Key to Octostruma species

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This worker key is based on Longino, J.T. 2013. A revision of the ant genus Octostruma Forel 1912 (Hymenoptera, Formicidae). Zootaxa 3699, 1-61. A discussion of the important characters for these species (see Octostruma characters) was included in this revision. These notes are useful for working through the key, especially if you are unfamiliar with this genus.

1

  • Face crossed by a clearly defined arcuate carina; anterior portion of face concave, differentiated from posterior portion, delimited by frontal carinae and facial arc (Fig. 1); HW < 0.65 . . . . . 2
  • Face lacking a sharp arcuate carina and differentiated anterior portion, occasionally a low arcuate convexity present, but not clearly defined and anterior portion not differentiated; HW variable . . . . . 8

2

return to couplet #1

  • Antennal scrobe deep and conspicuous with distinct posterior and ventral margins; vertex behind facial arc foveolate, punctate, or rugose (Fig. 1A, B) . . . . . 3
  • Antennal scrobe shallow and inconspicuous, fading posteriorly and ventrally; vertex behind facial arc smooth and shining with sparse puncta (Fig. 1C-F) . . . . . 4

3

return to couplet #2

  • Face anterior to facial arc uniformly densely punctate; posterior margin of vertex lacking erect setae; facial arc joins dorsal margin of scrobe posterior to eye; eye composed of > 5 ommatidia (Fig. 1A) (Venezuela, southern Brazil) . . . . . Octostruma rugifera
  • Face anterior to facial arc shallowly rugulose; posterior margin of vertex with pair of erect setae; facial arc joins dorsal margin of scrobe anterior to eye; eye minute, composed of < 5, partially fused ommatidia (Fig. 1B) (southern Mexico, Guatemala) . . . . . Octostruma rugiferoides
Octostruma rugiferoides, worker (top) and queen. Scale bar = 0.2 mm face view, 0.5 mm lateral view.

4

return to couplet #2

  • Facial arc continuous with frontal carinae (Fig. 1C, F) . . . . . 5
  • Facial arc extending laterally beyond frontal carinae, with shallow trough separating posterior ends of frontal carinae and facial arc (Fig. 1D, E) . . . . . 6

5

return to couplet #4

  • Facial arc more pronounced, arc relatively broader and less semicircular, frontal carinae proportionally longer and more convergent anteriorly, joining facial arc posterior to compound eye (Fig. 1C); pronotal dorsum smooth and shiny, differentiated from shallowly foveolate mesonotum (Central America) . . . . . Octostruma ascrobis
Octostruma ascrobis worker (top) and queen (bottom). Scale bar = 0.2 mm face views, 0.5 mm lateral views.
  • Facial arc less pronounced, arc more strongly curved and semicircular, frontal carinae shorter and less convergent, joining facial arc at level of compound eye (Fig. 1F); mesonotal foveolation extending onto pronotal dorsum, weakening anteriorly (Panama, Colombia) . . . . . Octostruma ascrobicula
Octostruma ascrobicula. Scale bar = 0.25 mm face view, 0.5 mm lateral view.

6

return to couplet #4

  • Facial arc strong throughout, much stronger than frontal carinae (Fig. 1D) (Panama) . . . . . Octostruma limbifrons
Octostruma limbifrons worker. Scale bar = 0.25 mm face view, 0.5 mm lateral view.
  • Facial arc distinct but less elevated, becoming irregular and somewhat weaker laterally, frontal carinae stronger than lateral portions of facial arc (Fig. 1E) . . . . . 7

7

return to couplet #6

  • Promesonotal dorsum uniformly foveolate; three pairs of spatulate setae along facial arc (Ecuador) . . . . . Octostruma convallisur
Octostruma convallisur worker (top) and queen. Scale bar = 0.2 mm face view, 0.5 mm lateral view.
  • Pronotum smooth and shiny, differentiated from foveolate mesonotum; one pair of spatulate setae on posterolateral corners of facial arc (Costa Rica) . . . . . Octostruma convallis
Octostruma convallis worker (top) and queen. Scale bar = 0.2 mm face view, 0.5 mm lateral view.

8

return to couplet #1

  • Antennal scrobe shallow, not or only feebly margined; base of scape without flattened anterior lobe . . . . . 9
  • Antennal scrobe deep and sharply demarcated with a carinate rim; base of scape with flattened anterior lobe . . . . . 10

9

return to couplet #8

  • Mandible elongate, with basal margin meeting masticatory margin at oblique angle, closed mandibles with pronounced gap between anterior margin of clypeus and masticatory margins; metanotal groove impressed; HW 0.50–0.56 (Panama to Ecuador) . . . . . Octostruma stenoscapa
  • Mandible subtriangular, basal margin meeting masticatory margin at close to right angle, closed mandibles flush with anterior margin of clypeus; HW > 0.68 (Colombia to Bolivia) . . . . . Octostruma inca
Octostruma inca worker (top) and queen. Scale bar = 0.25 mm face view, 0.5 mm lateral view.

10

return to couplet #8

  • Metanotal groove broadly impressed, promesonotum and dorsal face of propodeum forming two distinct convexities; basal tooth of mandible broad and blunt, differentiated from acute second tooth; HW > 0.66 (Colombia) . . . . . Octostruma impressa
  • Metanotal groove thin or absent, if more broadly impressed then basal tooth of mandible acute and similar to second tooth; other characters variable . . . . . 11

11

return to couplet #10

  • Mandible with basal tooth blunt/truncate or in the form of a rectangular lamina (Fig. 2D); second tooth from base acute, differ-entiated from basal tooth or lamina; vertex lobes uniformly punctate and opaque, never rugulose or smooth; face rarely obscured by a layer of soil; HW < 0.68 (O. balzani complex) . . . . . 12
Fig 2. Mandibles of Octostruma workers. Drawings are not to scale. A. O. triangulabrum, O. wheeleri, O. triquetrilabrum. B. O. cyrtinotum, O. gymnosoma, O. montanis, O. planities, O. schusteri. C. O. iheringi. D. O. balzanicomplex: O. amrishi, O. balzani, O. batesi, O. betschi, O. gymnogon, O. lutzi, O. megabalzani, O. stenognatha, O. trithrix. E. O. excertirugis, O. obtusidens. F. O. rugiferoides. G. O. ascrobis, O. convallis.
  • First and second basalmost teeth of mandible similar in shape, either acute or rounded (Fig. 2A, B, C, E); vertex lobes punctate, rugose, rugulose, or nearly smooth; face sometimes obscured by layer of soil; HW > 0.57 . . . . . 20

12

return to couplet #11

  • In most workers of a series, posterolateral vertex margin with an erect spatulate seta (Fig. 3A, B); mesosoma usually with one or two pairs of spatulate setae, rarely lacking them . . . . . 13
  • In most workers of a series, posterolateral vertex margin lacking erect seta (Fig. 3C, D); mesosoma usually lacking spatulate setae . . . . . 19
Fig 3. Full face view of Octostruma species, showing outline of headcapsule and disposition of spatulate setae. Depictions of seta number and location are species averages and individuals may vary. A. O. trithrix, O. lutzi (part). B. O. balzani, O. lutzi (part), O. megabalzani. C. O. amrishi. D. O. gymnogon. E. O. iheringi. F. O. obtusidens. G. O. excertirugis. H. O. leptoceps. I. O. wheeleri. J. O. triangulabrum. K. O. triquetrilabrum. L. O. cyrtinotum. M. O. gymnosoma. N. O. schusteri. O. O. planities. P. O. montanis. Note the enlarged seta pits on O. iheringi, O. cyrtinotum, and O. montanis. Drawings are not to same scale.

13

return to couplet #12

  • Posterior half of head with posterolateral margin shorter than anterolateral margin (Fig. 4A), giving head a more triangular appearance; basal lobe of scape weakly developed, margins proximal and distal to basal bend at approximate right angle . . . . . 14
Fig 4. Face views of (A) Octostruma betschi and (B) Octostruma balzani.
  • Posterior half of head with posterolateral margin subequal to or longer than anterolateral margin (Fig. 4B), giving head more transverse appearance; basal lobe of scape strongly developed, margins proximal and distal to basal bend at acute angle . . . . . 16

14

return to couplet #13

  • Mesosoma with one pair of spatulate setae, on mesonotum; anterior portion of face convex; dorsal surface of scape with thin ground pilosity; CI 99–103 (southern Brazil) . . . . . Octostruma stenognatha
  • Mesosoma with two pairs spatulate setae, one on pronotum and one on mesonotum; anterior portion of face convex or some-what concave; dorsal surface of scape with thin or spatulate ground pilosity; CI 106–111 . . . . . 15

15

return to couplet #14

  • Dorsal surface of scape with short appressed spatulate setae; anterior portion of face somewhat concave (Amazonian South America) . . . . . Octostruma betschi
Octostruma betschi worker. Scale bar = 0.25 mm face view, 0.5 mm lateral view.
  • Dorsal surface of scape with thin ground pilosity; anterior portion of face flat to convex (eastern Andean foothills) . . . . . Octostruma batesi

16

return to couplet #13

  • Mesosoma typically with two pairs spatulate setae, one pair in anterior half of promesonotum and one pair in posterior half (Lesser Antilles) . . . . . Octostruma lutzi
Octostruma lutzi workers and queen (bottom). Scale bar = 0.25 mm face view, 0.5 mm lateral view.
  • Mesosoma typically with one pair spatulate setae, on posterior half of promesonotum (mainland Neotropics) . . . . . 17

17

return to couplet #16

  • In most workers of a series, a spatulate seta present between anterior seta on side of head (near eye) and anteromedial vertex seta; in profile view, metanotal groove completely absent or weakly impressed (Fig. 5A); first gastral tergite typically with 8–16 erect setae (Mexico to Honduras) . . . . . Octostruma trithrix
Fig 5. Profile view of mesosoma, (A) Octostruma trithrix and (B) Octostruma balzani.
Octostruma trithrix worker. Scale bar = 0.25 mm face view, 0.5 mm lateral view.
  • In most workers of a series, no spatulate seta between anterior seta on side of head and medial vertex seta; in profile view, metanotal groove usually visible as a small notch (Fig. 5B); gastral setae variable . . . . . 18

18

return to couplet #17

  • Color red brown; typically < 16 erect setae on first gastral tergite (<10 in northern Central America, where sympatric with O. trithrix), more clustered posteriorly, apically broader, more spatulate (Fig. 6A); HW 0.50–0.63 (average 0.57, n=27) (wide-spread in Neotropics) . . . . . Octostruma balzani
Fig 6. Setae on first gastral tergite of (A) Octostruma balzani and (B) Octostruma megabalzani.
  • Color dark brown; typically about 20 erect setae on first gastral tergite, more evenly distributed across tergite, relatively linear and less spatulate (Fig. 6B); HW 0.61–0.66 (mountains of Panama, Peru, Bolivia) . . . . . Octostruma megabalzani
Octostruma megabalzani worker. Scale bar = 0.25 mm face view, 0.5 mm lateral view.

19

return to couplet #12

  • Posterior margin of vertex usually lacking spatulate setae (Fig. 3D); color dark brown (mountains of Central America) . . . . . Octostruma gymnogon
  • Posterior margin of vertex usually with medial pair of spatulate setae (Fig. 3C); color lighter red brown (Central America and northern South America) . . . . . Octostruma amrishi
Octostruma amrishi.Scale bar = 0.25 mm face view, 0.5 mm lateral view.

20

return to couplet #11

  • A single tall, transverse carina extends between propodeal spines, separating dorsal and posterior faces of propodeum, broad-ening laterally and extending to the apex of the propodeal spine, joining vertical plane of spine at a right angle, forming a roof-like structure over the posterior face of the propodeum and creating a thin-walled, concave propodeal spine that looks like a box corner (Fig. 7A) . . . . . 21
Fig 7. Oblique view of propodeal spines of (A) Octostruma iheringi and (B)Octostruma obtusidens.
  • Transverse carina between propodeal spines present or absent; if present, thin and fading medially, at most weakly expanded laterally (Fig. 7B) . . . . . 23

21

return to couplet #20

  • Pair of long filiform setae projecting from petiolar peduncle anterior to spiracle (these setae very fine and often difficult to see) (Fig. 8) (this character also present on Octostruma pexidorsum, elsewhere in key); shorter filiform setae projecting from sides of post-petiole; promesonotum with moderately impressed longitudinal sulcus; face with 8 spatulate setae, arranged around periphery of face, none medially (Fig. 3E); promesonotum and first gastral tergite lacking spatulate setae (widespread in Neotropics) . . . . . Octostruma iheringi
Fig 8. Octostruma iheringi, dorsal view of petiole and postpetiole, showing sensory setae.
Octostruma iheringi worker (top) and queen. Scale bar = 0.2 mm face view, 0.5 mm lateral view.
  • Filiform setae lacking on petiole and postpetiole; other characters variable. . . . . 22

22

return to couplet #21

  • Mesosoma with two pairs erect setae, one on pronotum and one on mesonotum; dorsal promesonotum with broad, shallow, longitudinal sulcus; HW 0.74 (Ecuador) . . . . . Octostruma onorei
  • Mesosoma lacking erect setae; dorsal promesonotum evenly convex; HW 0.85–0.90 (petiolar node in dorsal view strongly transverse) (southern Brazil) . . . . . Octostruma petiolata

23

return to couplet #20

  • Basal five teeth of mandible bluntly rounded (Fig. 2E); labrum about as wide as long; anterior labral lobes separated, about as wide as long (Fig. 9F); HW < 0.68 . . . . . 24
Fig 9. Labra of Octostruma workers. Drawings are not to scale. A. O. leptoceps, O. triquetrilabrum, O. triangulabrum, O. wheeleri. B. O. gymnosoma, O. schusteri. C. O. cyrtinotum, O. montanis, O. planities. D. O. iheringi. E. O. balzanicomplex: O. amrishi, O. balzani, O. batesi, O. betschi, O. gymnogon, O. lutzi, O. megabalzani, O. stenognatha, O. trithrix. F. O. excertirugis, O. obtusidens. G. O. rugiferoides. H. O. ascrobis, O. convallis. I. O. pexidorsum.
  • Basal five teeth of mandible acute (Fig. 2A, B); labrum longer than wide; anterior labral lobes separate and longer than wide or confluent (Fig. 9A, B, C, I); HW variable . . . . . 25

24

return to couplet #23

  • Face sculpture foveolate with at most faint longitudinal rugulae (Fig. 10A); when covered with soil, any rugulae usually not visible; face with 10–14 large spatulate setae projecting above ground pilosity; ground pilosity curved, projecting from sur-face; first gastral tergite with > 10 spatulate setae; first gastral tergite punctate over entire surface, although puncta somewhat smaller and sparser posteriorly; color orange brown (Guatemala to Colombia) . . . . . Octostruma obtusidens
Fig 10. Face views of (A) Octostruma obtusidens and (B) Octostruma excertirugis.
  • Face sculpture longitudinally rugose, interspaces matte but not foveolate (Fig. 10B); when covered with soil, rugulae often project through soil layer; face with at most 10 spatulate setae; ground pilosity fully appressed; first gastral tergite with < 10 spatulate setae; first gastral tergite punctate on anterior half, fading to nearly smooth and shining posteriorly; color red brown (Mexico to Ecuador) . . . . . Octostruma excertirugis

25

return to couplet #23

  • Ground pilosity abundant, subdecumbent, woolly; apical half of labrum with parallel sides, apex blunt but not bilobed (Fig. 8); pair of long filiform setae projecting from petiolar peduncle anterior to spiracle (these setae very fine and often difficult to see) (Fig. 8) (this character also present on the more common O. iheringi); shorter filiform setae projecting from sides of post-petiole and anteromedian first gastral sternite; HW 0.63–0.71; CI 106–111 (Amazonian Colombia) . . . . . Octostruma pexidorsum
Octostruma pexidorsum worker. Scale bar = 0.25 mm face view, 0.5 mm lateral view.
  • Ground pilosity sparse, fully appressed, not woolly; labrum shape variable; filiform setae lacking on petiole, postpetiole, first gastral sternite; HW and CI variable . . . . . 26

26

return to couplet #25

  • Labrum apex blunt, not distinctly bilobed; sides of labrum straight and evenly converging (Fig.9A) . . . . . 27
  • Labrum apex distinctly bilobed, sides of labrum straight or slightly concave, anterior lobes porrect, parallel and separate (Fig. 9B, C) . . . . . 30

27

return to couplet #26

Octostruma leptoceps worker. Scale bar = 0.2 mm face view, 0.5 mm lateral view.

28

return to couplet #27

  • Face with at most six spatulate setae (Fig.3I); promesonotum never with spatulate setae; face and dorsal pronotum matte but mostly smooth, not rugulose (Central America) . . . . . Octostruma wheeleri
Octostruma wheeleri worker (top) and queen. Scale bar = 0.2 mm face view, 0.5 mm lateral view.
  • Face with 8-10 spatulate setae (Fig. 3J, K); promesonotum lacking or with one pair of spatulate setae; face and dorsal prono-tum with stronger reticulate rugulose sculpture (although still faint compared to many other Octostruma) . . . . . 29

29

return to couplet #28

Octostruma triangulabrum worker. Scale bar = 0.2 mm face view, 0.5 mm lateral view.
Octostruma triquetrilabrum worker. Scale bar = 0.2 mm face view, 0.5 mm lateral view.

30

return to couplet #26

  • Anterior half of dorsal face of propodeum moderately to distinctly convex, demarcating broadly impressed metanotal groove (Fig. 11A); HW 0.6–0.7; face typically with 6 or 8 spatulate setae (as few as 2 on some Costa Rican O. montanis), seta-bearing pits along vertex margin large (Fig. 3L, P); mesosomal dorsum with or without spatulate setae . . . . . 31
Fig 11. Profile view of mesosoma of (A) Octostruma cyrtinotum and (B) Octostruma planities.
  • Dorsal face of propodeum flat or weakly convex over entire length; metanotal groove not impressed, or if slightly impressed, not as a result of a dorsal propodeal convexity (Fig. 11B); HW < 0.72 or > 0.80; face with 0 to 8 spatulate setae, seta-bearing pits not unusually large (Fig. 3M, N, O); mesosomal dorsum lacking spatulate setae . . . . . 32

31

return to couplet #30

  • Mesosomal dorsum lacking spatulate setae; face with 2 to 6 spatulate setae (Fig. 3P) (Nicaragua, Costa Rica) . . . . . Octostruma montanis
Octostruma montanis worker. Scale bar = 0.2 mm face view, 0.5 mm lateral view.
  • A single pair of spatulate setae on the mesonotum; face with 8 spatulate setae (Fig. 3L) (Honduras, Nicaragua) . . . . . Octostruma cyrtinotum
Octostruma cyrtinotum worker (top) and queen. Scale bar = 0.2 mm face view, 0.5 mm lateral view.

32

return to couplet #30

  • HW < 0.72; face typically with 2 spatulate setae on posteriomedian vertex margin (Fig 3O) (Mexico to Costa Rica) . . . . . Octostruma planities
Octostruma planities worker. Scale bar = 0.2 mm face view, 0.5 mm lateral view.
  • HW > 0.80; face typically with 0 or 6 spatulate setae . . . . . 33

33

return to couplet #32

Octostruma gymnosoma worker. Scale bar = 0.2 mm face view, 0.5 mm lateral view.
Octostruma schusteri worker. Scale bar = 0.2 mm face view, 0.5 mm lateral view.