Key to Rhopalothrix species

This key is based on Longino J. T. and Boudinot B. E. 2013. New species of Central American Rhopalothrix Mayr, 1870 (Hymenoptera, Formicidae). Zootaxa. 3616:301-324. doi:10.11646/zootaxa.3616.4.1

Key to the species of Rhopalothrix isthmica clade (workers).

The proposed R. isthmica clade with the following synapomorphies: (1) absence of squamiform setae on the face, and (2) development of shallow arcuate grooves and ridges on the face. All the species share a similar habitus, being small, nearly eyeless, pale brown, with uniformly matte surface, and a characteristically broad, lumpy face. The R. isthmica clade is strictly Neotropical, with the greatest abundance and diversity in Central America. The only Rhopalothrix species outside of the Rhopalothrix isthmica clade are Rhopalothrix ciliata from Colombia, Rhopalothrix diadema from New Guinea, and Rhopalothrix orbis from Australia. These latter three species all have squamiform setae on the face and their heads are more elongate and less flattened, more like other basicerotines.

Rhopalothrix kusnezovi is known from a single queen from Argentina. The worker is probably similar to Rhopalothrix plaumanni.

A synopsis of the important characters for the clade {see Rhopalothrix characters) can be helpful to read through before working specimens through the key, especially if you are unfamiliar with this genus.

1

 * Face with conspicuous squamiform setae; Old World and Colombia (Rhopalothrix incertae sedis) . . . . . Rhopalothrix diadema (New Guinea) . . . Rhopalothrix orbis (Australia) . . . Rhopalothrix ciliata (Columbia)


 * Face lacking large squamiform setae; Neotropics (Rhopalothrix isthmica clade) . . . . . 2

2
return to couplet #1
 * First gastral tergite completely lacking squamiform setae, or at most one pair at extreme posterolateral margins of tergite (Fig. 1A) . . . . . 3


 * First gastral tergite with at least six squamiform setae, at least one pair mid-disk, not all restricted to posterior margin of tergite (Fig. 1 B-F) . . . . . 4

3
return to couplet #2
 * Mandible with three or more acute teeth on masticatory margin; tooth at base of subapical tooth small, reclinate, directed anteriorly; apical and subapical teeth similar in length; southern Brazil . . . . . Rhopalothrix plaumanni


 * Masticatory margin of mandible with a tiny inconspicuous denticle near base and a large peg-like tooth at about one third distance from base; tooth at base of subapical tooth distinct and directed posteriorly; subapical tooth longer than apical tooth; Costa Rica . . . . . Rhopalothrix apertor

4
return to couplet #2
 * Labrum about as long as broad, medial sinus deep, distance from tip of labral lobe to base of sinus similar to distance from base of sinus to basal ridge (Fig. 2D,E) . . . . . 5


 * Labrum distinctly broader than long; distance from tip of labral lobe to base of sinus shorter than distance from base of sinus to basal ridge (Fig. 2A,B,C,F) . . . . . 7

5
return to couplet #4
 * Denticles of masticatory margin of mandible larger; HW > 0.41; subapical tooth of mandible at least twice as long as apical tooth; Costa Rica and Nicaragua (queen of R. subspatulata with pronounced concavity on face medial to compound eye, and with distinct carina between eye and concavity) . . . . . 6


 * Denticles of masticatory margin minute; HW < 0.40; subapical tooth of mandible about the same length or only slightly longer than apical tooth; Honduras to Mexico, Cuba (queen with face more evenly convex, without pronounced concavity or carina medial to compound eye) . . . . . Rhopalothrix weberi

6
return to couplet #5
 * First gastral tergite with four squamiform setae on posterior margin, two at mid-disk; HW < 0.50 . . . . . Rhopalothrix subspatulata


 * First gastral tergite with six or more squamiform setae on posterior margin, about six on disk between posterior border and midlength; HW > 0.50 . . . . . Rhopalothrix nubilosa

7
return to couplet #4
 * Anterior margin of labrum transverse, with at most a minute semicircular notch medially; masticatory margin of mandible with three small teeth; Guatemala and Mexico . . . . . Rhopalothrix stannardi


 * Anterior margin of labrum with conspicuous median notch; masticatory margin of mandible with two or three teeth . . . . . 8

8
return to couplet #7
 * Anterior labral lobes triangular, inner margins sloping evenly to small semicircular notch, medial lobule reduced to absent . . . . . 9


 * Anterior labral lobes moderately to distinctly bilobed, not triangular, with medial lobule well developed . . . . . 10

9
return to couplet #8
 * HW 0.58–0.68; propodeal tooth a right angle or acute; metanotal groove shallowly impressed . . . . . Rhopalothrix isthmica


 * HW > 0.73; propodeal tooth usually large and acute; metanotal groove moderately to deeply impressed . . . . . Rhopalothrix megisthmica

10
return to couplet #8
 * Lateral lobule of labrum rounded, shorter than medial lobule; masticatory margin of mandible with three distinct teeth; Costa Rica to Honduras . . . . . Rhopalothrix therion


 * Lateral lobule of labrum more triangular, longer than medial lobule; masticatory margin of mandible with two or three teeth; Honduras to Mexico . . . . . 11

11
return to couplet #10
 * Squamiform setae of first gastral tergite more elongate in profile view, with thin stem before thickened apex; HW 0.57–0.69 . . . . . 12


 * Squamiform setae of first gastral tergite shorter in profile view, more abruptly thickened near attachment point to gaster; HW 0.49–0.50; Guatemala . . . . . Rhopalothrix atitlanica

12
return to couplet #11
 * HW 0.57–0.65; masticatory margin of mandible with two teeth; propodeal tooth usually a short right angle, with infradental lamella relatively narrow; Mexico (Chiapas) . . . . . Rhopalothrix triumphalis


 * HW 0.63–0.69; masticatory margin of mandible with three teeth; propodeal tooth pronounced and acute, with infra-dental lamella broad and forming rounded convex lobe beneath tooth; Honduras . . . . .  Rhopalothrix andersoni