Leptomyrmex

These conspicuous ants are most often encountered individually or as small groups of 2 or 3 foragers on the surface of the ground any time of the day or night. Because of their long legs and thin bodies, they superficially resemble spiders. This is especially true when they are disturbed, as they extend their legs, raise their gasters, and run quickly to escape danger. This has led to their being given the common name "spider ants."

Nests are found in soil or in dead wood, either standing or on the ground, and are often at the base of trees. Colony sizes average a few hundred workers and a single queen. In all but a handful of species, the queen is wingless and worker-like ('ergatoid'), differing from workers only in being slightly larger and with an enlarged mesosoma. In a few species the queens are fully winged, as they are in most other ants.

When a large source of food is found, workers of Leptomyrmex will return to their nest and recruit additional workers to help utilise the newly found resource. They also use workers as "living storage vessels". These special workers, called repletes, accept liquids from returning foragers who transfer their liquid foods to these selected workers. These special workers continue to accept liquids until their gasters become greatly enlarged and extended. When enlarged, repletes cannot escape the nest and remain inside suspended from the ceiling. They can retain these fluids for extended periods and dispense it on demand when food is in short supply.

A published phylogeny for the genus (Lucky 2011) is reproduced here: Leptomyrmex phylogeny

Identification
The antennal scapes are elongate and surpass the rear margin of the head by more than one-half their length. The central area on the underside of the head near the mandibles has a U-shaped notch (medial hypostoma notched). The mandibles have 7 to 15 large teeth and 5 to 12 small denticles.

Species of Leptomyrmex are some of the most distinct in the subfamily Dolichoderinae. Most are morphologically similar, being relatively large with elongate legs and distinctive black, orange, or bicoloured black and orange colouration. However, several species are much smaller and superficially resemble Iridomyrmex. These small Leptomyrmex can be recognised using the same characters as used for the larger species.

Extended diagnosis

Worker: Antennal scapes elongate, surpassing the vertex by more than one-half their length; medial hypostoma notched; mandibles with 7 to 15 teeth and 5 to 12 denticles; anterior tentorial pit located laterally, near the mandibular insertion (mid-way between mandibular insertion and antennal socket in some species).

Queen: Usually, worker-like, differing as follows: mesosoma slightly enlarged; ocelli present.

Male: Funicular segments 2 and 3 more than twice as long as broad; anterior tentorial pit near the lateral junction of the gena and clypeus (near the mandibular insertion); medial hypostoma notched; pterostigmal appendage generally present (although sometimes reduced), but absent in at least one species.

Distribution
The genus is mostly limited to wet sclerophyll or rain forests of New Guinea (and nearby islands), eastern Australia and New Caledonia, with a single record from the Philippines. Previously Leptomyrmex was much more widely distributed based on fossils found in the Dominican Republic amber (Baroni Urbani 1980, Baroni Urbani and Wilson 1987). The recent description of Leptomyrmex relictus from Brazil is the lone extant member of the genus from the New World. This species also oddly lives in dry savanna.

Recent analysis show the genus originated in the Neotropics and likely experienced a single mid-Tertiary dispersal event from South America to southeastern Australia (Boudinot et al. 2016, Barden et al 2017).

Species by Region
Number of species within biogeographic regions, along with the total number of species for each region.

Fossils
Fossils are known from:, (an unidentified species, Wang et al., 2021).

Nomenclature

 *  LEPTOMYRMEX [Dolichoderinae: Leptomyrmecini]
 * Leptomyrmex Mayr, 1862: 695. Type-species: Formica erythrocephala, by monotypy.

Worker
HEAD. Vertex convex to very weakly concave. Compound eyes present, approximately round; relatively posterior on head. Ocelli absent. Antennae 12 segmented. Scape long, surpassing the vertex by about one-half its length or more. Anterolateral clypeal margin even with the mediolateral region (sometimes with a very weakly developed shoulder). Anteromedial clypeal margin entire, without a central notch or concavity of any type. Anterior clypeal setae 2-9; short, less than twice the maximum scape diameter; straight. Posterior clypeal margin even with or anterior to the anterior surfaces of the antennal socket cavities. Anterior tentorial pit near the lateral junction of the gena and clypeus (near the mandibular insertion). Frontal carina present. Anterolateral hypostoma reduced to a thin sclerite (rarely slightly expanded, but never tooth-like). Medial hypostoma notched. Psammophore absent. MOUTHPARTS. Palp formula 6:4 (very rarely 5:4). Third maxillary palp segment subequal in length to segment 4. Fifth maxillary palp segment at the apical extreme of segment 4. Mandible with 7-15 teeth and 5-12 denticles. Apical tooth slightly longer than the subapical tooth. Basal angle weakly defined by a denticle. Basal margin denticulate distally, smooth proximally. MESOSOMA. Posteroventral pronotum weakly expanded medially. Mesopleural process absent. Anteromedial mesosternum even with the lateral regions. Declivitous face of propodeum flat; dorsal face weakly convex to concave, longer than the declivitous face. Propodeal angle distinct (sometimes only weakly). Mesosomal spines and tooth absent. Erect pronotal hairs absent (rarely with numerous short, erect hairs). Dorsal pro-mesonotal junction with the pronotum and mesonotum even, or rarely with the mesonotum above the pronotum. Metanotal groove forming a distinct angle between the mesonotum and propodeum (often reduced and with the mesosomal dorsum nearly flat). Metanotal spiracle dorsal and lying on the dorsal surface when viewed in lateral profile, or rarely lateral and ventral of the dorsal surface when viewed in lateral profile. Propodeal spiracle lateral and ventral of the propodeal dorsum. Hind tibial spur with reduced barbules (barbules often absent from basal half). PETIOLE. Scale present; rounded and forming an even arch dorsally; strongly inclined anteriorly and with the anterior face much shorter than the posterior face. Venter with or without a slight or weakly developed lobe. GASTER. First tergite elongated posteriorly, smooth and without a groove or indentation. Anterior tergosternal suture of the first segment extending laterally from the helcium, without or with at most a very weak dorsal arch. Fifth tergite ventral, gaster with 4 apparent tergites. Gastral compression lateral. Fourth sternite keel-shaped posteriorly. GENERAL CHARACTERS. Worker caste monomorphic. Chromosome number 12 (n=12, L. erythrocephalus, Imai et al. 1977). Integument thin and flexible, weakly sculptured. PROVENTRICULUS. Cupola narrow relative to bulb; round; with long pile; smooth, without sculpture; and without phragma. Bulb exposed in lateral view. Longitudinal muscle No. 1 present. Occlusory tract present.

Queen
The majority of species have ergatoid queens which differ from the worker caste in being slightly larger, having ocelli, and in having an enlarged mesosoma. The fully-winged form is as follows: HEAD. Vertex flat to weakly concave. Compound eyes relatively posterior on head. Antennae 12 segmented. Scape long, surpassing the vertex by more than one-third scape length. Anterolateral clypeal margin even with the mediolateral region. Anteromedial clypeal margin entire, without a central notch or concavity of any type. Anterior clypeal setae about 20; short, less than twice the maximum scape diameter; straight. Posterior clypeal margin between the anterior and posterior surfaces of the antennal socket cavities. Anterior tentorial pit midway between the antennal socket and mandibular insertion. Anterolateral hypostoma reduced to a thin sclerite. Medial hypostoma notched. Psammophore absent. MOUTHPARTS. Palp formula 6:4. Third maxillary palp segment subequal in length to segment 4. Fifth maxillary palp segment at the apical extreme of segment 4. Mandible with about 5-7 teeth and about 14 denticles. Apical tooth elongate and much longer than the subapical tooth. Basal angle indistinct, with a relatively uninterrupted curve between the two margins and without a distinct tooth or angle. Basal margin with denticles limited to near the angle. MESOSOMA. Posteroventral pronotum lateral, rounded or angled. Episternal suture complete. Mesopleural process absent. Anteromedial mesosternum even with the lateral regions. Axilla constricted medially; entire. Anterior axillar suture straight. Declivitous face of propodeum flat to weakly concave. Dorsal face flat; longer than the declivitous face. Propodeal angle distinct. Propodeal suture absent. Mesosomal spines and tooth absent. Erect mesoscutal hairs absent. Propodeal spiracle lateral and ventral of the propodeal dorsum. Hind tibial spur with reduced barbules (absent from basal third). PETIOLE. Scale present; ridged and with a distinct angle dorsally; moderately inclined anteriorly but with the anterior and posterior faces approximately the same length. Venter with a well developed, rounded lobe. GASTER. First segment vertical and not concealing the petiole in dorsal view and smooth and without a groove or indentation. Fifth tergite ventral, gaster with 4 apparent tergites. Gastral compression absent (gaster circular in cross section). Fourth sternite keel-shaped posteriorly.

Male
HEAD. Inner margin of eye entire, flat. Scape length shorter than the length of funicular segments 2+3. First funicular segment cylindrical or cone-shaped. Second funicular segment cylindrical, straight. Funicular segments 2 and 3 more than twice as long as broad. Third and fourth funicular segments bent and forming an angle between them (angle either between segments, or at the distal end of segment 3). Anteromedial clypeal margin entire, without a central notch or concavity of any type. Anterior clypeal setae 0-6; when present short, about as long as the maximum diameter of the scape; straight. Posterior clypeal margin even with or anterior to the anterior surfaces of the antennal socket cavities. Anterior tentorial pit near the lateral junction of the gena and clypeus (near the mandibular insertion). Anterolateral hypostoma reduced to a thin sclerite. Medial hypostoma notched. MOUTHPARTS. Palp formula 6:4. Third maxillary palp segment subequal in length to segment 4. Fifth maxillary palp at the apical extreme of segment 4. Mandible with no teeth (except apical, if present) and about 0-26 denticles. Apical tooth absent (tip of mandible rounded, or occasionally angular) or rarely well defined and much longer than the subapical tooth. Basal angle either indistinct (with a relatively uninterrupted curve between the two margins and without a distinct tooth or angle) or weakly defined by a denticle. Basal margin smooth and without teeth or denticles. MESOSOMA. Posteroventral pronotum lateral, rounded or angled. Episternal suture present and complete (but weak posteriorly), or reduced and incomplete. Anteromedial mesosternum even with the lateral regions. Axilla absent dorsally. Anterior axillar suture straight. Declivitous and dorsal faces of propodeum flat; dorsal face longer than the declivitous face. Propodeal angle indistinct. WINGS. Radial cell closed. Fore wing with no cubital or discoidal cells (rarely with 2 cubital and 1 discoidal cell). Pterostigmal appendage present (although sometimes reduced), or rarely absent. Hind wing with 2 cells. PETIOLE. Scale present; rounded and forming an even arch dorsally; vertical and not inclined anteriorly. Venter with a slight or weakly developed lobe. Attachment to gaster narrow. GASTER. First segment elongated posteriorly, smooth and without a groove or indentation. GENITALIA. Pygostyles present. Posterior margin of subgenital plate convex to even across entire width. Paramere weakly divided (distinctly divided in a few small species). Digitus with a down-turned tip (sometimes also with a dorsal tooth). Cuspis parallel with digitus. Ventral lobe of volsella present as concave lobe (sometimes elongate and finger-like). Aedeagus with ventral teeth.

Larva
Shape leptomyrmecoid. Protuberances present or absent; when present, as a single boss located ventrally on prothorax. Body hairs numerous; simple; short. 10 spiracular pairs.