Solenopsis longicephala

The type collection, the only specimens of Solenopsis longicephala, was made from a nest found under a rock. This species is dimorphic and possesses a set of unusual characters for the genus that are likely convergent with traits of other Myrmicinae genera, such as Carebara.

Identification
Cuezzo & Fernández (2015) - This species can be differentiated from all other Solenopsis by the combination of the following traits: dimorphic ants; major worker with long and abundant hairs on the dorsal face of mesosoma and gaster; head in major and minor workers extremely elongated; eye reduced to 1-5 ommatidia; and palpal formula 2-1. According to Pacheco and Mackay (2013) most species of thief ants are monomorphic or weakly polymorphic; only five species are strongly dimorphic (Solenopsis iheringi; Solenopsis johnsoni; Solenopsis thoracica, Solenopsis tetracantha, and Solenopsis vinsoni) and none of the polymorphic species have the particular combination of character states mentioned above. All of the species described as dimorphic belong to the fugax complex except S. iheringi, which belongs to the wasmannii complex.

Solenopsis longicephala is a member of the pygmaea species complex of Pacheco and Mackay (2013). This complex comprises ants with elongated heads and strongly punctate integuments, often possessing a bicarinate clypeus; 4-5 well-developed clypeal teeth, with different degrees of development but always present, and extremely small minor workers with poorly developed eyes. Also, the notopropodeal suture is only slightly impressed and the postpetiole nearly circular. All the characters above mentioned are present in S. longicephala.

Some members of the Solenopsis wasmannii complex also have elongated heads, workers that are extremely small (total length up to 1.45 mm), and the head surface with very coarse punctures. However, those species possess a very particular form of the clypeal carinae different from that in S. longicephala Also, the compound eyes of their major workers have more ommatidia (10 or more) than those of S. longicephala (4 - 7).

South American species with minor workers similar to those of S. longicephala include Solenopsis leptanilloides and Solenopsis tetracantha, both also found in Argentina. The first species is monomorphic and much smaller than S. longicephala Other characters of S. longicephala useful for separating it from these two species include: clypeal carina well developed in major and minor workers (nearly absent in S. leptanilloides); anterior clypeal border with 4 teeth, two central and two extralateral (extralateral teeth are absent in S. leptanilloides). The elongated anterior clypeal margin of S. tetracantha and the lower process of the postpetiole (present in S. longicephala) are enough to differentiate major workers of both species. Minor workers of S. tetracantha and S. longicephala are extremely similar, but differ in size; S. longicephala is larger.

The major workers of S. longicephala are similar to those of Solenopsis wasmannii, but Solenopsis longicephala can be recognized by the extremely elongated head, whereas the head of S. wasmannii is only slightly longer than broad.

Distribution
Known only from Tucuman Pr., Argentina.

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina.

Biology
Cuezzo & Fernández (2015) - Most myrmicines possess heads not longer or slightly longer than wide. Some species of this subfamily, however, have majors with elongated heads. This trait is known in Adlerzia, Carebara, and some Pheidole (especially in the aberrans groups and Pheidole dispar, formerly in Machomyrma). Because these are distantly related to Solenopsis (Ward et al., 2015), the presence of an elongated head in Solenopsis longicephala is here interpreted as a convergent trait. The biological functions of elongated heads are obscure because little is known of the biology of the mentioned ants. Most Carebara appear to be soil-inhabiting ants, perhaps living near or inside the nests of other ants or termites, with supposedly lestobiotic habits. However, some Pheidole species in the aberrans group appear to be surface foragers and no data are available for Pheidole dispar and Adlerzia. Other subterranean myrmicines lack elongated heads, e.g., the “majors” of Tranopelta. Longer heads and cylindrical bodies could be associated with hard substrates or narrow passages in the soil or in twigs or may be associated with a specialization for defense in the soldier caste.

Castes
Known only from the worker caste.

Nomenclature

 *  longicephala. Solenopsis longicephala Cuezzo & Fernandez, 2015: 188, figs. 1, 2 (w.s.) ARGENTINA.

Worker
Major: Holotype (Measurements of paratypes in parentheses, n=14): HW 0.675 (0.670-0.700), HL 0.975 (0.950-1.025), SL 0.425 (0.375-0.500), SW 0.1 (0.1), EL0.050 (0.050-0.075), EW 0.050 (0.050), PW 0.450 (0.300- 0.425), WL 0.950 (0.950-1.025), FL 0.425 (0.450-0.525), PL 0.300 (0.300), PPL 0.375 (0.350-0.400), CI 69 (68-71), SI 63 (56-71), EI 100 (67-100).

Body and appendages golden yellow.

Head: Noticeably longer than wide (CI 68-71). Posterior cephalic border sinuous, concave in middle, sides straight. Mandibles conspicuous, smooth and shining with a few short setae scattered on dorsal face; masticatory margin with four stout teeth and denticle behind third tooth. Mandibular teeth darker than rest of mandible. Median part of clypeus bicarinate. Clypeal carinae short, divergent, ending at posterior margin of antennal sockets. Anterior clypeal margin with two teeth and two extralateral denticles (see Fig.1a). One central seta in clypeal anterior margin, besides 6 short setae present on anterior margin of clypeus reaching to halfway point of closed mandibles. Sides of clypeus with four to five extremely short setae. Palpal formula 2-1. Maxillary palp geniculate. Second segment of labial palp distally swollen. Both palps (labial and maxillary) with two long setae, distally attached. Compound eye small (EI 67-100) with four to seven ommatidia, placed anterior to cephalic midline. Median ocellus present (Fig 1a). Antennae 10-segmented with apical club of two articles. Scape exceeding posterior margin of compound eye, but failing to reach vertexal border by more than its maximum width. Scape stout and flattened, ventrally concave. All segments of funiculus wider than long, except last one. Frontal lobes well developed, rounded, completely covering antennal sockets.

Mesosoma: Straight in lateral view, with shallow notopropodeal suture. Propodeum convex, low, and unarmed. Propodeal spiracle circular, well developed, equidistant from propodeal border. Dorsopropodeum twice as long as posteropropodeum. From posterior view, posteropropodeum with central concavity, probably used to house anterior face of petiole.

Metasoma: Petiole wider than long in dorsal view, with short peduncle and well-defined high node. Sternopetiolar process well developed with short anterior tooth. Postpetiole dorsally convex, lower than petiole. Postpetiole, in dorsal view, rounded. Gaster with five visible segments, the anterior segment the longest (Fig 1b). Sting well developed, but generally withdrawn inside gaster.

Body smooth and shiny, except for the gena, which has fine parallel grooves, and for the side of the propodeum, which is covered by fine transverse striation. Cephalic dorsum with scattered punctures. Entire body covered by erect to subdecumbent light yellow hairs, more abundant on all gastral tergal plates. Body and appendages light brown. Minor: (n=15) HW 0.300- 0.400, HL 0.450-0.520, SL 0.250-0.300, SW 0.038-0.050, EL 0.025, EW 0.012, PW 0.200-0.275, WL 0.400-0.525, PL 0.125, PPL 0.150, CI 60-77, SI 75-83, EI 50.

Body and appendages light yellow.

Head: Longer than wide (CI= 60-77) with sides slightly convex. Cephalic dorsum with scattered punctures. Posterior cephalic margin concave in middle (Fig 2a). Extremely small eyes with only one ommatidium, placed at lateral cephalic margin in full-face view. Compound eyes difficult to see in dorsal view. Mandibles with four well-developed teeth along masticatory margin, with two diastema, one between subapical tooth and third tooth and the other between 3rd and 4th teeth. Dorsal face of mandible smooth and shining, covered with yellowish, subdecumbent hairs. Anterior clypeal margin with one central short seta. Central part of clypeus bicarinate, each carina ending in a well differentiated tooth. Two extralateral teeth, visible only at more than 60X magnification, each with an apical long hair (see Fig 2a). Gena without striation observed in major worker. Sides of clypeal margin with 5-6 short hairs. Antennae 10-segmented with 2-segmented apical club with two articles. Antennal sockets close to the posterior clypeal margin. Funicular segments 2x-5x wider than long. Palpal formula: 2-1.

Mesosoma, petiole and postpetiole (Fig 2b): in profile similar to major worker. Both sternopetiolar and sternopostpetiolar processes present, with antero-ventral denticle or projection.

Body smooth and shining, including gena. Sides of propodeum covered by fine, transverse striation. Mesosoma with more than 10 erect hairs. Gaster with scattered subdecumbent setae, more abundant in last two segments.

Type Material
Holotype major worker: Argentina, Tucumán, Dpto. Burruyacú. Las Lajitas (26° 41.74’ S- 64° 94.47’ W) 10- i-2011, F. Cuezzo coll. (deposited in IFML). Paratypes (same data): 10 major workers and 10 minor workers (deposited in ), 5 major workers and 10 minor workers (deposited in ), rest of the series (n= 33 major workers and 439 minor workers) deposited at IFML.