Gesomyrmex

A poorly known resident of tree canopies in Oriental tropics, showing a striking diversity of caste morphologies. Queens and supersoldiers share an elongate head with powerful mandibles, an adaptation to chew an entrance tunnel to the innermost pith of living branches (Peeters et al. 2017).

Gesomyrmex encompasses seven extant species, all endemic to southeast Asian rainforests, north of the Wallace line (Antmaps.org, Janicki et al. 2016), together with 12 fossil-based species. The genus was in fact first described by Mayr (1868) from Baltic amber fossils, assigned to Gesomyrmex hoernesi, some 24 years before the first extant species, Gesomyrmex chaperi, was published. Wheeler (1915) later provided a more thorough description of this species in his monograph on the ants of the Baltic amber, and synonymized Dimorphomyrmex with Gesomyrmex (Wheeler, 1929b), noting remarkable polymorphism among workers. Dlussky and colleagues (Dlussky et al. 2009, 2015) more recently raised the number of fossil species to eleven, following the description of eight new gynes preserved as compression fossils from Eocene limestones of Germany, Croatia and Russia (Aria et al., 2023).

Four extant species of Gesomyrmex (Gesomyrmex chaperi, Gesomyrmex howardi, Gesomyrmex kalshoveni and Gesomyrmex spatulatus) are known only from workers, while two species (Gesomyrmex luzonensis and Gesomyrmex tobiasi) were described from lone queens (Dubovikov, 2004).

Identification
Workers of the genus are easily recognised by the following features (Bolton, 1994):
 * eyes reniform and massive relative to head size
 * antennal scape passing below the eye in their resting position
 * masticatory margin of mandible with more than four teeth

The workers have 8-segmented antennae and soldiers of Gesomyrmex chaperi have 9-segmented antennae (Wheeler, 1916), whereas queens have 10-segmented antennae (Dubovikov, 2004).

Gary Alpert is of the opinion that all described species belong to Gesomyrmex chaperi (the name with priority). Caste polymorphism has led to great taxonomic confusion, both in extant and fossil species. Marked colour variations further complicate alpha taxonomy. Wheeler (1929) previously suggested that extant species may correspond to “sub-species or varieties”.

The genus is similar to Santschiella in that it possesses very large eyes, widely separated antennal insertions, and scapes that pass below the eyes.

Fossils
Fossils are known from:, , , , , , , ,.

Distinctions among fossil morphotypes based on body size, anterior clypeal margin and occipital cephalic margin support in general a radiation of the genus during the early Cenozoic. However, species erected based on dubious characters from compression fossils, such as colour or ‘head shape’ (prone to taphonomic deformation and changes due to orientation of entombment in these fossils) (Dlussky et al. 2009, 2015), do suggest some overestimation in palaeodiversity. Generic apomorphies are also missing or concealed in certain cases, such as in Gesomyrmex flavescens, questioning the affinity of these fossils with Gesomyrmex (Aria et al., 2023).

Biology
From Peeters et al. (2017) Gesomyrmex chaperi presents an intriguing division of labour: workers are the active hunters, with very distinct mandibles. Queens (as well as two kinds of soldiers) have different mandibles, indicating that they do not hunt during colony foundation. However, a foundress needs to chew an entrance tunnel through living wood, and then block this nest entrance for many months until the colony is strong enough to produce the first soldiers. Supersoldiers are presumably reared even later in colony ontogeny, because they are more costly. Relatively few supersoldiers are present and they show two queen-like behaviours: they stay inside nest chambers and block the entrances, and they chew entrance holes when starting other nests belonging to the same colony. Supersoldiers also store nutrients (trophic eggs) in their gaster.

Dubovikov (2004) - Members of this genus are very rare and ancient forms. The population of their nests are small and they live in small branches of trees (Cole, 1949b). Identification key to five living Gesomyrmex species was published by Cole (1949a).

Castes
In addition to winged queens, three sterile castes can be distinguished using discrete morphological traits, morphometry and total body size (Peeters et al. 2017). Observations of behaviour are challenging in tree canopies, and functional morphology can be used to predict the specialised functions of different castes.



Nomenclature

 *  GESOMYRMEX [Formicinae: Gesomyrmecini]
 * Gesomyrmex Mayr, 1868c: 50. Type-species: †Gesomyrmex hoernesi, by monotypy.
 * Gesomyrmex senior synonym of Gaesomyrmex: Forel, 1893a: 167.
 * Gesomyrmex senior synonym of Dimorphomyrmex: Wheeler, W.M. 1929a: 1.
 * DIMORPHOMYRMEX [junior synonym of Gesomyrmex]
 * Dimorphomyrmex André, 1892b: 49. Type-species: Dimorphomyrmex janeti (junior synonym of Gesomyrmex chaperi), by monotypy.
 * Dimorphomyrmex junior synonym of Gesomyrmex: Wheeler, W.M. 1929a: 1.
 * GAESOMYRMEX [junior synonym of Gesomyrmex]
 * Gaesomyrmex Dalla Torre, 1893: 175, unjustified emendation of Gesomyrmex.
 * Gaesomyrmex junior synonym of Gesomyrmex: Forel, 1893a: 167.

Ward et al. (2016) - The tribe Gesomyrmecini is here restricted to Gesomyrmex and two similar fossil taxa (Wheeler 1915). Bolton (2003) also placed Santschiella in Gesomyrmecini, but the molecular results do not support a close relationship between Gesomyrmex and Santschiella (Blaimer et al. 2015). The similarities between the two—very large eyes, widely separated antennal insertions, and scapes that pass below the eyes (Bolton 2003)-must be interpreted as due to convergence.