Linguamyrmex brevicornis

Identification
Perrichot et al. (2020) - Closely similar to Linguamyrmex vladi, but differs from this species by having a shorter clypeal horn (horn index HoI = 31-34 in L. brevicornis, 56 in L. vladi), with stalk much reduced, slightly broader than long, with the setose pad more hexagonal and aligned with stalk (circular and with a distinct bend between stalk and clypeal pad in L. vladi), and with clypeal pad coated in stout setae. It is also differentiated by the mandibles with two teeth on ventral corner of medioventral blade (teeth absent in L. vladi) and, in workers, with the apical mandibular portion shorter than basal portion (apical portion longer in L. vladi). In addition to the difference in the proportion of the mandible, the female castes of L. brevicornis differ by their size (body length ca. 7 mm in gyne, 4 mm in worker), by the antennae with flagellomeres compact in worker, more elongate in gyne, and by subpetiolar process present in worker.

Distribution
This taxon was described from.

Castes
Known from gynes and workers.

Specimens
Originating from amber mines located near Noije Bum Village, Tanai Town, Myitkina District of Kachin State, northern Myanmar. A radiometric dating of zircons from the amber-bearing bed gave a maximum age of 98.79 ± 0.62 Ma (Shi et al., 2012), thus corresponding to the latest Albiane-earliest Cenomanian (mid-Cretaceous), which may not be very different from the age of the amber itself (Smith and Ross, 2018; Yu et al., 2019).

Holotype NIGP172001. Worker. Preserved without apparent distortion, with cuticle cleared on head, pronotum and legs, and missing apical portions of all legs and apical gastral segments. In an oval piece of clear yellow amber measuring 13 x 9 x 3 mm, with two springtails and dipteran wings. NIGPAS, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Science, China.

LA01. Alate gyne. An exquisitely preserved specimen, without distortion, but missing right antennomeres VII-XII and tarsomeres of left mid leg and hind legs. Exposed in dorsal, profile and full facial views, in a quadrangular piece of clear yellow amber measuring 10 x 8 x 6 mm, with a staphylinid beetle (Coleoptera) and several domichnia (borings) from pholadid bivalves (Smith and Ross, 2018). LA, Mineral & Gem Research Center, Hong Kong.

RM4. Worker. A complete, nicely preserved specimen, without distortion. Exposed in profile views in an amber piece measuring 21 x 4 mm, with plant debris. RM, Ruipoxuan Museum, Jinan, China.

Nomenclature

 * † brevicornis. †Linguamyrmex brevicornis Perrichot, Wang & Barden, 2020: 13, figs. 6C-E, 7 (w.q.) MYANMAR (Burmese amber).

Description
Measurements (in mm) (worker holotype NIGP172001) [worker specimen RM4] {gyne specimen LA01}. HL [0.82] {0.82}; EL [0.21] {0.42}; HoL [0.17] {0.36}; MDL [0.59] {0.80}; length of antennomeres: I (scape) (0.57) [0.67] {1.04}, II (pedicel) (0.09) [0.12] {0.14}, III-XII (0.35, 0.17, 0.16, 0.15, 0.15, 0.13, 0.13, 0.12, 0.13, 0.17) {0.80, 0.40, 0.34, 0.32, 0.31, 0.30, 0.30, 0.32, 0.32, 0.35}; WL [1.29] {2.29}; PL {0.70}.

Queen
Head. As in Linguamyrmex vladi, but with compound eyes slightly reniform, emarginate in middle of posterior margin. Ocelli distinct, positioned on a prominent cuticular triangle, in dorsal view, with posterior margin of lateral ocelli aligned with posterior margin of eyes. Antenna filiform, with flagellomeres elongate, cylindrical except apical one, which has acute tip; third antennomere twice as long as following one. Antennal sockets immediately flanking a medial frontal triangle that is sharply expanded anteriorly above clypeal horn, and connecting ventrally with dorsal surface of horn by a cleared, laterally flattened cuticle; frontal triangle projection terminates anteriorly at approximately same length as clypeal horn with slight indentation just dorsad clypeal horn, giving the appearance of a second, small, broadly rounded horn. Clypeal horn originating at base of frontal triangle, short and straight, in profile view almost perpendicular to longest axis of head. Horn paddle-shaped, with short, compact stalk and hexagonal, setose pad; stalk apparently glabrous, 0.10 mm in length, 0.12 mm in width; setose pad 0.26 mm in length, 0.20 mm at greatest diameter, with anterior margin straight, transverse; dorsal surface of setose pad glabrous, ventral surface densely and uniformly coated with very short, velcro-like vestiture; trigger hairs not visible. Lateral clypeal margins extending from above mandibular insertions toward horn as gently curved lines. Anterior clypeal margin broadly concave. Labrum visible. Mandible scythe-shaped; basal portion linear, short, meeting apical curved portion nearly at right angle; medioventral triangular blade bearing one large apical tooth and one small preapical tooth, with concave dorsal surface coated with pointed setae; apical portion of mandible posterior to medioventral blade 1.6 x as long as blade, with tip broadly rounded. Mandibles preserved in closed position (with tips of apical portion reaching nearby clypeal setose pad), almost entirely parallel and closely approximated, except ventral corners of blades overlapping and tips slightly diverging. Maxillary palps 5-segmented, labial palps 3-segmented.

Mesosoma. Pronotal colar pronounced but not concealing entire propleuron in dorsal view; pronotal dorsum weakly convex. Promesonotal suture present, complete. In dorsal view, mesoscutum as long as pronotum (excluding neck), about as broad as long, with lateral and posterior margins carinate; posterior mesoscutal margin broadly concave; dorsal mesoscutal surface with long parapsidal furrows almost reaching anterior mesonotal margin, converging posteriorly but not touching. In profile view, pronotal dorsal outline feebly convex, mesoscutal dorsal outline nearly flat. Mesoscutellum prominent posteriorly; posterior mesoscutellar surfaces concave, their junction forming a sharp angle; dorsal mesoscutellar surface with a deep, broad, transverse groove immediately posterior to scuto-scutellar suture. Metanotum medially as high as long, with posterior surface forming distinct angle with pronotal dorsum. Propodeum 1.3 x as high as long; in profile view, propodeal dorsum distinctly lower than pronotal dorsum; propodeal dorsum nearly flat, declivitous surface feebly convex, their junction broadly rounded. Propodeal spiracle slit-like, oriented posteriad. Metapleural gland opening circular, gaping. Legs long and robust, mesocoxa distinctly shorter than pro- and metacoxae; small trochantellus present on mid- and hind legs; all femora distinctly swollen in their basal half, tibiae swollen in their apical half; ventral margin of protibia apically with large calcar and two simple, stout setae less than half as long as calcar; mesotibia apically with two long, straight, pectinate spurs, and two short, straight, stout setae; metatibia apically with one long, pectinate spur and one long, simple spur. Tarsomeres I-IV of all legs with pairs of stout setae along entire ventral surface (8-10 pairs on tarsomere I, 4-5 pairs on tarsomere II, 3 pairs on tarsomere III, 2 pairs on tarsomere IV), and apically with 2 pairs of stout setae each flanking a spatulate spine; additionally the ventral surface of tarsomeres I-IV covered by dense brush of fine, erect setae; pretarsal claws thick, with a distinct subapical tooth.

Fore wing with veins M∙f4 and Cu1 nebulous, not reaching wing margin; remaining veins tubular; cross-vein 1r-rs absent; Rs∙f1 faintly arched; M∙f1 distinctly arched, twice as long as Rs∙f1; Rs∙f2 nearly at right angle with RsþM, half as long as M∙f2; RsþM and M∙f2 not aligned, so that discal cell pentagonal, with vein 1m-cu distinctly longer than M∙f1; second submarginal cell long, crossvein 2rs-m situated far beyond apex of pterostigma; cross-vein cu-a arising from Cu, shortly distal to M∙f1. Hind wing with jugal lobe present.

Metasoma. Petiole short-pedunculate; petiolar tergite forming a broadly convex node, with anterior surface approximately twice as long as posterior surface; subpetiolar process present, in profile a high, transverse, tooth pointing ventrally, with anterior surface concave, posterior surface vertical; transverse sulcus visible across petiole near basal margin. First gastral segment with helcium pronounced, forming a post-petiolar peduncle, with anterior surface posterior to helcium high, oblique, and dorsal surface strongly convex, short; anteriormost part of first gastral sternite with a distinct mesal process pointing anteroventrally below helcium. Second gastral segment distinctly longer than first, with presclerite largely exposed to form a deep, broad constriction between first and second gastral segments (abdominal segments III and IV). Posterior margin of gastral segments II onward with sparse, elongate setae. Pygidium nearly glabrous while hypopygium with sparse setae; pygidium apparently broadly acute towards sting shaft.

Worker
Smaller than gyne (total length of body around 4 mm), and with the following differences: compound eyes oval, ocelli faintly visible, not positioned on a prominent cuticular triangle. Antenna more compact, with flagellomeres II-IX gradually broadened apically, apical flagellomere with rounded apex. Clypeal setose pad more quadrangular, the ventral surface with a pair of trigger hairs originating at pad base. Mandible with medioventral triangular blade larger and apical portion (posterior to blade) reduced, 0.6 x as long as blade.

Mesosoma and metasoma as in Linguamyrmex vladi except propodeum higher than long, petiole with subpetiolar process present, and girdling constriction deeper between abdominal segments III and IV (AIII with an oblique posterior surface). Protibia with one large apical spur (calcar) and two short stout setae; mesotibia with two simple spurs and three conspicuous setae positioned along anterior margin of tibial apex; metatibia with one pectinate and one simple spur, additionally with a brush of 8-10 setae flanking the pectinate spur. Sparse, erect setae positioned on dorsum of pronotum, propodeum, petiolar tergite, and first and second gastral tergites and sternites. Pygidium not preserved.

Type Material
Holotype. NIGP172001, worker. Additional specimens. LA01, alate female, and RM4, worker. Horizon and locality. Upper Cretaceous, upper Albianelower Cenomanian (ca. 99 Ma); in amber from the Hukawng Valley, Kachin State, Myanmar. Holotype - NIGPAS, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Science, China.

Etymology
The specific epithet is a combination of brevi (Latin, meaning ‘short’) and cornus (Latin, meaning ‘horn’), and refers to the smaller clypeal horn compared to the type species L. vladi.