Taxonomic revision of the genus Prionopelta (Hymenoptera, Formicidae) in the Malagasy region

Abstract In this study we revise the taxonomy of the genus Prionopelta for the Malagasy region, treating seven species, six of which are newly described (Prionopelta laurae sp. n., Prionopelta seychelles sp. n., Prionopelta subtilis sp. n., Prionopelta talos sp. n., Prionopelta vampira sp. n., Prionopelta xerosilva sp. n.), and one redescribed (Prionopelta descarpentriesi Santschi). One species, Prionopelta seychelles, is restricted to Seychelles, while the six remaining species treated are endemic to Madagascar.


Introduction
The genus Prionopelta Mayr, 1866 contains 21 species, including six that are newly described here. In his species-level treatment of the genus, Brown (1960) recognized ten species scattered throughout the Old and New World tropics, and later returned P. marthae to the genus after an erroneous removal based on mislabeled specimens (Brown 1965). Subsequently, Terron described two new species from the Afrotropics (1974), and Shattuck described two new Indo-Pacific species and clarified diagnoses and geographic distributions for P. kraepelini and P. opaca (2008). Total species counts to date are thus: five from the New World tropics (P. punctulata Mayr, P. antillana Sculpture of the head and dorsum of the mesosoma is of high diagnostic value in identifying Malagasy Prionopelta. In describing sculpture we use the same terminology as Harris (1979) and Shattuck (2008). We use the term "foveae" to refer to the shallow, circular, flat-bottomed depressions present on the integument, and the terms "punctations" or "punctures" to mean minute, point-like depressions in the surface of the integument that appear as tiny pinpricks even under high magnification. All Malagasy Prionopelta have foveae that vary in both size and density on both head and mesosoma, and most species also have punctations on the dorsum of the mesosoma. Additionally, all taxa possess a band devoid of foveae on the head that runs medially and longitudinally. This area, extending from just posterior to the antennal sockets to near the posterior margin of the head, varies in size and shape among species. Throughout the text, this area is excluded from consideration when statements are made concerning the density and extent of sculpture on the head. In some individuals a linear, scarlike, coronal suture is present in this region running longitudinally along the dorsum of the head.
When identifying Malagasy Prionopelta, it is helpful to recognize three qualitative conditions concerning the arrangement of foveae on the head. The first condition occurs when foveae are relatively equally spaced from one another at a distance that is greater than the diameter of an individual fovea, with smooth, shining integument present between foveae (Fig. 1A, B). The next condition is one of variable foveal placement, with some foveae as described above but others directly adjacent to one another ( Fig. 1C). At low magnification, these adjacent foveae appear to touch directly, where-as under high magnification they appear very close together (much closer together than the diameter of an individual fovea), causing the area between each fovea to form a distinct raised margin that rises above what would otherwise be the smooth surface of the integument. There is a general tendency in both of the above-described conditions for cephalic foveae to increase in density moving laterally to medially in full-face view. In the final of these three general conditions, foveae are placed extremely densely on the head so that virtually no shining integument is present between foveae (except for the median cephalic band mentioned above). In this condition, foveae appear as a dense system of directly adjacent pits with only netlike boundaries of thin, raised margins separating individual fovea (Fig. 1D).

Prionopelta Mayr, 1866
Prionopelta can be recognized from other Malagasy genera through the following combination of characters: petiole which lacks a posterior face due to its broad attachment to the gaster; elongate, subtriangular mandibles with three teeth positioned distally on a distinct mandibular face; apical tooth the longest, and second tooth the shortest with a mid-length third tooth; antennal segments variable (either 9 or 12 for Malagasy species) but always with a four-segmented club; body overall never more than 3 mm in length.

Identification key for Malagasy Prionopelta species (workers)
1 Nine antennal segments present, palest and smallest of the Malagasy Prionopelta (HL < 0.4 mm and HW < 0.3 mm); entire body pale yellow (Fig. 7) .. Cephalic foveae widely and evenly spaced such that they are only extremely rarely adjacent to one another; all cephalic foveae appear as if scooped out of a flat, shining surface, and completely lack raised margins at their perimeter ( Fig. 1A, B Cephalic foveae of variable spacing but never as sparse as above; at a minimum, head with several clusters of two or more foveae which are directly adjacent in full-face view (Fig. 1C) and often with many to most foveae on the head directly adjacent to one another (Fig. 1D); integument between adjacent foveae appears to bulge, and when foveae are densely placed, these bulged areas form a network of raised margins (Fig. 1D)  3 Metanotal suture absent in dorsal view, in its place a smooth surface with no clear boundary dividing mesonotum and propodeum; posterior margin of the propodeum convex and crescent-shaped in dorsal view ( Fig. 2A); lamellae of the posterior propodeum present; apical tooth of the mandible extremely long (Fig. 2C)  Vast majority of cephalic foveae densely positioned so they are directly adjacent to one another; vein-like ridges present running between foveae producing the appearance of a netlike pattern across the entire head ( Fig. 3A Cephalic foveae small and densely positioned so that the vast majority are directly adjacent to one another with swollen ridges between; median cephalic band devoid of foveae is wide, usually wider at its base and narrowing posteriorly (Fig. 3C) Figs 1C,D,3D,4B,C,E,F,5,6 Prionopelta descarpentriesi Santschi, 1924b:195. . Diagnosis. P. descarpentriesi can be identified by the following combination of characters: twelve antennal segments; median cephalic band lacking a thin suture that is swollen above the surrounding integument; placement of cephalic foveae ranging from sparse to dense, but always at minimum, with at least several clusters of foveae directly adjacent to one another (if nowhere else, then medially in full-face view); if all foveae on the head are directly adjacent so that no flat, shining space is present between foveae, then foveae are large and accompanied by pronotal sculpture which is characterized as being both similar in size to that on the head and not consisting of smaller foveae or punctures; eye appearing as either an asymmetrical dark patch which appears to be a stain in the cuticle that is flush with its surrounding integument, or a single, slightly rounded glob with no definable subunits. Worker description. Posterior margin of the head straight to weakly concave in full-face view; spacing of cephalic foveae highly variable, ranging from individuals with dense, directly adjacent foveae covering the entire head (known only from far eastern and northern Madagascar, see morphotype descriptions below), to individuals with foveae more widely spaced so that shining areas are visible between; median cephalic band devoid of foveae ranging from wide to extremely narrow but never appearing as a linear suture that is uniformly swollen above the level of the surrounding integument; apical tooth intermediate in length; evenly-space pronotal foveae range from shallow to deep; shallow foveae present on mesonotum and propodeum.

Prionopelta descarpentriesi Santschi
Distribution and ecology. This widespread species has been collected from leaf litter from 10-1860 meters of elevation. While found most commonly in rainforest and montane rainforest, it has also been collected in Uapaca woodland, littoral rainforest, and tropical dry forest (Fig. 13). P. descarpentriesi has been collected in forest litter, under moss, rocks, and logs, as well as inside rotten logs and underground in soil.
Taxonomic notes. Three generalized morphotypes can be distinguished within this taxon; these vary in density of cephalic foveae and other co-occurring traits (Fig.  6). In morphotype A, the majority of foveae are equidistantly spaced and separated by a span of shining integument of around one foveal diameter (Figs 1C, 5A, 6A). These foveae appear cleanly scooped from the surface of the integument and largely lack raised margins. Morphotype B, which is intermediate between A and C, has denser cephalic foveae covering almost the entire head, however foveae are smaller and more delicate than those of morphotype C and raised ridges between foveae are less pronounced (Fig. 6B). Pronotal sculpture in morphotype B usually consists of several sizes of foveae along with some punctures. Morphotype C possesses large, dense cephalic foveae that cover the entire head, accompanied by a pronounced network of raised, jagged ridges between foveae (Figs 1D, 3D, 4C, 6C). Morphotype C additionally has large, deep, and evenly spaced pronotal foveae that are similar in size to those on the head, and this morphotype lacks smaller foveae or punctures on the pronotum. Width of the median cephalic band devoid of foveae is widest in morphotype A and narrowest in morphotype C.
P. descarpentriesi is much more morphologically variable than the other species treated in this revision and very possibly represents a species complex. This could explain why P. descarpentriesi is abundant, geographically widespread, and morphologically variable. Morphotype C, which is restricted to a band bordering the coast of eastern and northern Madagascar (Fig. 14), is recognizably distinct from the majority of individuals from interior populations of the species. At several localities where morphotype C is present, individuals from morphotype A and B are also present with little to no evidence of character blending between morphotypes: Galako, Makirovana, Morojejy Nature Reserve, and Sahafina. However, at other locales such as Ambohijanahary, Montagne d'Ambre, and Vohemar, a bewildering array of intermediate forms have been collected, blurring the lines between the three morphotypes. Collecting nest series from the aforementioned locations to determine whether this population-level variation is intra-or intercolonial is an important first step in ultimately understanding how morphological variation is partioned in this taxon as currently dilineated.
Under the above scheme, the lectotype and paralectotype of P. descarpentriesi would be considered morphotype A. Jules Descarpentries (1881Descarpentries ( -1927 collected the types for this species on 30 September 1923 and the collection locale was subsequently noted as "Ikelivia", making it difficult to determine a more precise location based on modern place names. Historical records indicate that Descarpentries resided in Tulear, and worked as a topographic surveyor and entomologist. According to records, he was active around Tsaratanana and Fianarantsoa (specifically Andringitra) and often traveled with H. Perrier de la Bâthie. Given that morphotype A is reasonably widespread across Madagascar, including Tsaratanana and Andringitra, it is possible that the type specimens from "Ikelivia" were collected in either of these localities. To further complicate matters, most of Descarpentries's specimens, which were destined to the Paris Museum, were sold on the side by a member of the staff. Before the specimens were sold, the labels were changed to hide their true ordinance (Jeannel 1951, (Anonymous 1970).
Non    (Fisher, Griswold et al.); Toliara, Réserve Spéciale Kalambatritra, Ampanihy, 23.4635°S, 46.4631°E, 1270m, montane rainforest, 9.ii.2009; Bongolava Prefec. de Tsiroanomandidy, 6.xii.1974 (A.Peyrieras). Worker description. Head much longer than wide with lowest cephalic index on average of all Malagasy Prionopelta (mean CI 72.71); posterior head margin straight in full-face view; cephalic foveae small and very dense, with no space for additional cephalic foveae present; median cephalic band devoid of foveae is long and thin and appears slightly swollen or raised above the surrounding integument, forming a scarlike suture; apical tooth of the mandible over four times the length of third tooth in fullface view; nine antennal segments; eye greatly reduced, appearing as a tiny, dark gray patch; majority of marks on pronotum are densely spaced, tiny punctures; mesonotum and propodeum consisting of tiny shallow foveae; metanotal groove visible dorsally; smallest of the Prionopelta from Malagasy region; body distinctly pale yellow in color.

Prionopelta laurae
Etymology. The name of this species is a patronym dedicated to Laura D. Steger for her continual support during the course of this work and her either completely genuine-or expertly feigned-enthusiasm for being endlessly bombarded by information about ants. Distribution and ecology. This species has been collected in leaf litter primarily in rainforest with some collections from littoral rainforest and one in tropical dry rainforest, at elevations between 10-600 meters. Its range is restricted to eastern Madagascar and is seemingly disjunct, with most individuals collected from the northeast and only two locales known from the southeast near the coast. No individuals have been collected between Sahafina Forest in the north and Mahabo forest in the south, a distance of 500 km (Fig. 13). The current range of P. laurae is such that it may once have been distributed along the entire eastern coast of Madagascar.
Taxonomic notes. This species of Prionopelta is unmistakable as it is the only Malagasy species with nine antennal segments. It is also the smallest known species of Malagasy Prionopelta and is a distinct pale yellow color, which is much lighter than the fully-sclerotized workers of any other Malagasy species. Diagnosis. P. seychelles is the only known species from Seychelles. It can be distinguished from all other Malagasy Prionopelta through the following characters: twelve antennal segments; densely arranged cephalic foveae with virtually no space for additional foveae and no shining integument visible between; pronotum consisting of shallow foveae much larger in diameter than those on the head with punctures between; median cephalic band which is devoid of foveae not swelling above the surrounding integument, and often characterized as being wider anteriorly and narrower posteriorly. Worker description. Twelve antennal segments; posterior margin of the head weakly concave in full-face view; small cephalic foveae densely positioned so that no flat, shining integument is present between; median cephalic band which is devoid of foveae does not appear to swell above the surrounding integument, but rather appears as a smooth, shining surface which is widest anteriorly, narrowing posteriorly; apical tooth intermediate in length; pronotum with foveae which range from shallow to deep and are interspersed regularly with punctures; mesonotum and propodeum consisting of large, shallow foveae; metanotal groove strongly visible, and mesopropodeal suture visible to barely visible, but some depression always present; posterior propodeal edge viewed dorsally is straight or only very slightly concave; no protruding lamellae of the posterior propodeum.
Etymology. This species is named after the Seychelles archipelago, to which it is endemic. The species epithet is a noun in apposition, and thus invariable.
Distribution and ecology. This species is known only from Seychelles and is found between 15-660 meters of elevation on the islands of Mahé, Conception, Thérèse, Silhouette, Praslin, La Digue, Félicité Island, and the Little Sister island group. It does not appear to have strict habitat preferences as it has been collected in mixed forest, littoral forest, non-native forest, palm forest, and coastal scrub. P. seychelles has also been collected from a diversity of microhabitats including from leaf litter, inside rotten logs, under rocks, under moss on live trees, and under root mats.
Taxonomic notes. P. seychelles is most likely to be confused with P. subtilis, as both have very small cephalic foveae that are densely arranged on the head. However, the two species are not sympatric, as P. seychelles is known only from Seychelles, and P. subtilis only from Madagascar. The median cephalic bands contrast markedly between these taxa, as that of P. seychelles is wide anteriorly, often tapering posteriorly, with an interrupted border caused by aberrantly placed foveae which break up the margins which define the smooth shining area (Fig. 8A). That of P. subtilis, on the other hand, is very thin throughout its length, with clearly defined borders which are swollen above the surrounding integument (Fig. 3A, B) On average, P. subtilis also has a wider head than P. seychelles: 0.39-0.45 (0.42) vs. 0.33-0.37 (0.35), respectively. Additionally, the cephalic foveae of P. subtilis are smaller and the cephalic sculpture overall appears more delicate than that of P. seychelles.
Non Diagnosis. P. subtilis can be recognized by the following combination of characters: twelve antennal segments; minute, densely placed cephalic foveae with raised margins where foveae touch so that the entire head is covered in a delicate mosaic of connected foveae with ridges between; well-defined, uniformly narrow, coronal suture that swells above the level of the surrounding integument; shallow foveae on the pronotum are much larger than those on the head, and more widely spaced, with tiny punctures between.
Worker description. Posterior head margin slightly concave with a noticeable notch medially; cephalic foveae dense and minute; virtually no area of the head lacking foveae in full-face view except at the extreme posterolateral corners; median cephalic band devoid of foveae is thin, linear, and slightly but uniformly swells above the surrounding integument; apical tooth intermediate in length; foveae on the pronotum are shallow, as well as more widely spaced and obviously larger than those on the head, with punctures present between; mesonotum and propodeum consisting of shallow foveae and punctures; metanotal groove visible dorsally, and mesopropodeal suture strongly visible in lateral view.
Etymology. The name of this species comes from the Latin adjective meaning "fine", "thin", or "slender" and refers to the very delicate, net-like patterns produced by the sculpture on the head.
Distribution and ecology. This common and widespread species is found in rainforest, montane rainforest, lowland rainforest, tropical forest, littoral forest, degraded forest, and marsh edge from 5-1325 meters of elevation (Fig. 13). On the ground it has been collected from inside rotten logs and sticks, as well as under moss, rocks, logs, and in litter. It has also been collected from above-ground sites including canopy moss and leaf litter, as well as inside above-ground twigs and branches.
Diagnosis. P. talos can be recognized by the following combination of characters: twelve antennal segments; large globular eye which appears as a half-sphere emerging from the surface of the head, composed of several asymmetrical subunits visible under high magnification; pronounced, tricolored body with a dark brown head, light brown body, and yellow/pale legs; known only from the Anjanaharibe-Sud Reserve in northeastern Madagascar.
Worker measurements (N=9).  Worker description. In full-face view, cephalic foveae become denser medially, so that laterally, foveae are separated by more than the diameter of one fovea, and medially, many foveae are directly adjacent and tend to form longitudinal chains of connected foveae; areas between foveae dark brown and shining; median cephalic area devoid of foveae not swelling above the surface of the surrounding integument and characterized as being widest anteriorly, narrowing posteriorly; apical tooth relatively short in length; eyes largest of all Malagasy Prionopelta, uniformly globular in shape, and under very high magnification, appear to be composed of several subunits which form a conglomerate half-sphere; both large, shallow foveae and small punctures present on pronotum and mesonotum; propodeal surface possesses only large shallow foveae; strong metanotal groove in dorsal view; strong mesopropodeal suture; distinctly tricolored body with uniformly dark brown head, tan mesosoma and gaster, and pale yellow legs and antennae.
Etymology. The rich brown color of the shiny integument of this taxon inspired the name "Talos", after the living bronze statue that protected Europa from invaders in Greek mythology. The species epithet is an arbitrary combination of letters, and thus invariant.
Distribution and ecology. This rare ant is known from a single locality in the Anjanaharibe-Sud Reserve in the province of Antsiranana (Fig. 13). It is found in montane rainforest at an elevation of 1260 meters.
Taxonomic notes. P. talos is most likely to be confused with P. descarpentriesi as it has a similar arrangement of cephalic foveae to some individuals of P. descarpentriesi (morphotype A). However, P. talos has distinctly shaped and larger eyes and a striking color pattern that distinguishes it quite easily from all Malagasy congeners. Worker description. Highest cephalic index on average of Malagasy Prionopelta ; posterior margin of the head with slight notch medially in full-face view; cephalic foveae shallow, large, and widely spaced; directly adjacent cephalic foveae either completely lacking, or very rare; if any foveae are adjacent, then always with only 2-3 foveae connected, and these usually always medially on the head in full-face view; majority of cephalic foveae separated by 1-3 foveal diameters, appear cleanly scooped from the shining integument, and lack raised margins; median cephalic band devoid of foveae is uniformly broad, and not swelling above the integument; apical tooth very long, longest of all Malagasy Prionopelta, over four times the length of the third apical tooth measured from base to tip (Fig. 2C); sculpture of the dorsum of the mesosoma consisting of large, shallow foveae which are widely spaced at 2-3 foveal diameters with punctures present between foveae; no metanotal suture present in dorsal view, but rather a shining surface with no clear distinction between propodeum and mesonotum; in a few specimens, a slightly perceptible depression is sometimes visible at the site of the metanotal suture, with associated notches on the lateral edges of the dorsum of the mesosoma, but this depression always lacks scarring; in lateral view, mesopropodeal suture weak, appearing as a gradual depression rather than a scar; posterior propodeal edge seen dorsally strongly concave; sharp lamellae of the posterior propodeum present.

Prionopelta vampira
Etymology. The name of this species is inspired by the vampire-like nature of its exceptionally long apical tooth. The species epithet is a Latinized adjective of the German and Hungarian word "vampir".
Distribution and ecology. This species is almost entirely restricted to northern Madagascar where it is found in litter in rainforest, littoral rainforest, and montane rainforest from 25-1200 meters of elevation. Intriguingly, P. vampira has also been collected at a single, highly disjunct site in far southeastern Madagascar 1048 km to the south near Enakara in the province of Toliara (Fig. 13).
Taxonomic notes. This species, which has similar cephalic scupturing to P. xerosilva, is otherwise unmistakable due to its extremely long apical tooth, lack of a metanotal suture, and strongly concave posterior propodeal edge when viewed dorsally.
Non-type material. MADAGASCAR: Antsiranana, Ampasindava, Andranomatavy Forest, 13.669°S, 47.9877°E, 149 m, disturbed dry forest, 6.x.2013 (B.L.Fisher et al.); Antsiranana, Ampasindava, Andranomatavy Forest, 13.663°S, 47.9794°E, 543 m, disturbed dry forest, 6.x.2013 (B.L.Fisher et al.) (Fisher, Griswold et al.); Toliara, 11 km NW Enakara, Rés. Andohahela, 24.5667°S, 46.8333°E, 800 m, rainforest, 17.xi.1992 (B.L.Fisher). Worker description. Cephalic foveae shallow, large, and most widely spaced of the Malagasy Prionopelta; directly adjacent cephalic foveae either completely lacking, or very rare; if any foveae are adjacent, then always with only 2-3 foveae connected, and these usually located medially on the head in full-face view; majority of cephalic foveae are separated by 1-3 foveal diameters, appear cleanly scooped from shining integument, and lack any raised margins at their perimeter; median cephalic band devoid of foveae is wide along its length, not tapering posteriorly; coronal suture absent medially on the head; apical tooth intermediate in length, never more than 4 times as long as the third tooth from base to tip (Fig. 2D); very weak, shallow, foveae present on the pronotum which are noticeably less dense than those of other Malagasy Prionopelta; foveae of the dorsum of the mesosoma interspersed with punctures; pronounced metanotal suture visible in dorsal view, mesopropodeal suture weaker but present; posterior edge of the propodeum straight or only very slightly concave in dorsal view; protruding lamellae of the posterior propodeum absent.
Etymology. The name of this new species is a combination of the Greek adjective "xero" meaning dry and the Latin noun "silva" for forest, as this species is known only from tropical dry forests in western Madagascar.
Distribution and ecology. P. xerosilva is known only from tropical dry forests in the province of Mahajanga in western-central Madagascar (Fig. 13). It has only been collected from forest litter and ranges between 50-300 meters of elevation.
Taxonomic notes. With its uniformly and widely spaced cephalic foveae, P. xerosilva could only be confused with P. vampira. However, the latter lacks a metanotal suture and possesses a very long apical tooth, a strongly concave posterior edge of the propodeum in dorsal view, and lamellae protruding from the posterior propodeum, all of which P. xerosilva lacks.  White diamonds represent type localities whereas dark circles represent non-type localities. Type locality for P. descarpentriesi is unknown. P. seychelles, known only from Seychelles, is not pictured here.