Formicidae

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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?

Remarks
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).

Diagnosis
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).