Haidomyrmecinae

Hell ants, the haidomyrmecines, are a group of putatively predatory ants that are known exclusively from three Cretaceous amber deposits in France, Myanmar, and Canada. Hell ant cranial morphology is unlike any modern group of ants, a reﬂection of ancient diversification ultimately bound for extinction. Phylogenetic analyses have recovered haidomyrmecines as a stem-group lineage that diverged from modern ants prior to the most common recent ancestor of all living ants (Barden and Grimaldi, 2016; Barden et al., submitted). This phylogenetic placement, molecular divergence estimates (Moreau and Bell, 2013) and the presence of crown ants in Cretaceous amber (Grimaldi and Agosti, 2000; McKellar et al., 2013b; Zheng et al., 2018; Perrichot, 2019) indicate that hell ants and early members of extant lineages overlapped for tens of millions of years. The extinction of haidomyrmecines following their diversification remains an outstanding question in ant evolution, as is the function and evolutionary history responsible for this striking expansion into unparalleled phenotypic space. (Perrichot, Wang & Barden, 2020)

Identification
Perrichot et al. (2020) - Defined by mandibles that are dorsoventrally expanded and highly modified heads with a variety of cranial appendages.

(females). Mandibles scythe- or sickle-shaped, with linear basal portion leading to an elongate and dorsally curved apical portion tapering, with inner margin usually developed in a triangular blade pointing medially and ventrally (exception in Aquilomyrmex where the inner margin is simple); mandibles uniquely articulating in a vertical plane oblique to longitudinal axis of body, in addition to a moderate lateral opening. Clypeus elongate, with anterior margin broadly concave, smooth, and lateral margins leading posteriorly to an elevated brushy lobe just ventral to antennal insertion, or to a horn expanded anteriorly between toruli. Antennae 12-segmented, filiform, usually with third antennomere longest of basal three flagellomeres (exceptions are in Haidomyrmex where fourth antennomere is longest, and in Haidomyrmodes where basal flagellomeres are of equal length). Petiole with a short anterior peduncule, nodiform. Gastral constriction between AIII and AIV generally present, faintly to deeply impressed. Pygidium simple, unarmed. Sting robust, dorsally curved. Legs with procoxa distinctly longer than meso- and metacoxae, with trochantellus present on mid- and hind legs; tibial spur formula 1-2-2, rarely 1-1-2, and tibiae additionally with 1e4 subapical, stout setae. In gynes, the fore wing with 8 closed cells, with cross-vein 1r-rs absent or incomplete (present as a short tubular or nebulous stub not reaching ScþR); cross-vein 2rs-m present, tubular; and crossvein cu-a arising from MþCu or Cu. Hind wing with jugal lobe present, with costal, basal and subbasal cells enclosed by tubular veins.

Even as Dlussky remarked on the unique cranio-mandibular system of hell ants, his bewilderment was related to a single taxon. Discoveries over the last decade and the taxa described here expand the boundaries of the group's morphology. Most striking is the extent to which the clypeus and mandibles are exaggerated. While all previously known hell ants possess cranial nodes or horns (Dlussky, 1996; Perrichot et al., 2008a, 2016; Barden and Grimaldi, 2012; McKellar et al., 2013a; Barden et al., 2017; Miao and Wang, 2019), these appendages are the product of elevations that originate in the posterior region of the clypeus in Haidomyrmex, Haidomyrmodes, Haidoterminus, Ceratomyrmex, and Linguamyrmex. The clypeus itself is drawn out dorsoventrally, matching the elongation present in the head capsule. This scheme is echoed in Protoceratomyrmex. However, the cranial horns present in Aquilomyrmex, Chonidris, and Dhagnathos are the product of an anterior clypeal margin that is extended dorsally as well as posteriad, resulting in a furrowed clypeal sclerite with a medial depression and ventrally concave horn. The visible epistomal sutures of Chonidris and Protoceratomyrmex highlight the two distinct cuticular origins of horns in haidomyrmecines. In Aquilomyrmex, the ventrally concave clypeus comprises the entire anterior expansion of the head capsule, and this expanded clypeus is matched by an equally extended labrum that is coated in thick denticles. The parallel modifications of the clypeus and mandibles in all haidomyrmecine taxa strongly suggest that these two features interacted during mouthpart movement, most likely to aid in prey capture.

Evolution
Perrichot et al. (2020) - Hell ant cranial morphology is unlike any modern group, a reflection of ancient diversification ultimately bound for extinction. Phylogenetic analyses have recovered haidomyrmecines as a stemgroup lineage that diverged from modern ants prior to the most common recent ancestor of all living ants (Barden and Grimaldi, 2016; Barden et al., submitted). This phylogenetic placement, molecular divergence estimates (Moreau and Bell, 2013) and the presence of crown ants in Cretaceous amber (Grimaldi and Agosti, 2000; McKellar et al., 2013b; Zheng et al., 2018; Perrichot, 2019) indicate that hell ants and early members of extant lineages overlapped for tens of millions of years. The extinction of haidomyrmecines following their diversification remains an outstanding question in ant evolution, as is the function and evolutionary history responsible for this striking expansion into unparalleled phenotypic space.

Nomenclature

 * † [tribe of †Sphecomyrminae]
 * †Haidomyrmecini Bolton, 2003: 74, 261. Type-genus: †Haidomyrmex Dlussky, 1996: 84.

Genera

Taxonomic History

 * †Haidomyrmecini as tribe of †Sphecomyrminae: Bolton, 2003: 74, 261; Perrichot, et al. 2008: 92; McKellar, et al. 2013b: 457; Borysenko, 2017: 17.
 * †Haidomyrmecinae as subfamily of Formicidae: Perrichot, Wang & Barden, 2020: 3.

Taxonomic References
McKellar, et al. 2013b: 457 (genera and all species key); Perrichot, Wang & Engel, 2016: in supplemental information (not paginated) (genera and all species key); Barden, et al. 2017: 838 (synopsis of species); Borysenko, 2017: 19 (diagnosis); Perrichot, Wang & Barden, 2020: 3 (status as subfamily).