Key to Malagasy Erromyrma, Monomorium, Syllophopsis and Trichomyrmex species

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This worker key is based on: Heterick, B.E. 2006. A revision of the Malagasy ants belonging to genus Monomorium Mayr, 1855. Proceedings of the California Academy of Sciences. 57:69-202. PDF

After the publication of this key the Monomorium species given here were determined to be members of the now reorganized genera: Monomorium, Royidris, Syllophopsis and Trichomyrmex. This key reflects this new nomenclature.

The genus Royidris is endemic to Madagascar and has been revised by Bolton and Fisher (2014). This more contemporary study described more than 10 new species and these are not included in the key below. If specimens run through the key on this page suggest you have an unresolved Royidris species, there is a 2014 worker key for all the species in this genus: Key to Royidris workers.

Couplet 6 in the key has been removed due to taxonomic changes in Royidris.

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1

  • Antenna 11-segmented (M. monomorium species group) . . . . . 2
  • Antenna 12-segmented (various species groups) . . . . . 3

2

return to couplet #1

  • Metanotal groove weakly impressed; in profile, propodeum rather elongate and petiolar node always conical, its dorsum tapered (depigmented yellow to brown) . . . . . Monomorium exiguum
Monomorium exiguum casent0470747 head 1.jpg
Monomorium exiguum casent0470747 profile 1.jpg
  • Metanotal groove strongly impressed; in profile, propodeum evenly rounded and petiolar node may be thick, broadly rounded (dark reddish-brown to black) . . . . . Monomorium nigricans
Monomorium nigricans casent0020200 head 1.jpg
Monomorium nigricans casent0020200 profile 1.jpg

3

return to couplet #1

  • Palps long (PF 5,3); propodeum usually lacking standing setae, where present these short and fine; occipital carina strongly defined and conspicuous along rear margin of head when head viewed in profile; mandible armed with five distinct teeth (M. shuckardi species group) . . . . . 4
  • Palps shorter (PF 3,3 or less); propodeum usually with standing setae, but if these lacking, then mandible armed with four teeth, or with three distinct teeth and a small to minute denticle or angle; occipital carina vestigial or short, not conspicuous along rear margin of head when head viewed in profile . . . . . 7

4

return to couplet #3

  • Head and mesosoma finely striolate; in profile, propodeum without a distinct angle between its dorsal and declivitous faces . . . . . Royidris robertsoni
Monomorium robertsoni casent0035010 head 1.jpg
Monomorium robertsoni casent0035010 profile 1.jpg
  • Head and mesosoma finely striolate-microreticulate, microreticulate-punctate with or without unsculptured areas, or head and dorsum of mesosoma smooth and shining; propodeum in profile with blunt to sharply defined propodeal angle . . . . . 5

5

return to couplet #4

  • Antennal scape very long, extending beyond vertex of head capsule (SI 113–132) larger (HW 0.62–0.72 mm), depigmented yellow ants . . . . . Royidris notorthotenes
Monomorium notorthotenes casent0002259 head 1.jpg
Monomorium notorthotenes casent0002259 profile 1.jpg
  • Antennal scape shorter, barely or not attaining vertex (SI 90–108); in full-face view, frons of head capsule finely striolate-microreticulate or smooth and shining . . . . . Royidris clarinodis
Royidris clarinodis casent0019565 h 1 high.jpg
Royidris clarinodis casent0019565 p 1 high.jpg

No couplet 6 (see preface to key)

7

return to couplet #3

  • Mandible distinctly longitudinally striate, maximum number of teeth four . . . . . 8
  • Mandible smooth and shining, except for piliferous pits, with three to five teeth . . . . . 12

8

return to couplet #7

  • In full-face view, frons of head capsule smooth, with exception of piliferous punctures, fine rugulae on vertex (M. destructor) and fine, circular striolae around antennal insertions (M. destructor species group) . . . . . 9
  • In full-face view, frons of head capsule sculptured, sculpture varying from superficial reticulation to shagreenate-punctate (M. salomonis species group) . . . . . 10

9

return to couplet #8

Monomorium robustior casent0008626 head 1.jpg
Monomorium robustior casent0008626 profile 1.jpg
Monomorium destructor casent0073621 head 1.jpg
Monomorium destructor casent0073621 profile 1.jpg

10

return to couplet #8

  • Mesosoma possessing one or more pairs of erect and/or sub-erect setae . . . . . Monomorium pharaonis
Monomorium pharaonis casent0104095 head 1.jpg
Monomorium pharaonis casent0104095 profile 1.jpg

11

return to couplet #10

  • In full-face view, frons of head capsule shining, with surface sculpture confined to faint reticulation, the vertex more noticeably shagreenate; scape shorter (SI < 100) . . . . . Monomorium willowmorense
Monomorium willowmorense casent0902199 h 1 high.jpg
Monomorium willowmorense casent0902199 p 1 high.jpg
  • In full-face view, frons of head capsule opaque, reticulate to shagreenate-punctate; scape longer (SI 100 >) . . . . . Monomorium subopacum
Monomorium subopacum casent0064820 head 1.jpg
Monomorium subopacum casent0064820 profile 1.jpg

12

return to couplet #7

  • Either frontal lobes separated by a strip of posterior sector of clypeus that is clearly wider than either of them, or, if frontal lobes closely approximated, metanotal groove strongly indented and propodeum smooth; propodeal dorsum rounding into declivitous face without distinct angle; propodeal denticles always absent; fine, transverse ridges always absent from underside of petiolar peduncle and node; anteroventral postpetiolar process commonly vestigial or absent, rarely an inconspicuous lip and prominent only in E. latinodis . . . . . 13
  • Frontal lobes usually closely approximated, and separated by a strip of posterior sector of clypeus that is narrower than either of them, or, if frontal lobes well separated, and metanotal groove deeply impressed, then propodeal dorsum meeting declivitous face at a distinct angle; propodeum often armed with flanges or denticles; several to many fine, transverse ridges present on underside of petiolar peduncle and node; anteroventral postpetiolar process usually distinct as sharp angle, conspicuous lip or spur (M. hanneli and M. hildebrandti species groups.) . . . . . 26

13

return to couplet #12

  • Mandible armed with five teeth; PF 3,3; propodeum elongate, its dorsum and sides finely and densely striolate (M. latinode species group) . . . . . Erromyrma latinodis
Monomorium latinode casent0132440 head 1.jpg
Monomorium latinode casent0132440 profile 1.jpg
  • Mandible armed with three or four teeth (five teeth very rarely present in M. chnodes); PF 2,2 or 1,2; propodeum not as above, usually smooth on dorsum and sides (M. monomorium species group) . . . . . 14

14

return to couplet #13

  • In full-face view, clypeus strongly bidentate, each clypeal carina terminating in a sharp angle or denticle, the anteromedian clypeal margin between the clypeal projections often straight; eyes large, eye length between 1/4th and 1/5th length of side of head capsule (body of ant concolorous yellow or brown, gaster may be slightly darker shade than mesosoma) . . . . . 15
  • In full-face view, clypeus usually weakly emarginate to weakly convex, if clypeus tending to bidentate then anteromedian margin of clypeus usually indented between anteromedian clypeal projections, body of ant is distinctly bicolored or eye is smaller, eye length 1/5th < length of side of head capsule . . . . . 16

15

return to couplet #14

  • Propodeal spiracle very large, ≈ same width as antennal segments a3–9; petiolar node higher, tending to cuneate . . . . . Monomorium madecassum
Monomorium madecassum casent0490190 head 1.jpg
Monomorium madecassum casent0490190 profile 1.jpg
  • Propodeal spiracle minute, pinhole-like, width << antennal segments a3–9; petiolar node lower, conical . . . . . Monomorium denticulum
Monomorium denticulum casent0470359 h 1 high.jpg
Monomorium denticulum casent0470359 p 1 high.jpg

16

return to couplet #14

  • Mandible always clearly armed with three teeth with no trace of a smaller, basal tooth or denticle . . . . . 17
  • Mandible characteristically armed with four teeth, albeit basal tooth may be reduced to a minute, offset denticle or angle (rarely absent in individuals of M. flavimembra, whose workers have a bright reddish- or yellowish-brown body and yellowish appendages), rarely with five teeth (some M. chnodes) . . . . . 18

17

return to couplet #16

  • In profile clypeus sloping towards mandibles at acute angle; in full-face view clypeus projecting forward so that its anterior margin adjoins or partially obscures basal margin of mandibles; metanotal groove deeply impressed but almost always without broad cross-ribs; variously colored from orange to dark reddish-black or bicolored, but where head is light colored, antennal scape is conspicuously darker than head capsule . . . . . Monomorium versicolor
Monomorium versicolor casent0433301 head 1.jpg
Monomorium versicolor casent0433301 profile 1.jpg
  • In profile, clypeus almost at right angle to mandibles; in full-face view, anterior margin of clypeus largely straight, clypeus not projecting forward, leaving a gap between clypeus and mandibles; metanotal groove deeply impressed, broad cross-ribs always present; concolorous orange, with antennal scape of same color as head capsule . . . . . . Monomorium platynode
Monomorium platynode casent0026538 h 1 high.jpg
Monomorium platynode casent0026538 p 1 high.jpg

18

return to couplet #16

  • Head and gaster uniformly dark brown to black, contrasting with yellowish or bicolored yellow-and-brown mesosoma; petiolar node low, broadly conical to tumular, scarcely higher than postpetiole . . . . . Monomorium floricola
Monomorium floricola casent0053986 head 1.jpg
Monomorium floricola casent0053986 profile 1.jpg
  • Either concolorous, or head and gaster not both uniformly dark in color with a lighter colored mesosoma; petiolar node variable but usually distinctly higher than postpetiole . . . . . 19

19

return to couplet #18

  • In profile, mesosoma straight and at higher level than propodeum; propodeal spiracle very large, ≈ diameter of antennal segments a3–a9; five teeth occasionally present, all non-apical mandibular teeth usually of same size; propodeum short, about as long as high; antennal scape relatively short (SI 67–76) (densely hairy, bright yellow ants) . . . . . Monomorium chnodes
Monomorium chnodes casent0025716 head 1.jpg
Monomorium chnodes casent0025716 profile 1.jpg
  • In profile, mesosoma, if straight, on same level as propodeum; propodeal spiracle generally smaller, commonly pinhole-like; maximum of four teeth, basal tooth frequently distinctly smaller than other non-apical teeth and reduced to minute denticle or angle in several taxa; propodeum usually longer than high; antennal SI usually 80> . . . . . 20

20

return to couplet #19

  • In full-face view, antennal scapes very long, scapes normally reaching or even slightly exceeding vertex of head capsule (SI 100 >), in rare, very small individuals with scape index slightly less than 100, head darker than mesosoma; profile of combined mesosoma and propodeum straight (except for indentation of metanotal groove), propodeum elongate with indistinct angle between propodeal dorsum and its declivitous face; mesosoma often with 8> pairs of setae; mesopleuron usually with distinct sculpture, often reticulate-punctate (reddish-brown ants which normally have darker head and gaster) . . . . . Monomorium sakalavum
Monomorium sakalavum casent0475908 head 1.jpg
Monomorium sakalavum casent0475908 profile 1.jpg
  • In full-face view, antennal scapes usually shorter, if exceeding vertex, and profile of combined mesosoma and propodeum straight, then mesopleuron smooth, or head of same color intensity as mesosoma and mesosoma with only three prominent pairs of standing setae, or propodeum rounded in a convex curve towards its declivitous face . . . . . 21

21

return to couplet #20

  • With combination of: head and mesosoma brownish-yellow to orange (normally concolorous, head may be of slightly darker shade than mesosoma), gaster brown; clypeus projected forward, clypeal carinae strongly developed and moderately divergent anteriad; propodeal spiracle relatively large, ≈ diameter of antennal segments a3–a9; in profile, postpetiole visibly higher than wide . . . . . Monomorium xuthosoma
Monomorium xuthosoma casent0016361 head 1.jpg
Monomorium xuthosoma casent0016361 profile 1.jpg
  • Without this combination of characters (similar species most easily distinguished by smaller, pinhole- like propodeal spiracle, or rounded postpetiole which is as high as wide, or weakly developed or indistinct clypeal carinae) . . . . . 22

22

return to couplet #21

  • In full-face view, clypeal carinae well developed and strongly divergent anteriad, anteromedian clypeal margin straight and broad, sometimes reaching almost to genae; in profile, clypeus meeting mandibles at angle of ≈ 90°; petiolar node often very small and low for size of ant, with a distinct ventral lobe in some specimens; head rather broad (CeI 86–94) (gracile, yellow or brown species) . . . . . Monomorium bifidoclypeatum
Monomorium bifidoclypeatum casent0173547 head 1.jpg
Monomorium bifidoclypeatum casent0173547 profile 1.jpg
  • In full-face view, if clypeal carinae well developed, then clypeus with carinae sub-parallel or only slightly divergent, or, if anteromedian clypeal margin straight and broad, then clypeal carinae obsolete or only weakly developed . . . . . 23

23

return to couplet #22

  • Concolorous reddish-brown to yellowish-brown species with yellow to brownish-yellow appendages, clypeus also yellowish, in contrast with color of head capsule; clypeal carinae obsolete or feebly indicated, clypeus only weakly projected forward and depressed medially, anteromedian clypeal margin broad and straight to weakly emarginated . . . . . Monomorium flavimembra
Monomorium flavimembra casent0041614 head 1.jpg
Monomorium flavimembra casent0041614 profile 1.jpg
  • Either color pattern not as above, or clypeal carinae distinct, anteromedian clypeal margin narrow, and clypeus strongly projected forward . . . . . 24

24

return to couplet #23

  • Bicolored species with head and mesosoma yellow to orange, gaster brown to shining black, sometimes with paler, yellowish or orange area at base of first gastral tergite; in full-face view, clypeal carinae weak or obsolete, anteromedian clypeal margin broad and straight or slightly emarginate; head generally broader (CeI 82–88); basal tooth reduced to minute angle and strongly offset . . . . . Monomorium lepidum
Monomorium lepidum casent0442208 head 1.jpg
Monomorium lepidum casent0442208 profile 1.jpg
  • Concolorous yellow or brown to bicolored yellow and brown or black, but if bicolored as above, then clypeal carinae distinct in full-face view, often sharply defined and sub-parallel, anteromedian clypeal margin narrow, its outline slightly convex, straight or concave; head of these bicolored specimens generally narrower (CeI 74–83); basal tooth variably developed, but usually a distinct denticle (several specimens may need to be examined as mandibular teeth are sometimes abraded) . . . . . 25

25

return to couplet #24

  • Eye very small (8 ommatidia <), its diameter ≤ greatest width of antennal scape; in profile, clypeus broadly convex, angle formed by anterior clypeal margin usually indistinct; in full-face view clypeus bulbous, clypeal carinae weakly developed and frontal carinae separated by barely greatest width of antennal scape (hairy, concolorous, depigmented yellow ants) . . . . . Monomorium micrommaton
Monomorium micrommaton casent0044969 head 1.jpg
Monomorium micrommaton casent0044969 profile 1.jpg
  • Eye almost always larger (10 ommatidia ≥), its diameter > greatest width of antennal scape (extremely rarely, eye small with some ommatidia reduced in size , in which case gaster and sometimes head have brownish tinge, totally lacking in M. micrommaton); in profile, descending outline of clypeus in most cases sloping rather than broadly convex, with distinct angle at anterior clypeal margin; in full-face view, clypeus usually less bulbous and clypeal carinae often weakly to strongly developed; frontal carinae most commonly separated by more than width of antennal scape at its widest point (body color highly variable, ranging from pale yellow through various shades of brown to chocolate or bicolored yellow or orange and brown/black) . . . . . Monomorium termitobium
Monomorium termitobium blf1070(40)-9 head 1.jpg
Monomorium termitobium blf1070(40)-9 profile 1.jpg

26

return to couplet #12

  • In full-face view, clypeus projected forward with well-defined clypeal carinae, median seta positioned at or slightly posteriad of (i.e., above) the projecting anteromedian clypeal margin; petiolar node smooth, thin, tapered and often scale-like; postpetiole elevated, smooth, usually distinctly higher than wide; underside of petiolar peduncle and node smooth, without hint of fine, transverse ridges (concolorous yellow to tawny-orange species) (M. hanneli species group) . . . . . Monomorium hanneli
Monomorium hanneli casent0010764 head 1.jpg
Monomorium hanneli casent0010764 profile 1.jpg
  • In full-face view, either clypeus not projected forward with strong carinae, or anteromedian seta set well anteriad of (i.e., underneath) apparent anteromedian clypeal margin; petiolar node variable but never thin, tapered and scale-like; postpetiole usually rounded, if elevated and distinctly higher than wide, then strongly sculptured; underside of petiolar peduncle with several to many fine, transverse ridges in medium-sized and larger species (M. hildebrandti species group) . . . . . 27

27

return to couplet #26

  • PF 3,2; four mandibular teeth, apical tooth twice as large as t3, sharp and downcurved (depigmented, pale yellow species) . . . . . Syllophopsis ferodens
Monomorium ferodens casent0173588 head 1.jpg
Monomorium ferodens casent0173588 profile 1.jpg
  • PF 2,2; four or five mandibular teeth, with apical tooth usually same size as t3; sometimes with broad base but never downcurved (color variable, often brown, bright yellow-orange or bicolored) . . . . . 28

28

return to couplet #27

  • Promesonotum, petiolar node and postpetiole coarsely striate; in full-face view, mandibles broadly triangular with at least five distinct teeth arranged vertically on masticatory margin, additional denticle may be present; eye size moderate, diameter of eye ≈ equal to greatest width of antennal scape (very large, blackish-red or orange-and-yellow ants with HW > 0.85 mm, often >1mm) . . . . . 29
  • Promesonotum and petiolar node, at least, without coarse striae; either smooth, or with weak striolae posteriad (promesonotum) or with faint, lateral, longitudinal striolae (petiolar node); in full-face view, mandibles linear-triangular in most specimens, number of teeth and denticles four to five, but four is most common; HW 85 <, except in very large workers of M. fisheri, in which eye is very small (eye width < greatest diameter of antennal scape.) . . . . . 30

29

return to couplet #28

Monomorium infuscum casent0010867 head 1.jpg
Monomorium infuscum casent0010867 profile 1.jpg
Monomorium aureorugosum casent0044971 head 1.jpg
Monomorium aureorugosum casent0044971 profile 1.jpg

30

return to couplet #28

  • Eye point-like, consisting of one or two ommatidia (small, depigmented, yellow species) . . . . . 31
  • Eye often small, but of normal appearance, consisting of four ommatidia . . . . . 33

31

return to couplet #30

Monomorium sechellense casent0033849 head 1.jpg
Monomorium sechellense casent0033849 profile 1.jpg

32

return to couplet #31

  • In profile, metanotal groove deeply impressed; propodeal angle distinct with clear separation of dorsal and declivitous faces; smaller ants (HW 0.27–0.30 mm) . . . . . Syllophopsis cryptobia
Monomorium cryptobium casent0060392 head 1.jpg
Monomorium cryptobium casent0060392 profile 1.jpg
  • In profile, metanotal groove usually weakly impressed; propodeal angle indistinct, propodeum without clear separation of dorsal and declivitous faces; larger ants (HW 0.36–0.42 mm) . . . . . Syllophopsis modesta
Monomorium modestum casent0026710 head 1.jpg
Monomorium modestum casent0026710 profile 1.jpg

33

return to couplet #30

  • In full-face view, humeri of promesonotum armed with a small, bluntly rounded angle, tubercle or rugosity . . . . . Syllophopsis gongromos
Monomorium gongromos casent0173590 head 1.jpg
Monomorium gongromos casent0173590 profile 1.jpg
  • In full-face view humeri of promesonotum not armed with a small, bluntly rounded angle, tubercle or rugosity, though they may be flattened and project as flanges over sides of mesopleuron . . . . . 34

34

return to couplet #33

  • With combination of: eyes relatively large (eye width ≥ greatest width of antennal scape, and number of ommatidia ≥ 16); postpetiole attenuated anteriad; weak, longitudinal striolae nearly always present on sides of petiole; maximum of four mandibular teeth and denticles; promesonotum always smoothly rounded on to humeri (HW 0.63–0.70 mm; SI 88–100) . . . . . Syllophopsis adiastolon
Monomorium adiastolon casent0172864 head 1.jpg
Monomorium adiastolon casent0172864 profile 1.jpg
  • Without above combination of characters (eyes usually small, eye width < greatest width of antennal scape, other characters vary with species) . . . . . 35

35

return to couplet #34

  • Petiolar node thick, posterior face descending at right angle to petiole, dorsum broadly rounded and sides lacking longitudinal striolae; five mandibular teeth and denticles often present; promesonotum flattened and projecting as flange in smooth, shining specimens; anteromedian margin of clypeus indented, straight or narrowly convex with indistinct clypeal carinae; propodeal angle always square, lacking short denticles; nest series show allometric monophasy, some southern populations distinctly polymorphic with large-headed major workers; average size larger (HL 0.64–1.23 mm; HW 0.50–1.26 mm) . . . . . Syllophopsis fisheri
Monomorium fisheri casent0491610 head 1.jpg
Monomorium fisheri casent0491610 profile 1.jpg
  • Petiolar node usually distinctly asymmetrical or narrow tending to cuneate, if node thick and erect, then often finely longitudinally striolate with dorsum narrowly rounded and posterior surface distinctly sloping at angle of 90° <; maximum of four mandibular teeth or denticles; promesonotum always rounded; anteromedian margin of clypeus straight to indented, extension of clypeal carinae usually forming blunt angles; propodeal angle often armed with short denticles or flanges; nest series monomorphic with, at most, slight variation in size; average size smaller (HL 0.44–0.73 mm; HW 0.36–0.61 mm) . . . . . Syllophopsis hildebrandti
Monomorium hildebrandti casent0023410 head 1.jpg
Monomorium hildebrandti casent0023410 profile 1.jpg