|List of Subfamilies|
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Below is a recent diagnosis of the Family, which is based on Boudinot (2015). For those of you that are not taxonomists or have knowledge about animal classification and diversity - the following is the taxonomic answer to the question - What is an ant?
The Formicidae is an unequivocally monophyletic group, previously defined by Bolton (1994, 2003) as eusocial, sexually dimorphic aculeate Hymenoptera bearing metapleural glands and geniculate antennae, among other characters. Several previously unreported synapomorphies exist for the family, including a suite of adaptations for terrestrial locomotion (characters 6 and 7). The “low and lateral” propodeal spiracle placement may also be an adaptation for terrestrial locomotion, as it may reduce the distance oxygen would need to diffuse to leg locomotor muscles. While this does not clarify whether the ancestral ant was hypogaeic or epigaeic, it does indicate that terrestrial locomotion was a crucial transition for the Formicidae, as these apomorphies are present in all adult castes of the family. Previous diagnoses of the family (Brothers 1975; Gauld & Bolton 1988; Goulet & Huber 1993) were significantly improved by Bolton (1994, 2003). Characters indicated in the family diagnosis by Bolton (2003) and above will be valuable to evaluate for critical fossil taxa such as †Armania Dlussky and other fossils assigned to the †Armaniidae whose relationship to the Formicidae is uncertain (see Dlussky 1975:†Archaeopone, †Dolichomyrma, †Poneropterus, †Pseudarmania; Dlussky 1983: †Armaniella; Dlussky 1999: †Khetania; Dlussky et al. 2004: †Orapia; also see discussion in LaPolla et al. 2013).
Aculeate Hymenoptera with the following apomorphies:
1. Eusocial, wingless worker caste present, colonies perennial (note 1).
2. Sexuals with synchronous nuptial flights (note 2)
3. Head capsule prognathous (worker, gyne) (note 1).
4. Infrabuccal sac present between labium and hypopharynx (note 1).
5. Antenna geniculate between long scape and funiculus (worker, gyne) (notes 1, 3).
6. Disticoxal foramen directed laterally and completely enclosing protrochanteral base, including protrochanteral condyles, such that all disticoxal membrane concealed (all castes, Fig. 3C) (note 4).
7. All meso- and metacoxal cavities small, circular, monocondylic, ventrally-directed, and disticoxae strongly produced laterally (all adult castes, Fig. 3C) (note 5).
8. Metapleural gland present (adult castes, but see note 6).
9. Propodeal spiracle located on lateral propodeal face distant from the anterodorsal propodeal corner, often near propodeum midlength (all adult castes) (note 7).
10. Wings of alate gyne deciduous, being shed after copulation (note 1).
11. Forewing 3rs-m and 2m-cu absent (note 1).
12. Hindwing C not extending along anterior margin, even spectrally (note 8).
13. Hindwing basal/radial cell not produced distally (alate castes) (note 9).
14. Metasoma petiolate (abdominal segment II differentiated from segment II,I which is strongly constricted between the pre- and postsclerites) (all castes), extremely rarely (~ 1 species) abdominal segment III not constricted between pre- and postsclerites (notes 1 and 10).
Additional, non-synapomorphic characters of value for diagnosis and identification include: Antenna with 4–12 antennomeres (female) or 5–13 antennomeres (male) (note 11). Bulbus neck (= radicle) and scape with common axis. Epicnemium extremely reduced, not visible in situ (note 12). Abdominal segment II with sternum and tergum equally sclerotized. Pterostigma present or absent (note 13). Wing venation variable, may be extremely reduced, with at minimum no closed cells (note 14). Jugal lobe present or absent; abdominal sternum IX may be complex and modified apically (including prongs, teeth, and lobes).
1. Noted as apomorphic by Bolton (2003).
2. Bolton (2003) indicated that “sexuals with mass nuptial flight” was an apomorphy of the Formicidae. Although mass flights do occur in several lineages of ants, it is not clear if the ancestral condition for the Formicidae is to release large quantities of sexuals. The wording has been specifically rephrased here to account for this uncertainty.
3. Males of many species have derived geniculate antennae with elongate scapes, including numerous Myrmicinae, most Formicinae, and Tapinoma (Dolichoderinae). Most males, including poneroids and numerous formicoids, however, have antennae which are not geniculate and have very short scapes.
4. The procoxa of Formicidae is characteristically modified. The trochanteral foramen (situated apically on the procoxa) is directed laterally and entirely enclosed, revealing no membrane in undamaged specimens (Fig. 3C, left column, top row). Medially, the foramen is closed by an unfused seam of the anterior and posterior apical coxal lobes, which completely surround the anterior trochanteral process. The axis of coxal-trochanteral articulation, rather than being lateromedial as in Symphyta (Fig. 3A), or rotated obliquely as in many Aculeata (Fig. 3B), is almost entirely anteroposterior. Leg adduction and abduction occurs along this anteroposterior axis in more-or-less one plane of motion, with the trochanter rotating within the closed disticoxal foramen. The coxae and their articulations with the mesosoma and trochanters are poorly studied and show promise for valuable systematic characters. Previous work on hymenopteran coxae include Johnson (1988), which solely focused on the basicoxite and its musculature, Michener (1981), which focused on the meso- and metacoxae of the Apoidea, and Vilhelmsen et al. (2010), which operationalized several coxal characters. This character is unique to the Formicidae.
5. The meso- and metacoxal foramina are monocondylic, bearing only the medial coxal articular processes and lacking the lateral coxal articular processes of the meso- and metapleurae. Lateral condyles are lacking in the examined species of Chyphotinae, Bradynobaenidae s. str., Mutillidae, and Myrmosidae.
6. The metapleural gland, so distinctive of the female castes, is variably developed in males and has been lost in various taxa.
7. The “high and far forward” placement of the propodeal spiracle remarked upon by Bolton (2003) as a plesiomorphy for the Formicidae is actually an apomorphy for the family. In non-formicid Aculeata (including Apoidea, Scoliidae, and Bradynobaenidae s. str.) the propodeal spiracle is usually situated at the extreme anterodorsal corner of the propodeum, usually within a propodeal spiracle length from the metanotum, and often on the dorsal propodeal face. Some Pompilidae and Tiphiidae (Tiphiinae) have the spiracle situated more posteriorly. Although the propodeal spiracle of †Sphecomyrma freyi is situated high—but laterally—and rather anteriorly (Wilson et al. 1967), it is clearly not at the extreme of other Aculeates. Other †Sphecomyrma species have more posteriorly situated spiracles which are clearly situated laterally (Wilson 1985; Engel & Grimaldi 2005). The potential male of †Sphecomyrma identified by Grimaldi et al. (1997) has a low spiracle situated at about segment midlength.
8. Reduction of the hindwing costal vein occurs sporadically in other aculeate families.
9. The basal/radial cell has been convergently reduced or lost in several ant subfamilies, and has been lost in Mutillidae, Myrmosidae, Bradynobaenidae s. str., and Chyphotidae. The generality of this trait in these families was not evaluated.
10. The male of an unidentified Protanilla (Leptanillinae) from Thailand has secondarily lost petiolation, where the third abdominal segment is no longer constricted between the pre- and postsclerites (Fig. 10A). These males are still recognizable as ants by the closed apical procoxal foramen, ventrallydirected meso- and metacoxal cavities, and low and lateral propodeal spiracle. Other Protanilla species (even in sympatry) retain the constriction, while yet others have petiolation of the third abdominal segment (Fig. 10B). Some males of the Dolichoderinae (e.g., Azteca) and other unidentified males of the Leptanillinae have very reduced petioles, but these are still distinctly differentiated from the third abdominal segment and are slightly posteriorly constricted.
11. Antennomere count for males usually 13, less often 8–12 (count of 8 observed in Acropyga and Stenamma; counts of 10+ more common). Antennomere counts may be extremely reduced in inquilines, for example in Pheidole acutidens, which occasionally have an antennomere count of 5, although this is variable infraspecifically, and indeed may vary between the left and right antennae.
12. Brothers (1975) contends that the form of the formicid epicnemium is unique, being highly reduced, fused to and extending over the height of the mesepisternum, and obscured by the pronotum. This putative homology was not evaluated in the present work.
13. The pterostigma is lost in most Leptanillinae, some myrmicine genera, and some species of Leptomyrmex (Dolichoderinae).
14. No closed cells are observed in some males of Leptanillinae and Myrmicinae.
Boudinot, B.E. 2015. Contributions to the knowledge of Formicidae (Hymenoptera, Aculeata): a new diagnosis of the family, the first global male-based key to subfamilies, and a treatment of early branching lineages. European Journal of Taxonomy 120, 1-62 (http://dx.doi.org/10.5852/ejt.2015.120).