Gracilidris

Gracilidris pombero and a tentatively assigned Dominican amber fossil species, Gracilidris humiloides, are the only known species of this genus.

Identification
Wild and Cuezza 2006 - Small, slender dolichoderine ant. Petiole distinct, bearing a tall dorsal scale with apex narrowed and distinctly curved posteriad. Compound eyes located near longitudinal midpoint of head and touching lateral margins in full frontal view. Anterior margin of clypeus projecting forward in a broad convexity. Antennal scapes longer than head length. Dorsum of head, mesosoma, and metasoma exclusive of abdominal tergite 6 devoid of erect setae.

In Bolton’s Identification guide to the ant genera of the world (Bolton 1994), Gracilidris will work through to the final couplet of the Neotropical dolichoderine key separating Linepithema and Azteca. Gracilidris will not key easily to either genus as it lacks the broad medial anterior clypeal concavity of Linepithema and does not match the mandibular dentition of Azteca. In Shattuck’s Generic revision of the ant subfamily Dolichoderinae (Shattuck 1992b), Gracilidris will key through to couplet 17 of the worker key but does not match either option given in that couplet for clypeal configuration. Of all the Neotropical ant genera, Gracilidris most closely resembles Linepithema, but can be distinguished by a more lateral position of the eyes on the head, the relatively longer antennal scapes, and the distinct posteriad curve of the petiolar apex.

Other common dolichoderine genera may be separated from Gracilidris as follows. Azteca: worker caste polymorphic, petiolar node low, and clypeus not projecting forward. Dolichoderus: integument more sclerotized, declivitous face of propodeum concave in lateral view, and possessing hypostomal teeth. Dorymyrmex: with a distinct process or spine on propodeum, anterior clypeal margin not strongly projecting forward, and with a much stronger apical mandibular tooth. Forelius: anterior clypeal setae as long as the closed mandibles, petiole with a much smaller scale and obscured in dorsal view by abdominal tergite 3. Iridomyrmex: compound eyes in full frontal view in most species not touching the lateral margins of the head, anterior clypeal margin not projecting forward as a single convexity. Leptomyrmex: body and appendages much more elongate, petiolar scale lower and less developed. Tapinoma: petiole lacking a distinct scale and obscured in dorsal view by abdominal tergite 3. Technomyrmex: as for Tapinoma, petiole lacking a distinct scale and obscured in dorsal view by abdominal tergite 3.

Fossils
Fossils are known from:.

Biology
Feitosa et al. (2015) - Gracilidris was originally described based on specimens collected primarily along the so called South American dry diagonal, which includes the savannah lands of northeastern and central Brazil (caatinga and cerrado, respectively), and the chaco areas of Paraguay and Argentina (Prado & Gibbs, 1993). Until recently, it was thought that the extant lineage of Gracilidris was restricted to the low scrub forests and open fields of this dry diagonal. However, Guerrero and Sanabria (2011) provided the first record of G. pombero for the northern portion of South America, more precisely for the foothills of the Colombian Amazon Basin.

Gracilidris pombero specimens were identified among the ants collected in a long term ecological research (LTER) network (PELD Campos Sulinos – CNPq) at southern Brazilian grasslands. These grasslands occupy a transitional zone between tropical and temperate climates, with hot summers and cool winters, and are composed of several phytophysiognomies and extremely high plant species richness. Vegetation is characterized by an association of C4 and C3 grasses and a variety of forbs and shrub species. Cattle grazing is the traditional land use, which ensures the maintenance of typical grassland vegetation (Overbeck et al., 2007; Pillar et al., 2009).

Gracilidris pombero individuals were sampled in experimental sites distributed solely over the Pampa biome. Eighteen workers were recovered from pitfall traps. The specimens studied here came specifically from the traps installed in the experimental plots of Lavras do Sul (30º42’02”S, 53º58’53”W) and Aceguá (31°38’55”S, 54°09’26”W), near the border with Uruguay. Both sites are placed in private farms that use traditional grazing management. Workers were captured in 16th December, 2011 (one individual in Lavras do Sul, and eight individuals in Aceguá), 8th December, 2012 (one individual in Lavras do Sul, and seven in Aceguá), and 4th December, 2013 (one individual in Aceguá).

The records of Gracilidris for Rio Grande do Sul represent the southernmost register of the genus in the Neotropics and the first for the Pampa Biome.

Nomenclature

 *  GRACILIDRIS [Dolichoderinae: Leptomyrmecini]
 * Gracilidris Wild & Cuezzo, 2006: 59. Type-species: Gracilidris pombero, by original designation.

Worker
Worker morphology generalized, somewhat elongate. Worker caste monomorphic. Integument thin and covered in a dense, fine pubescence. Proventriculus small, weakly developed, and unsclerotized, without phragma or visible pile, and with a dome-shaped cupola divided into 4 quadrats. Proventricular cupola only partly obscuring the bulb in lateral view, and about as wide or only slightly wider than the bulb.

Head

In full frontal view longer than broad and quadrate-oval in shape. Lateral margins convex, posterior margin straight. Compound eyes situated near longitudinal midpoint of head, touching lateral margins, and large in size, with at least 50 ommatidia (>130 in G. pombero). Ocelli absent. Antennae 12-segmented, scapes noticeably longer than head length (SI 138–167). Clypeus large and entire, without a median notch or concavity, and projecting forward with a broad, convex anterior margin. Hypostoma with a strong medial notch. Psammophore absent.

Mouthparts

Palp formula 6:4. Maxillary palp segments generalized and of roughly equal length, except for a short proximal segment. Mandibles triangular with dentition as follows: strong apical tooth, a smaller subapical tooth, and a series of 8–12 small teeth or denticles on masticatory margin. Basal angle indistinct and often bearing several denticles.

Mesosoma

Pronotum and mesonotum freely articulating, not fused, and in lateral view forming a single continuous convexity. Pronotum rounded laterally. Propodeum depressed below the level of promesonotum. Dorsal and declivitous faces of propodeum subequal in length and meeting in an indistinct convexity. Propodeal spiracle located postero-laterally, at about 2/3 height of declivitous face.

Petiole

(abdominal segment 2). With a distinct dorsal scale that is taller than broad, inclined anteriorly, and achieving height of propodeal spiracle. Apex of scale narrowed into a point and distinctly curved posteriad, so that in lateral view anterior face of apical third of scale is convex and posterior face is slightly concave. Venter of petiole with a weakly developed lobe.

Gaster

(abdominal segments 3–7). Gaster in dorsal view with four visible terga. Abdominal tergite 7 small, concealed under tergite 6, and oriented ventrally. Gaster in posterior view roughly triangular and slightly compressed laterally. First gastral segment (abdominal segment 3) with anterior tergosternal suture extending laterally from helcium. Ultimate abdominal sternite (abdominal sternite 7) folded medially in shape of a longitudinal keel.

Etymology
Gracilidris refers to the slender habitus of this ant.