Solenopsis is a moderately large genus with over 180 described species (Bolton 1995) and a large but unknown number of undescribed species. Most Solenopsis species are small, cryptic and poorly known, ecologically unrelated to the small number of large, open-foraging, notorious species such as S. invicta Buren, S. geminata (Fabricius) and S. saevissima (F. Smith). Some small subterranean species live next to nests of other ants and prey on the larvae of their neighbors (Hölldobler & Wilson 1990). A few South American Solenopsis species are workerless parasites, whose peculiar morphology caused them to be placed in the genera Labauchena or Paranamyrmex until Ettershank (1966) synonymized those genera with Solenopsis. Recently, a new species of Solenopsis, S. phoretica, was described, based on a single dealate queen found clinging to the petiole of a queen of Pheidole dentata Mayr (Davis & Deyrup 2006). It is suspected that this queen represents a parasitic species, considering the situation of the specimen and certain morphological features. At the time of the description of S. phoretica, this species did not appear to be closely related to the known parasitic Solenopsis, or any other known species of Solenopsis (Davis & Deyrup 2006). Soon after the description of S. phoretica was published, another species of inquiline Solenopsis was discovered on the island of Dominica in the West Indies. This species appears to be closely related to S. phoretica, and is described below.

Character states defining Solenopsis are provided by Ettershank (1966), or can be derived from the keys in Bolton’s guide to ant genera (1994). Members of the genus can usually be recognized by the following combination of character states: 2-segmented petiole; 2-segmented antennal club; propodeum lacking spines or angles; petiolar node well developed; head and body shining, sculpture restricted to setigerous punctures and localized striae; clypeus longitudinally carinate, with a median, apical marginal seta. The clypeal features are lacking in S. phoretica and the species described below.

We have designated a dealate queen as the holotype because of the apparent close relationship between this species and S. phoretica. Diagnosis of the new species must be by reference to character states of queen S. phoretica, as workers of that species are unknown.

Solenopsis enigmatica, Deyrup and Prusak new species

Diagnosis of Dealate Female (Fig. 1). Distinguished from most other Solenopsis by the form of the mandibles, which are falcate, with the teeth not in an oblique series, as normal in species of Solenopsis; the basal angle is strongly developed and weakly bidentate, the concave inner margin with a low, median double tooth; S. phoretica lacks this median double tooth.

Description of Holotype Dealate Female. Features visible in lateral view, described from left side. Measurements in mm. Total length (length of head excluding mandibles + length of mesosoma + length of petiole + length of postpetiole + length of gaster): 3.92; head length: 0.60; head width at rear margins of eyes in frontal view: 0.62; length of mesosoma: 1.20; length of petiole: 0.37; length of postpetiole: 0.20; length of gaster: 1.55. Color: yellowish brown, ocellar triangle dark brown, appendages yellow. Head: smooth, shining, sparsely covered with setigerous punctures separated by 2-8 times the width of a puncture in frontal area, 2-4 times the width of a puncture on sides, setae suberect, directed posteriorly in frontal area, elsewhere directed anteriorly; ocelli not enlarged, each ocellus about the width of antennal scape near base; malar distance about half (4/7) length of eye; mandibles elongate, a little less than half the length of the head at midline; mandible with basal angle strongly developed, bidentate; inner margin of mandible strongly concave distal to basal angle, with a low, median, marginal double tooth, the proximal cusp blunt but conspicuous, the distal cusp reduced to a low ridge; clypeus declivitous, concave, without carinae, with 4 subapical elongate setae; antennae 10-segmented, scapes reaching outer corner of head in frontal view, antennal club 2-segmented, club about as long as remainder of funiculus. Mesosoma: smooth and shining, with sparse setigerous punctures on pronotum and near margins of mesonotum, mesopleura, sides of propodeum; katepisternum with sparse setigerous punctures; disc of mesonotum and declivity of propodeum not punctate; propodeum evenly declivitous in lateral view; legs smooth, shining, with sparse, strong, semidecumbent, curved, distally directed hairs; hind basitarsus longer than hind tibia; a dense, coarse, ventral brush of hairs on distal half of front basitarsus. Petiole: peduncle short, less than 0.25 length of base of petiole in lateral view; node of petiole in lateral view triangular, apex broadly and smoothly rounded, in posterior view semicircular; ventral process narrowly expanded, with a small ventral projection that is like a low ridge rather than a tooth. Postpetiole: low and rounded above in lateral view, in posterior view about 1.5 times as wide as long. Gaster: in dorsal view first tergite with prominent, rounded anterior corners, projecting forward from midline; first gastral tergite and sternite each with a broad, median, basal concavity; first tergite with sparse, long, suberect, posteriorly-directed, hairs that are generally distributed; tergites 2-4 smooth, each with an irregular row of large, suberect, subapical hairs.

Type Locality and Associated Information. Collecting data on labels (3) of holotype: DOMINICA, W.I., Pont Casse, 30 April 2006, M., N., L. Deyrup; Trail to Trois Pitons, in nest of Pheidole antillana in rotten log; rainforest near base of trail, N15°22.80’, W61°20.46’.

Diagnosis of Worker (Fig. 2). Distinguished from other Solenopsis workers by clypeal morphology: clypeus lacking carinae, in lateral view rounded and strongly declivitous. Pale yellow, antennal scapes short, in frontal view failing to reach occipital margin of head by a distance about equal to the combined length of the first 4 segments of the funicle. If S. phoretica produces workers, they might be similar to those of enigmatica, so this diagnosis may not be definitive.

Description of Paratype Worker. Measurements in mm. Total length (length of head excluding mandibles + length of mesosoma + length of petiole + length of postpetiole + length of gaster): 2.27; head length: 0.50; head width at rear margins of eyes in frontal view: 0.45; length of mesosoma: 0.60; length of petiole: 0.20; length of postpetiole: 0.12; length of gaster: 1.55. Color: pale yellow, eyes black. Head: smooth, shining, with scattered setigerous punctures, including a series of 4 large, posteriorly-directed, suberect, slightly convergent setae on each side of frontal area, and several small, decumbent, convergent setae on each side of frontal area; eye composed of a single, large, dark facet; mandibles unlike those of queen, more typical of workers of other small Solenopsis species: apical tooth long and sharp, remaining 3 teeth small, sharp, in a strongly oblique series, with a long, toothless proximal inner margin; clypeus without carinae, convex in lateral view, with four subapical elongate setae and a long median apical seta; antennae 10-segmented, scapes in frontal view failing to reach occipital margin by a distance about equal to the first 4 segments of the funicle; antennal club 2-segmented. Mesosoma smooth, shining, pronotum with 3 large, semierect, posteriorly-directed setae on each side, and 1 smaller, semierect, anteriorly-directed seta on each side at posterior margin of pronotum; propodeal spiracle large, as in Fig. 2; legs smooth, shining, with sparse, strong, semidecumbent, distally-directed hairs; hind basitarsus about the length of hind tibia. Petiole: apex of node in posterior view somewhat flattened; node with two long, posteriorly-directed setae on each side near apex; ventral process a low, triangular tooth, somewhat rounded apically. Postpetiole: low and rounded above in lateral view; in dorsal view 5/7 as long as wide. Gaster: first tergite with sparse, long, suberect, posteriorly-directed hairs that are generally distributed; tergites 2-4 smooth each with an irregular row of large, suberect, subapical hairs; sting slightly extruded.

Type locality and associated information as in holotype female.

Additional Paratype Material. Two workers, one a pharate adult in pupal skin, associated with the holotype female. One dealate queen: collecting data as on labels: DOMINICA, W.I., Cochrane, west of Morne Macaque, 29 April 2006. Z. Prusak. Middleham Falls Trail, in nest of Pheidole antillana in rotten log. 15°20.852’N, 61°20.698’W.

All specimens were collected in Dominica rain forest habitat as defined by Lack et al. (1998), with evergreen shrubs, canopy trees and abundant epiphytes.

Holotype and other type material to be deposited in the Museum of Comparative Zoology, Harvard University, Cambridge, MA.

Etymology: from aenigma (Latin), meaning “mystery,” referring to the many unknown aspects of the natural history of this unusual Solenopsis.

Discussion

There is a striking resemblance between the queens of enigmatica and phoretica (Fig. 1, Fig. 3). With only one specimen of phoretica and two of enigmatica, little can be said about the variation in either species. It seems unlikely, however, that these specimens represent a single, widespread, variable species. The differences in mandibular structure are probably functionally significant, considering that the specimen of phoretica was found with her mandibles locked around the petiolar peduncle of the host queen. Inquiline ants tend to be host-specific, exploiting a single host, or a few very closely related hosts (Hölldobler & Wilson 1990). Pheidole antillana Forel is in the tristis species group of Pheidole, dentata in the fallax group, and the two species are not closely related (Wilson 2003). It also seems more likely that these specimens represent 2 rare, localized species, rather than a widely distributed species that has never been found anywhere but Florida and Dominica.

The S. phoretica-enigmatica group is known from only 3 queens and a few associated workers, but all 3 were apparent inquilines in the nest of Pheidole species, and we are assuming that this is a normal association. It is rare for an inquiline ant to be so distantly related to its host, but there is a relevant example in Oxyepoecus, a genus related to Solenopsis, with hosts in both Pheidole and Solenopsis (Hölldobler & Wilson 1990). We are not suggesting that there is any reason to think that phoretica and enigmatica could be transferred to Oxyepoecus, or that they represent some sort of transition between Solenopsis and Oxyepoecus. Several species of small Solenopsis may live in nests adjacent to those of other ants, which they exploit in what one might call a peri-inquiline way (Hölldobler & Wilson 1990), and it could be that the phoretica-enigmatica lineage took such a relationship a few steps further.

The exact nature of the relationship between S. enigmatica and its host is unknown. It is reasonable to speculate that queen enigmatica spend some time in close association with the host queen, as enigmatica has the elongate mandibles and concave first gastral tergite found in phoretica. No queen Pheidole was found in either of the 2 colonies that contained enigmatica, but it is possible that the host queens were overlooked. The presence of 3 workers associated with 1 queen enigmatica is consistent with the hypothesis that enigmatica is a temporary nest parasite, but no mature colony has been found. Worker enigmatica are conspicuously larger than the other 3 species of yellow Solenopsis that we found on Dominica, and it is unlikely that we would have discarded a colony of enigmatica, mistaking it for one of the common little yellow Solenopsis. Parasitic ants in general are notoriously rare. The abundance of another parasitic Solenopsis, S. daguerrei (Santschi) has been studied by Calcaterra et al. (2000). With 4,131 potential host colonies examined, parasitization rates were 3.9%. If this degree of rarity is at all typical of parasitic Solenopsis, we were fortunate to find 2 parasitized colonies during the short time we were in Dominica.

It is highly unlikely that the phoretica-enigmatica lineage of Solenopis is confined to the island of Dominica and Alachua County in Florida. One possibility is that members of this group occur throughout the range of Pheidole in the New World, but have escaped notice because they are rare and host-specific. It is also possible that this group of inquilines can only succeed in communities in which a few species of Pheidole are especially abundant, with persistent and accessible colonies. This might favor the success of inquilines on islands with relatively low Pheidole diversity, and near the edges of the range of Pheidole. The relentless raiding of Pheidole nests by army ants in the humid Neotropics would seem to decrease the effective resource base for inquilines.

The potential host range for S. enigmatica on Dominica is probably limited. The species most similar to P. antillana is P. laudatana Wilson, a slightly smaller member of the tristis group that is common on Dominica, but not known from other islands. Both species are common in primary forest on Dominica. Pheidole antillana nests in rotten sticks and logs on the forest floor, while colonies of laudatana nest in clay soil and have a small nest opening. It seems probable that antillana is more vulnerable to inquilines than laudatana. If it turns out that S. enigmatica is a host-specific inquiline, it must have a restricted distribution, as P. antillana is known only from St. Vincent, Grenada and Dominica. We found antillana common in the extensive forest reserves on Dominica, but absent from disturbed areas. Other islands of the Lesser Antilles have less extensive forest reserves, in some cases only small fragments. It is too soon too speculate about the rarity of S. enigmatica, but its conservation status is one of several interesting questions pertaining to the species.