ecuador, Brazil) …. boopis (Roger)
  4. Head with scattered, fine punctulae on a smooth, shiny background; punctulae on upper third of head separated by several times their diameters or more …. 5
    Head opaque to sublucid, more coarsely and densely punctulate, the punctulae subcontiguous on most parts of the head …. 6
  5. Larger species (HW > 1.20), with broad head (CI 1.08-1.16) and abundant long pilosity (Fig. 3) (Mexico to Argentina, Brazil) …. kuenckeli (Emery)
    Small species (HW < 0.72), with elongate head (CI < 0.86) and shorter, sparser pilosity (Fig. 4) (Mexico) …. hesperius Ward
  6. Eyes relatively large and elongate (Fig. 5b), eye length more than one-half head length (worker and queen REL 0.52-0.65); pronotum laterally submarginate; outer surfaces of tibiae usually with standing pilosity (may be very short); larger species, worker HW > 1.20; palp formula 6,4 …. 7
    Eyes smaller (Fig. 11), usually less than one-half head length (worker and queen REL 0.38-0.50); pronotum laterally rounded; outer surfaces of tibiae without standing pilosity; medium-sized species, worker HW < 1.28; palp formula 5,3 or 4,3 (ferrugineus group) …. 11
  7. Petiole long and slender, with a well developed anterior peduncle (worker and queen PLI 0.42-0.57) (Fig. 6) …. 8
    Petiole less elongate, with a short anterior peduncle (PLI > 0.59) ( Fig. 7) …. 9
  8. Head densely punctulate-coriarious, presenting a matte appearance; head and mesosoma black, with a contrastingly pale orange petiole, postpetiole, and gaster; petiole very slender, worker PLI 0.42-0.47 (Fig. 5a) (southern Mexico, Guatemala) …. opaciceps Ward
    Head densely punctulate but retaining a subopaque to sublucid (not matte) appearance; color variable but without the preceding pattern in Mexico or Guatemala; petiole usually less slender, worker PLI 0.46-0.57 (Fig. 6) (throughout the Neotropics) …. gracilis (Fabricius)
  9. Larger species (worker HW 1.47-1.54, queen HW 1.66), with broad head (worker CI 1.00-1.02, queen CI 0.92) (Nicaragua) …. reconditus Ward
    Smaller species (worker HW 1.21-1.41, queen HW 1.15-1.36); head more elongate (worker CI 0.84-0.90, queen CI 0.77-0.80) …. 10
  10. Standing pilosity short, pale and inconspicuous (Fig. 9); pronotum sharply margined laterally; petiole longer, worker PLI 0.61-0.66, queen PLI 0.63-0.68; color black (Panama) …. simulans Kempf
    Standing pilosity long and conspicuous, with long curved black setae arising from the propodeum and petiole (Fig. 7); pronotum with blunter lateral margination; petiole short and high, worker PLI 0.69-0.77, queen PLI 0.68-0.75; color variable, usually pale or bicolored (Mexico to Costa Rica) …. nigropilosus (Emery)
  11. Median clypeal lobe of worker concave, with sharp lateral angles or teeth (Fig. 10, Fig. 11); legs long in relation to body size; larger species (worker HW > 0.92, worker LHT > 0.88, queen HL > 1.40, queen LHT > 1.05; frontal carinae closely contiguous, worker FCI2 0.24-0.42; propodeum punctulate to coriarious-imbricate, posterolaterla portions sublucid with little or no overlying coarse rugulo-punctate sculpture… 12
    Median clypeal lobe of worker laterally rounded or subangulate (without sharp angles or teeth) (e.g., Fig. 12, Fig. 18); legs shorter in relation to body size; size variable but if as large as the preceding (worker HW > 0.92, etc.) then frontal carinae relatively well separated, worker FCI2 > 0.43, and posterolateral portions of propodeum opaque to subopaque, overlain by coarse (although often weak and ill-defined) rugulo-punctate sculpture …. 13
  12. Larger species (worker HW > 1.09, queen HW > 1.20); head broader, its posterior margin straight and rounding rather sharply into the sides (Fig. 10); median clypeal lobe of worker longer and narrower; worker with a conspicuous, pit-like impression on midline of head, anterior to the median ocellus; palp formula 4,3 (Panama)… satanicus (Wheeler)
    Smaller species (worker HW 0.94-1.15, queen HW 0.94 – 1.14), with head a little less broad and its posterior margin rounding more gently into the sides (Fig. 11); median clypeal lobe of worker notably shorter and broader; worker usually lacking a pit-like impression on mid-line of head; palp formula almost invariably 5,3, rarely 5p4,3 (Honduras to Colombia) ….  spinicola (Emery)
  13. Smaller species (worker HW 0.74-0.90, queen HW 0.76-0.85); head, propodeum, and petiole more elongate, for a given head width (Fig. 36)… 14
    Larger species (worker HW 0.85-1.21, queen HW 0.84-1.19); head, propodeum, and petiole shorter, for a given head width (Fig. 36)…. 16
  14. Petiole and postpetiole very broad (worker PWI 0.63-0.75, worker PWI3 0.33-0.46, worker PPWI 1.41-1.83; queen PWI2 0.69-0.78), the petiolar node with conspicuous posterolateral angles, in dorsal view (Fig. 24b); head very finely and densely punctulate-coriarious, presenting a matte (opaque) appearance; palp formula 4,3 (Mexico to Nicaragua)…peperi (Forel)
    Petiole and postpetiole relatively narrow (worker PWI 0.49-0.61, worker PWI3 0.50-0.61, worker PPWI 1.03-1.30; queen PWI2 0.57-0.63), the petiolar node without conspicuous posterolateral angles (Fig. 23b); head densely punctulate, sublucid to subopaque, but without a matte appearance; palp formula 5,3 …. 15
  15. Workers and queens light orange-brown, with a fuscous patch on anterior third of abdominal tergite IV (first gastric tergite); eyes relatively short (worker EL/LHT 0.56-0.61, queen REL2 0.58-0.66) (Fig. 13); queen head less elongate (queen CI 0.67-0.72) (Guatemala to Costa Rica)…nigrocinctus (Emery)
    Workers and queens entirely dark brown; eyes longer (worker EL/LHT 0.59-0.64, queen REL2 0.69-0.70) (Fig. 14); queen head more elongate (CI approx. 0.61, in the two known specimens) (Costa Rica) …. particeps Ward
  16. Small species (worker HW 0.85-0.95, queen HW 0.84-0.96) with head, gaster, and at least part of mesosoma very dark brown to black; body pubescence dense, decumbent to suberect, and conspicuous, especially on the petiolar node (Fig. 28a); standing pilosity often (not always) sparse; head weakly shining (western Mexico)…veneficus (Wheeler)
    Body pubescence dense but predominantly appressed, petiolar node without conspicuous suberect pubescence; usually larger (worker HW 0.89-1.21, queen HW 0.96-1.19) with more conspicuous standing pilosity; color and head sculpture variable …. 17
  17. Head and gaster (typically also mesosoma) very dark brown to black; head densely punctulate and opaque…18
    Body lighter in color: light orange-brown to medium brown, rarely dark brown; head at least weakly sublucid between ocelli and upper margin of the compound eye …. 19
  18. Smaller species, worker HW 0.89-1.03, queen HW 0.96-1.01; petiole relatively longer and higher (Fig. 45) (southern Mexico)…mixtecus Ward
    Larger species, worker HW 0.99-1.21, queen HW > 1.10; petiole relatively shorter and lower (Fig. 45) (Guatemala to Costa Rica) …. flavicornis (F. Smith)
  19. Head and mesosoma light orange-brown, the gaster similar or slightly darker; underside of head with conspicuous suberect pubescence (Fig. 19b); profile of worker mesosoma as in Fig. 29a; smaller species (worker HW 0.93-1.03, queen HW 0.96-1.00) with shorter, higher petiole (western Mexico)… janzeni Ward
    Gaster (and usually head) medium to dark brown, mesosoma variable; gular pubescence usually more appressed and inconspicuous; in profile worker mesosoma usually with basal face rounding more gradually into declivitous face (Fig. 27a); size variable but larger on average (worker HW 0.92-1.15, queen HW 0.92-1.12), with longer and lower petiole (eastern and southern Mexico to Honduras) …. ferrugineus (F. Smith)

Literature Cited

  • Janzen, D. H. 1966. Coevolution of mutualism between ants and acacias in Central America. Evolution 20:249-275.
  • Janzen, D. H. 1967. Interaction of the bull’s-horn acacia (Acacia cornigera L.) with an ant inhabitant (Pseudomyrmex ferruginea F. Smith) in eastern Mexico. University of Kansas Science Bulletin 47:315-558.
  • Ward, P.S. 1993. Systematic studies on Pseudomyrmex acacia-ants (Hymenoptera: Formicidae: Pseudomyrmecinae).  Journal of Hymenoptera Research 2:117-168.

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