Key to New World Eurhopalothrix

This worker key is based on Longino J. T. 2013. A review of the Central American and Caribbean species of the ant genus Eurhopalothrix Brown and Kempf, 1961 (Hymenoptera, Formicidae), with a key to New World species. Zootaxa. 3693:101-151. doi:10.11646/zootaxa.3693.2.1

Longino also provides the following details regarding distinctive characters for the genus. Reading through this will greatly facilitate making determinations with the key.

Two character systems are normally hidden from view on dry-mounted specimens: mandibular dentition and shape of the labrum. Identification is greatly facilitated if the mandibles are spread and the labrum pulled outward prior to dry mounting. Care must be taken in doing so, because the mandibular teeth are fragile and easily broken, and the labrum easily detaches when flexed outward.

The mandible is a complex three-dimensional structure that is triangular in cross section, with three faces: dorsal, ventral, and interior. The dorsal face is usually in the same plane as the clypeus when the mandibles are closed, and there is a prominent basal ridge that abuts the clypeus when the mandible is closed. The dorsal face rounds into the ventral face which is flat to shallowly concave. The ventral face meets the interior face at a sharp, well-demarcated angle. The interior face is moderately to strongly concave. The masticatory margin, with serial teeth, is at the juncture of the interior and dorsal faces.

Mandible dentition among Central American and Caribbean species is of two forms. One form is a simple row of 11 similar, low, triangular teeth. The other form has an inner row of 3–4 longer, needle-like teeth in addition to an outer row of 10 triangular teeth. Snelling (1968) was the first to describe and illustrate this character, on Eurhopalothrix apharogonia. It is a surprisingly discrete character but is not highly correlated with other characters that are typically used to differentiate species. For example, Eurhopalothrix gravis has a double tooth row and Eurhopalothrix bolaui does not. These are two species that are considered very similar, appearing in the same couplet in dichotomous keys (Brown and Kempf, 1960; Ketterl et al., 2004).

The labrum is a moveable, tongue-like structure that is highly variable. The basal portion is more or less flat dorsally, with the ventral surface concave. The basal portion is delimited anterolaterally on the ventral face by gibbosities that are developed into short, ventrally-projecting teeth. The basal portion is usually surmounted by an apical portion that may be in the same plane as the basal portion or more often bent dorsally. The apical portion may be elongate and subtriangular or short and broad. The apex may be unilobed or divided into two apical lobes. Bilobed apices vary from being no more than a minute apical notch to having a deep cleft separating the two lobes. The apex of the labrum is usually fringed with thick, translucent, soft setae. These setae are filiform, or somewhat capitate, and in two species they are curiously flattened. These setae may also be restricted to a dorsal patch on the labrum (e.g., Eurhopalothrix cimu).

Most species have traditionally been separated based on the distribution and abundance of pilosity on the face, dorsal mesosoma, and first gastral tergite. Pilosity can nearly always be separated into ground pilosity consisting of small short setae, variously appressed to suberect, and much larger and fewer specialized setae that project above the ground pilosity. The ground pilosity varies from thin, sparse, and barely visible, to distinctly spatulate, abundant, and conspicuous. The specialized setae are always at least somewhat thickened and stiff (never flexuous) but vary from being thin and nearly linear, to clavate, to strongly spatulate. In the extreme they are nearly as broad as long, subspherical and "pompon-like" (Brown and Kempf, 1960). More spatulate to subspherical setae are also typically strongly curved, but still projecting above the ground pilosity.

Specialized setae have consistent numbers and locations that vary among species. Placement is typically bilaterally symmetrical. On the face, a typical arrangement is an anterior row of 8 (4 on each side) in an arc between the compound eyes, a median row of 4, and a posterior row of 6 on the posterior vertex margin. In a few species the specialized setae are small and the ground pilosity is large and conspicuous, such that the specialized setae grade into the ground pilosity and are difficult to distinguish. But knowledge of the typical condition allows one to see the specialized setae, slightly larger, "hiding" among the ground pilosity. The promesonotum has up to 6 specialized setae: 2 on the anterior pronotum, 2 on the anterior mesonotum, and 2 on the posterior mesonotum. The first gastral tergite often has two parallel rows of specialized setae down the center, more or less in pairs. There are up to 4 such pairs. The posterior pair may be flanked by another outer pair, forming a row of 4 setae along the posterior margin. For highly setose species, additional setae may occur as irregular rows lateral to the medial rows. Unfortunately, setal counts and locations are averages and vary within species. Some of this variation appears to be ontogenetic, with the occasional extra seta appearing in an atypical location. These are often revealed by being unpaired. However, the larger problem is presumed loss of setae due to wear. The variation of seta number is asymmetrical, with more specimens lacking setae than possessing supernumerary setae. The identification key is developed based on the typical seta count and arrangement for a species, but when keying single specimens (when multiple collections from a locality are not available) one could easily be misled by a worn specimen.

A few species have a distinctive transverse arc on the face, separating the face into anterior and posterior regions. One species has a transverse arc on the clypeus.

1

 * Face, dorsal mesosoma, and first gastral tergite covered with uniform ground pilosity of appressed spatulate setae, lacking differentiated erect or suberect setae projecting above ground pilosity; posterior face of propodeum with broad, rectangular lamella, not differentiated into acute propodeal spine; HW < 0.5 (Argentina) . . . . . Eurhopalothrix bruchi


 * Face, dorsal mesosoma, and first gastral tergite with thin, non-spatulate ground pilosity or, if with conspicuous spatulate ground pilosity, then accompanied by differentiated larger suberect to erect spatulate setae; posterior face of propodeum as above or with distinct acute propodeal spines; HW variable . . . . . 2

2
return to couplet #1
 * Face with well-defined transverse ridge that separates posterior and anterior portions of face, or clypeus with transverse ridge that separates small posterior triangular portion and larger, concave anterior portion . . . . . 3


 * Neither face nor clypeus with transverse ridge . . . . . 5

3
return to couplet #2
 * Transverse ridge on face, not on clypeus . . . . . 4


 * Transverse ridge on clypeus, not on face (Guyana) . . . . . Eurhopalothrix clypeata

4
return to couplet #3
 * Face with 16 large spatulate setae (Brazil) . . . . . Eurhopalothrix depressa


 * Face lacking spatulate setae (Cuba) . . . . . Eurhopalothrix cimu

5
return to couplet #2
 * Larger erect to suberect setae of face 12–20, distinctly differentiated from ground pilosity (A, below) or grading into and difficult to distinguish from spatulate ground pilosity (B, below) . . . . . 6


 * Larger erect setae at most 10; ground pilosity thin, distinctly differentiated from erect setae (C, above) . . . . . 22

6
return to couplet #5
 * Propodeal spines in the form of broad, foliaceous crests that extend down posterior face of propodeum (A, below); face and dorsal surface of scapes covered with ground pilosity of conspicuous, broad appressed spatulate setae . . . . . 7


 * Propodeal spines in form of acute triangles, with a narrow carina extending ventrally (B, above); dorsal surface of scapes with spatulate or narrower appressed setae, ground pilosity of face variable . . . . . 8

7
return to couplet #6
 * HW < 0.6, CI 89–97 (Central America) . . . . . Eurhopalothrix pilulifera


 * HW > 0.9, CI > 100 (southern Brazil) . . . . . Eurhopalothrix lenkoi

8
return to couplet #6
 * HW < 0.56; CI 96; ground pilosity conspicuous, spatulate; differentiated erect setae strongly spatulate, as broad as long; first gastral tergite with spatulate setae in double row of 4–5 pairs down the middle, flanked by 2 or 3 additional setae (Trinidad) . . . . . Eurhopalothrix alopeciosa


 * HW > 0.55; CI > 95; other characters variable . . . . . 9

9
return to couplet #8
 * Eye large, about 9 ommatidia across long axis (A, below); legs relatively long, hind tibia length about 0.6 HW; angle between anterior and dorsal faces of petiolar node obtuse; first gastral tergite with 4 pairs spatulate setae; ground pilosity of face uni-formly distributed; masticatory margin of mandible with single tooth row (Costa Rica) . . . . . Eurhopalothrix megalops


 * Eye small, 3–6 ommatidia across long axis (B, below); legs relatively short, hind tibia length about 0.4–0.5 HW; other charac-ters variable . . . . . 10

10
return to couplet #9
 * Masticatory margin of mandible with double row of teeth, outer series of lower triangular teeth, inner row of 3–4 long, spiniform teeth; promesonotum rugose and puncta obliterated and frons with a distinct raised median carina . . . . . 11


 * Masticatory margin with single row of triangular teeth; promesonotum smooth (and sometimes shining) between distinct puncta and frons without a distinct raised median carina . . . . . 15

11
return to couplet #10
 * Erect setae on face filiform to weakly clavate, not strongly spatulate (A, below) (Cuba) . . . . . Eurhopalothrix mabuya


 * Erect setae on face strongly spatulate (B, above) . . . . . 12

12
return to couplet #11


 * Basal lobe of scape strongly developed, SLI > 15 . . . . . 13


 * Basal lobe of scape less developed, SLI < 14 . . . . . 14

13
return to couplet #12
 * Smaller, HW 0.57–0.66; basal lobe of scape more developed, SLI 21–25; color orange; posterior mesonotum lacking pronounced longitudinal keel (A, below) (Honduras to Costa Rica) . . . . . Eurhopalothrix xibalba


 * Larger, HW 0.69–0.83; basal lobe of scape less developed, SLI 15–21; color dark brown; posterior mesonotum with a pronounced, short longitudinal keel between the posteriormost pair of spatulate setae (B, above) (Mexico, Guatemala) . . . . . Eurhopalothrix hunhau

14
return to couplet #12
 * Petiolar peduncle relatively thin in lateral view (A, below); head narrower (C, below), CI 97–99; size smaller, HW 0.71–0.74 (Guadeloupe, Dominica) . . . . . Eurhopalothrix guadeloupensis


 * Petiolar peduncle thicker (B, above); head broader (D, above), CI 110–115; size larger, HW 0.83–0.94 (widespread) . . . . . Eurhopalothrix gravis

15
return to couplet #10
 * Promesonotal dorsum with 3 pairs of large, broadly spatulate setae; projecting setae of face prominent, spatulate, of uniform size, strongly differentiated from ground pilosity*; ground pilosity of face more or less uniformly distributed across frons, vertex lobes, and frontal carinae; first gastral tergite with 4 pairs spatulate setae, these of similar size and shape to those on face, distributed in two longitudinal rows . . . . . 16


 * Promesonotal dorsum with 0–2 pairs of filiform to spatulate setae, clearly differentiated from ground pilosity; projecting setae of face as above or differing in size and grading into ground pilosity; ground pilosity of face as above or variously restricted to symmetrical patches on each side of the midline; first gastral tergite with projecting setae variable . . . . . 17


 * singleton from Costa Rica will fail at this couplet and may be a distinct species. See Comments under Eurhopalothrix oscillum.

16
return to couplet #15
 * HW 0.56–0.70; base of propodeal spines relatively broad, grading into broad infradental lamella; propodeal spines relatively longer and close-set, in dorsal view length about one and one-half as long as distance between their bases; color orange brown (A, below) (widespread) . . . . . Eurhopalothrix bolaui


 * HW 0.78–0.85; base of propodeal spines narrower, more abruptly meeting narrower infradental lamella; propodeal spines rela-tively shorter and separated, in dorsal view length less than or equal to distance between their bases; color darker red brown (B, above)(Guatemala, Honduras) . . . . . Eurhopalothrix zipacna

17
return to couplet #15
 * Projecting setae on first gastral tergite and mesosomal dorsum stiff but nearly filiform, not spatulate (A, below); puncta on first gastral tergite narrower than interspaces, interspaces smooth, giving gaster shiny appearance; face with shallowly-impressed longitudinal median trough that is devoid of ground pilosity, sculpture of trough densely punctatorugose; apex of labrum forming single projecting lobe, not bifid (Costa Rica; intergrades with E. schmidti in Monteverde, Costa Rica) . . . . . Eurhopalothrix ortizae


 * Projecting setae on first gastral tergite and mesosomal dorsum, when present, clearly clavate or spatulate (B, above); puncta on first gastral tergite similar to above or often more closely spaced, with interspaces narrower than puncta diameters; face with or without median trough; apex of labrum typically bifid . . . . . 18

18
return to couplet #17
 * Ground pilosity of face more or less uniformly distributed across frons, vertex lobes, and frontal lobes (A, C, below) . . . . . 19


 * Ground pilosity of face divided into two patches separated by medial area devoid of ground pilosity (D-F above) . . . . . 20

19
return to couplet #18
 * Ground pilosity of face relatively thin and sparse; face often with pair of spatulate setae on outermost angles of sides of head, such that full complement of projecting setae is often 20; first gastral tergite with 4 pairs projecting spatulate setae (Costa Rica) . . . . . Eurhopalothrix schmidti


 * Ground pilosity of face relatively thicker and denser; face without pair of spatulate setae on outermost angles of sides of head, full complement 18; first gastral tergite with 3 pairs spatulate setae (Honduras to Costa Rica) . . . . . Eurhopalothrix oscillum

20
return to couplet #18
 * Ground pilosity of face ends abruptly in front of anterior row of projecting spatulate setae; posterior border of ver-tex in full-face view with 4 clavate to spatulate setae; petiolar node typically lacking spatulate setae; first gastral tergite always lacking spatulate setae . . . . . 21


 * Ground pilosity of face extends anteriorly beyond anterior row of spatulate setae; posterior border of vertex in full-face view with 6 spatulate setae; posterior border of petiolar node with pair of spatulate setae; first gastral tergite with 0–3 pairs spatulate setae (Costa Rica) . . . . . Eurhopalothrix semicapillum

21
return to couplet #20
 * In full face view, ground pilosity extends evenly to posterolateral margins of vertex lobes; ground pilosity more strongly flattened (Nicaragua to Costa Rica) . . . . . Eurhopalothrix circumcapillum


 * In full face view, ground pilosity fades posterolaterally, becoming obsolete on vertex lobes; ground pilosity rela-tively narrower (Guatemala) . . . . . Eurhopalothrix machaquila

22
return to couplet #5
 * No specialized setae on face, dorsal mesosoma, petiole, postpetiole, and first gastral tergite; clypeus smooth, highly polished and glassy; face uniformly punctate but with the same highly polished integument (southern Brazil, Paraguay) . . . . . Eurhopalothrix spectabilis


 * At least two larger erect to suberect setae on face; other characters variable . . . . . 23

23
return to couplet #22
 * Face with a single pair of erect spatulate setae on a somewhat protruding eminence on the medial vertex; size large, HW > 0.90; first gastral sternite smooth and shiny anteriorly, with prominent median keel (New Guinea, Philippines, multiple Pacific islands east of New Guinea, Costa Rica) . . . . . Eurhopalothrix procera


 * Face setae variable, but face smoothly convex, without median eminence; HW < 0.90; first gastral sternite punctate anteriorly, lacking median keel . . . . . 24

24
return to couplet #23
 * Face usually with a single pair of erect clavate setae on posterior vertex margin (may have 2 pairs in medial cluster, like E. apharogonia); 3 pairs of erect clavate hairs on promesonotum, including pair on anterior pronotum (Florida, Puerto Rico, Dominican Republic) . . . . . Eurhopalothrix floridana


 * Face with 3–5 pairs erect setae on face; mesonotum with 1–3 pairs erect setae, if < 3 pairs, lacking erect setae on pronotum . . . . . 25

25
return to couplet #24
 * Face with 4 or 5 pairs erect setae, arranged as a posteromedial rectangle of 2 pairs flanked anterolaterally by 1 or 2 pairs . . . . . 26


 * Face with 2 or 3 pairs erect setae, arranged in a close rectangle on posteromedian vertex with third pair (if present) on postero-lateral vertexal corners . . . . . 27

26
return to couplet #25
 * Pronotum lacking erect setae; ground pilosity on face extremely thin and sparse, not easily visible (southern Mexico) . . . . . Eurhopalothrix sepultura


 * Pronotum with pair of erect setae; ground pilosity on face flattened, conspicuous (Guatemala) . . . . . Eurhopalothrix vulcan

27
return to couplet #25
 * Face with 3 pairs erect clavate setae, arranged in rectangle on vertex; mesonotum with 2 pairs erect setae (southern Brazil) . . . . . Eurhopalothrix speciosa


 * Face with 2 pairs erect clavate setae, arranged in rectangle on vertex; mesonotum with 1 pair erect setae (El Salvador) . . . . . Eurhopalothrix apharogonia