Gracilidris Wild & Cuezzo, 2006

Wild, A. L. & Cuezzo, F., 2006, Rediscovery of a fossil dolichoderine ant lineage (Hymenoptera: Formicidae: Dolichoderinae) and a description of a new genus from South America., Zootaxa 1142, pp. 57-68 : 59-62

Gracilidris Wild & Cuezzo , gen. nov.

(worker Figs. 1-8, 12-14; map Fig. 11)

Type species: Gracilidris pombero (by present designation).

Included species

pombero Wild & Cuezzo 2006

humilioides (Wilson 1985; Dominican amber)

Worker diagnosis

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.

Worker description

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 (Fig. 7). 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.

GENERAL. 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.

Discussion

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.

The relationship of Gracilidris to other dolichoderines is the subject of ongoing research by the authors (Cuezzo & Wild, in preparation) and will not be reported in detail here. In support of the establishment of a new genus, however, preliminary results from three nuclear protein-coding genes indicate that the sequence divergence between G. pombero and other dolichoderines is of a level similar to that observed among other dolichoderine lineages currently treated as genera. Additionally, the establishment of Gracilidris does not appear to render any currently-recognized genus paraphyletic.

Etymology

Gracilidris refers to the slender habitus of this ant.