Key to Pheidole tristis group

This worker key is based on:

This exclusively Neotropical group ranges from lowland Mexico to Argentina and throughout the West Indies. It rivals the other giant Pheidole assemblages of the New World, the diligens, fallax, and flavens groups, in its species diversity and magnitude of adaptive radiation. Like these other groups, it is vaguely bounded and connected to other assemblages by phenetically intermediate species. Nonetheless, a suite of traits allows reasonably secure placement of the great majority of species classified with the Pheidole tristis group in the present treatment. They include medium to large size, 4 or 5 hypostomal teeth, comparatively short antennal scapes in the major and moderately long ones in the minor, relatively small eyes set well back from the anterior genal border, and a mesonotal convexity either weakly developed or altogether absent. Also present among a third of the species are hornlike protrusions from the frontal lobes and mid-clypeal carina in the major caste and, in many, also decoration of the pronotum with conspicuous parallel transverse carinulae in the minor caste. Three very distinctive Cuban species (Pheidole alayoi, Pheidole macromischoides, and Pheidole naylae) as well as Pheidole avia and Pheidole bucculenta from Brazil, Pheidole bruchi from Argentina, Pheidole laeviventris from Colombia, Pheidole macracantha from Panama, Pheidole severini from French Guiana, and Pheidole microps and Pheidole minax from Peru, are so far known only from minor workers and are not included in the key that follows. However, they can be readily identified from the drawings and diagnoses provided in their individual species descriptions.

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 * Pheidole
 * "New World Pheidole" Wilson 2003

1

 * Very large species, Head Width of major about 2.6 mm and of minor 0.8-1.0 mm. Major: in side view, either the ventral profile of the postpetiole bulges forward to form a convexity (separate from the rest of the postpetiolar sternite) as large as the petiolar node, or else the ventral profile is covered with a dense brush of medium-length hairs . . . . . 2


 * Smaller species, Head Width of major usually under 2.0 mm and rarely over 2.5 mm. Major: ventral profile of postpetiole concave, straight, or convex, but if convex, the protrusion is much smaller than the postpetiolar node, and at most it bears 3 or 4 hairs . . . . . 3

2
return to couplet #1
 * Major: profile of postpetiolar venter bearing a dense brush of medium-length hairs, and profile of petiolar venter completely lined with a dense row of uniformly short hairs; postpetiolar venter bearing convexity much smaller than the petiolar node; seen from above, lateral margins of postpetiolar node drawn into spines (southern Mexico to Argentina) . . . . . Pheidole fimbriata


 * Major: postpetiolar and petiolar venters at most with several hairs; and the profile of the postpetiolar venter bulges forward as a convexity as large as the petiolar node; seen from above, lateral margins of postpetiolar node smoothly rounded (southern Mexico to Amazonian Bolivia and Brazil and east to the Guianas and Tobago) . . . . . Pheidole cephalica

3
return to couplet #1
 * Major: in side view, profile of mid-clypeal longitudinal carina drawn out from the surface of the head into a lobe, angle, or spike, sometimes small but still conspicuous; or else the frontal lobe is drawn out in profile forward or downward to create a horn, tooth, or lobe whose upper margin and lower margin if extended as lines that meet would form a right angle or acute angle; or both . . . . . 4


 * Major: in side view, neither the mid-clypeal carina (if there is one) nor the frontal lobe is drawn out into protrusions as described above; in particular, even if the frontal lobe is drawn out in a short lobe, its upper margin and lower margin if extended as lines would meet to form an obtuse angle, not a right or acute angle . . . . . 40

4
return to couplet #3
 * Major: the profile of the mid-clypeal longitudinal carina (as opposed to the frontal lobes) drawn outward into at least a small protruding hornlike angle, lobe, or spike . . . . . 5


 * Major: the mid-clypeal profile, even if a longitudinal carina is present, is flat or slightly convex, with no part of it drawn outward as an angle, lobe, or spike (but the frontal lobes are extended this way) . . . . . 18

5
return to couplet #4
 * Major: entire dorsum of head and mesosoma (thorax plus propodeum) covered by mostly transverse carinulae . . . . . 6


 * Major: posterior half of dorsal head surface and at least the center of the promesonotal dorsum smooth, or foveolate and opaque, or partly covered by longitudinal (not transverse) carinulae . . . . . 7

6
return to couplet #5
 * Major: in profile, the entire clypeus bulges out into a nose-like protrusion larger than the eye; in full-face view, longitudinal carinulae originating on the frontal lobes tum away from the midline of the head just anterior to the occiput and travel horizontally to the margins of the head (Amazonian Peru) . . . . . Pheidole nasifera


 * Major: in profile, the posterior half of the mid-clypeal carina protrudes as a blunt obtuse angle much smaller than the eye; all carinulae on the dorsal surface of the head posterior to the eyes are horizontal (Veracruz, Mexico) . . . . . Pheidole triumbonata

7
return to couplet #5
 * Major: shallow antennal scrobes present, even if each is represented only by a clear or foveolate space marked posteriorly with a curving posterior carinula . . . . . 8


 * Major: antennal scrobes absent . . . . . 11

8
return to couplet #7
 * Major: carinulae and/or rugulae cover all of posterior half of dorsal head surface except occiput . . . . . 9


 * Major: carinulae and/or rugulae absent from posterior half of dorsal head surface, with the exception of a single curving carinula that marks the posterior limit of the antennal scrobe on each side . . . . . 10

9
return to couplet #8
 * Major: lateral anterior margins of mesonotum and dorsal anterior sides of propodeum carinulate. Minor: occiput rugoreticulate (Argentina, southern Brazil) . . . . . Pheidole risii (in part - also )


 * Major: lateral anterior margins of mesonotum and dorsal anterior sides of propodeum lack carinulae. Minor: occiput foveolate only, lacking rugoreticulum or other sculpture (Argentina) . . . . . Pheidole rosula (in part - also )

10
return to couplet #8
 * Major: in side view, margin of mesonotal convexity as high as pronotal margin; anterior pronotal strip with multiple carinulae; median ocellus present. Minor: nuchal collar absent (Venezuela) . . . . . Pheidole gravida


 * Major: in side view, margin of mesonotal convexity much lower than pronotal margin; anterior pronotal strip with only a single carinula; ocelli absent. Minor: nuchal collar present (Venezuela) . . . . . Pheidole rhytifera

11
return to couplet #7
 * Major: in full-face view, frontal lobes square in shape, completely smooth and shining, and the distance between their outer margins 2 X as great as the distance from the outer margin of either one to the nearby lateral margin of the head (Costa Rica, Colombia) . . . . . Pheidole vallifica


 * Major: in full-face view, the frontal lobes are lobe-shaped or angular in front, not square-shaped, and they are at least partly covered by longitudinal carinulae . . . . . 12

12
return to couplet #11
 * Major: in side view, center of clypeus extended as a large nose-like lobe longer than the eye, and its visible part is rugoreticulate (Choco, Colombia) . . . . . Pheidole binasifera


 * Major: in side view, the protrusion of the clypeus either is much smaller than the eye or, if as large, the visible part is not rugoreticulate . . . . . 13

13
return to couplet #12
 * Major (possibly the ant species of any genus with the most elaborate cephalic armament): the anterior clypeal margin is extended downward as 4 large teeth, one pair to a side; the center of the clypeus (viewed from the side) protrudes as a nose-like appendage larger than the eye; a sharp spike projects from the uppermost margin at the center of the clypeus; and the frontal lobes project outward somewhat as additional lobes (montane Colombia) . . . . . Pheidole mirabilis


 * Major: in side view, bearing only a comparatively small central projection on the clypeus and, in some species, a projection of the frontal lobe . . . . . 14

14
return to couplet #13
 * Major: the space between eye and antennal fossa is at least partly filled by rugoreticulum (Central America, Colombia) . . . . . Pheidole rhinoceros


 * Major: the space between eye and antennal fossa is entirely filled by longitudinal carinulae, and lacks a rugoreticulum . . . . . 15

15
return to couplet #14 . . . . . Pheidole unicornis
 * Major: strong parallel transverse carinulae cover the anterior third of the pronotal dorsum and all of the basal face of the propodeum (montane Colombia and Ecuador)


 * Major: at most, several broken transverse carinulae occur on the anterior third of the pronotal dorsum; carinulae absent from the basal face of the propodeum . . . . . 16

16
return to couplet #15
 * Major: in side view, profile of mesonotal convexity is triangular with an acute apex. Minor: seen from above and obliquely, propodeal spine reduced to an obtuse angle formed by the two propodeal faces (Bolivia, Costa Rica) . . . . . Pheidole cornicula


 * Major: in side view, profile of mesonotal convexity surmounted by at most a low secondary convexity. Minor: seen from above and obliquely, propodeal spine developed at least as a well-formed triangular denticle . . . . . 17

17
return to couplet #16
 * Major: moderately large species (Head Width of major l.9-2.0 mm, of minor 0.6 0.7 mm); in side view, propodeal spine rises perpendicular to the basal face of the propodeum and is half as long; mesonotum foveolate and opaque. Minor: dorsal surface of head foveolate and opaque (Argentina, southern Brazil) . . . . . Pheidole schwarzmaieri


 * Major: smaller species (Head Width of major about 1.5 mm, of minor 0.6 mm); in side view, propodeal spine forms a l30-degree angle with the propodeal basal face and is only one-third as long. Minor: dorsal surface of head smooth and shiny (Costa Rica, Nicaragua) . . . . . Pheidole celaena

18
return to couplet #4
 * Major: head completely lacks rugoreticulum; its sculpture is limited to carinulae and foveolae . . . . . 19


 * Major: head with at least small patches of rugoreticulum, especially laterad to or behind the antennal fossae . . . . . 26

19
return to couplet #18
 * Major: in full-face view, longitudinal carinulae on both sides of the midline extend posteriorly all the way to the occipital border. Major and minor: in side view, propodeal spine is more than half as long as the basal face of the propodeum anterior to it, and, in the minor, the spine is curved posteriorly (montane Colombia) . . . . . Pheidole pidax


 * Major: in full-face view, at least the posterior third of the head capsule free of carinulae, and smooth and shining. Major and minor: in side view, propodeal spine less than half as long as the basal face of the propodeum, and straight, not curved posteriorly . . . . . 20

20
return to couplet #19
 * Major: propodeal spiracle nearly half as wide as length of the eye and also as wide as base of propodeal spine; seen from above and obliquely, humerus subangulate; head capsule bicolored, with anterior half yellow and posterior half yellowish brown. Minor: in side view, propodeal spine one-third as long as propodeal basal face and pointed backward, almost level with the face (Costa Rica to Brazil and Peru) . . . . . Pheidole cramptoni


 * Major: propodeal spiracle much less than half as wide as eye length and not as wide as base of propodeal spine; seen from above and obliquely, humerus smoothly rounded; head capsule concolorous, variously yellow to brown, or, if bicolored, anterior half is dark reddish brown. Minor: in side view 5 propodeal spine either is much shorter than one-third the basal propodeal face, or, if longer, it rises nearly perpendicularly to the basal face . . . . . 21

21
return to couplet #20
 * Major: seen from above, lateral margins of postpetiolar node are drawn out into sharp spines . . . . . 22


 * Major: seen from above, lateral margins of postpetiolar node are right-angular or convex, not spinose . . . . . 23

22
return to couplet #21
 * Major: in side view, frontal lobes extend as acute angles downward almost to the level of the anterior clypeal border (Para, Brazil) . . . . . Pheidole allarmata


 * Major: in side view, frontal lobes point outward, perpendicular to the head surfaces and do not approach the anterior clypeal border (Brazil) . . . . . Pheidole moseni

23
return to couplet #21
 * Major: seen from the side, promesonotal profile as a whole forms a smooth semicircle; in side view also, frontal lobe forms a triangle whose height (away from head surface) is about equal to the width of its base along the head surface. Major and minor: body yellow (Guyana) . . . . . Pheidole synarmata


 * Major: seen from side, promesonotal profile bilobed or flattened, not semicircular; in side view also, frontal lobe forms a triangle only half as high as the base is wide. Major and minor: yellowish brown or dark brown . . . . . 24

24
return to couplet #23
 * Major: longitudinal carinulae immediately mesad to the eye extend posteriorly to halfway between the level of the eye and level of the occipital margin; a well-defined suture extends all around the boundary between prothorax and mesothorax, and in side view it divides the promesonotal profile into 2 lobes; dorsum of head concolorous dark brown (Costa Rica) . . . . . Pheidole hasticeps


 * Major: longitudinal carinulae immediately mesad to the eye extend posteriorly only one-tenth or so the distance from level of the eye to level of the occipital margin; suture separating the prothorax and mesothorax absent from the promesonotal dorsum, the profile of which is a single flattened curve; either light yellowish brown or tricolored (head brown, mesosoma and waist yellow, gaster brown) . . . . . 25

25
return to couplet #24
 * Major: light yellowish brown, with a darker, indistinct brownish spot just posterior to center of the head (abundant in disturbed habitats, southern Mexico to Brazil and throughout the West Indies) . . . . . Pheidole subarmata


 * Major: tricolored, head medium brown, mesosoma, waist, and legs yellow, gaster light brown (montane Trinidad) . . . . . Pheidole triplex

26
return to couplet #18
 * Major: posterior half of head capsule, and all of pronotal and propodeal dorsa covered by parallel transverse carinae (Costa Rica) . . . . . Pheidole arietans


 * Major: transverse carinulae cover at most the pronotal dorsum and/or a small part of the occiput . . . . . 27

27
return to couplet #26
 * Major and minor: all of pronotal dorsum or at least its anterior half rugoreticulate . . . . . 28


 * Major and minor: at most, the humeri alone are rugoreticulate in major, and pronotum dorsum of minor lacks rugoreticulum entirely . . . . . 31

28
return to couplet #27
 * Major: in full-face view, posterior half of dorsal surface of head smooth and shiny (montane Venezuela) . . . . . Pheidole hortonae


 * Major: in full-face view, posterior half of dorsal surface of head mostly or entirely rugulose to rugoreticulate, with non-reticulate portion foveolate and opaque . . . . . 29

29
return to couplet #28
 * Major: occiput and sides of pronotum entirely rugoreticulate (southeastern Brazil) . . . . . Pheidole aper


 * Major: occiput and sides of pronotum rugose or foveolate, not reticulate . . . . . 30

30
return to couplet #29
 * Major: in full-face view, occiput longitudinally rugulose; sides of pronotum with scattered, broken longitudinal rugulae; anterior fringe of central strip of first gastral tergite foveolate and opaque. Minor: sides of pronotum rugoreticulate; in side view, propodeal spine perpendicular to the basal propodeal face and one-third as long (Panama, Costa Rica) . . . . . Pheidole traini


 * Major: in full-face view, occiput foveolate and opaque, lacking rugulae and carinulae; sides of pronotum covered by widely spaced, continuous transverse rugulae; first gastral tergite entirely smooth and shiny. Minor: sides of pronotum longitudinally carinulate; in side view, propodeal spine a triangular denticle less than one-tenth as long as the basal propodeal face (Sao Paulo, Brazil) . . . . . Pheidole sigillata

31
return to couplet #27
 * Major: in side view, frontal lobes drawn out into forward-directed, sharp, acute angular, and upward-curving horns; seen from above and obliquely, humerus acute-angulate (Choco, Colombia) . . . . . Pheidole diabolus


 * Major: in side view, frontal lobes at most right-angular, round-tipped, and not curving upward; humerus smoothly rounded . . . . . 32

32
return to couplet #31
 * Major: shallow antennal scrobes present and clearly demarcated by surrounding sculpture on the dorsal head surface, although not deep enough (nor are the frontal lobes expanded enough laterally) to receive the antennae . . . . . 33


 * Major: at most a slight trace of antennal scrobes present . . . . . 38

33
return to couplet #32
 * Major: rugoreticulum limited to a small patch between eye and antennal fossa (Santa Catarina, Brazil) . . . . . Pheidole rosae


 * Major: rugoreticulum present or not between eye and antennal fossa, but either way also occurs in a small patch either just posterior to the antennal scrobe, or just behind and above the eyes, or both . . . . . 34

34
return to couplet #33
 * Major: in full-face view, carinulae on dorsum of head surface extend all the way to occipital border, including occipital lobes (Ecuador) . . . . . Pheidole fullerae


 * Major: in full-face view, occiput lacking carinulae, its surface smooth and shiny, with or without scattered foveae . . . . . 35

35
return to couplet #34
 * Major: rugoreticula present as small patches behind and above the eye and also behind the posterior margin of the scrobe (Santa Catarina, Brazil) . . . . . Pheidole hetschkoi


 * Major: rugoreticulum present either as a small patch just behind and above the eye or else as a small patch just behind the antennal scrobe, but not both . . . . . 36

36
return to couplet #35
 * Major: rugoreticulum present as a small patch just behind and above the eye, but absent from just behind the posterior margin of the antennal scrobe (Ecuador) . . . . . Pheidole napoensis


 * Major: the reverse of above, i.e ., rugoreticulum present as a small patch just behind the margin of the antennal scrobe but absent from behind and above the eye . . . . . 37

37
return to couplet #36
 * Major: seen in dorsal-oblique view (mesosoma rotated around long axis 45 degrees from top view), profile of pronotal dorsum forms the near-perfect arc of a circle; mesonotal dorsum completely covered by transverse carinulae. Minor: entire promesonotal dorsum covered by transverse carinulae (Choco, Colombia) . . . . . Pheidole securigera


 * Major: seen in dorsal-oblique view, anterior half of profile of pronotal dorsum raised as a separate lobe, and posterior half nearly flat; mesonotal dorsum mostly bare of carinulae. Minor: transverse carinulae are absent from mesonotum, and only several occur near anterior and posterior margins of pronotum (Ecuador) . . . . . Pheidole guayasana

38
return to couplet #32
 * Major: rugoreticulum mesad to the eye extends posteriorly almost to the occipital border (Costa Rica) . . . . . Pheidole arachnion


 * Major: rugoreticulum mesad to the eye restricted to the space between the eye and the antennal fossa . . . . . 39

39
return to couplet #38
 * Major: very large (Head Width 2.3 mm); reddish yellow; propodeal dorsum longitudinally carinulate; seen from above and obliquely, humerus right-angulate (southeastern Brazil, Paraguay) . . . . . Pheidole praeses


 * Major: small (Head Width 0.8 mm); medium to dark brown; propodeal dorsum lacking carinulae; seen from above and obliquely, humerus smoothly and evenly rounded (Bolivia) . . . . . Pheidole mosenopsis

40
return to couplet #3
 * Major: in full-face view, entire posterior third of head, up to and including occipital lobes, rugoreticulate . . . . . 41


 * Major: in full-face view, at least the occipital lobes and usually the entire posterior third of the head, variously smooth and shiny, foveolate, rugose, or carinulate, but not reticulate . . . . . 48

41
return to couplet #40
 * Major and minor: in side view, propodeal spine as long as or longer than the basal face of the propodeum anterior to it; very large (Head Width of major 2.6 mm, of minor 0.9 mm). Major: entire dorsal surface of gaster foveolate and opaque (Mexico) . . . . . Pheidole ursus


 * Major and minor: in side view, propodeal spine only one-third as long as the propodeal basal face, or shorter; medium-sized to large (Head Width of major 1.1- 2.2 mm, of minor 0.6-0.7 mm). Major: at least the posterior three-quarters of first gastral tergite smooth and shiny . . . . . 42

42
return to couplet #41
 * Major: entire mesosoma enveloped in groups of variously transverse, longitudinal, or even concentric semicircular carinulae. Minor: dorsal surfaces of head and mesosoma covered by transverse or semicircular carinulae . . . . . 43


 * Major and minor: body and head mostly rugose and rugoreticulate, not enveloped by carinulae as described above . . . . . 44

43
return to couplet #42
 * Major: in side view, entire area of head behind (ventral) and posterior to the eye rugoreticulate. Minor: propodeal spine present; nuchal collar (behind occiput) absent (Costa Rica to Ecuador) . . . . . Pheidole ectatommoides


 * Major: in side view, almost all of the area of head behind (ventral) and posterior to the eye smooth and shining, bearing only a few longitudinal carinulae. Minor: propodeal spines absent; nuchal collar present (Choco, Colombia) (placed in the fallax group; also keyed out here to ensure identification) . . . . . Pheidole tigris

44
return to couplet #42
 * Major: in full-face view, head capsule square; long axes of the frontal lobes pointed away from each other at right angles and in the direction of the anterior corners of the head (montane Atlantic forest, Brazil) . . . . . Pheidole dyctiota


 * Major: in full-face view, head subrectangular or else somewhat heart-shaped; long axes of frontal lobes are approximately parallel and point forward toward the middle of the anterior clypeal border . . . . . 45

45
return to couplet #44
 * Major: dorsal surface of head capsule entirely rugoreticulate, except for antennal fossae, clypeus, and frontal triangle. Minor: in side view, anterior portion of basal face of propodeum drops precipitously through a right angle followed by steeply vertical descent to the metanotal groove (Sao Paulo, Brazil) . . . . . Pheidole lutzi


 * Major: frontal lobes lack rugoreticulum; they are covered instead by parallel longitudinal carinulae or rugulae. Minor: basal face of propodeum descends anteriorly to the metanotal groove through an even convexity followed by a gently oblique slope . . . . . 46

46
return to couplet #45
 * Major: longitudinal carinulae cover frontal lobes and extend posteriorly only to the level of the posterior margin of the eye; center of side of pronotum rugoreticulate (Sao Paulo, Brazil, and Misiones, Argentina) . . . . . Pheidole bambusarum


 * Major: longitudinal carinulae cover frontal lobes and extend posteriorly over central dorsal surface of head to halfway between the level of the posterior margin of the eye and the level of the occipital margin; center of side of pronotum smooth and shiny . . . . . 47

47
return to couplet #46
 * Major: lower half of mesopleuron entirely rugoreticulate; seen from above and obliquely, humerus subangulate. Minor: entire side of mesosoma rugoreticulate; propodeal spine well developed (southeastern Brazil) . . . . . Pheidole brevicona


 * Major: lower half of mesopleuron entirely smooth and shiny; seen from above and obliquely, humerus smoothly rounded. Minor: entire side of mesosoma smooth and shiny; propodeal spine reduced to simple right angle (Costa Rica) . . . . . Pheidole iracunda

48
return to couplet #40
 * Major: in full-face view, parallel longitudinal carinulae that cover the frontal lobes continue posteriorly over the dorsal head surface halfway or more to the occiput, while turning laterally away from the midline and curving transversely to the lateral margins of the head above the eye . . . . . 49


 * Major: in full-face view, either the parallel longitudinal carinulae that cover the frontal lobes do not tum laterally, or if they do turn, they fail to reach the sides of the head . . . . . 50

49
return to couplet #48
 * Major: seen from the side, carinulae posterior to the eye continue curving downward to cover all of the gena, hence the entire side of the head; carinulae arising from the frontal lobes reach the occiput; transverse carinulae completely cover the promesonotum. Minor: head and pronotum foveolate and opaque (Costa Rica) . . . . . Pheidole glyphoderma


 * Minor: seen from the side, carinulae posterior to the eye do not continue behind (laterad to) the eye, leaving most of the gena smooth and shiny; promesonotum with only a few scattered transverse carinulae, mostly smooth and shiny. Minor: head and pronotum smooth and shiny (Honduras) . . . . . Pheidole walkeri