Gesomyrmex

A poorly known resident of tree canopies in Oriental tropics, showing a striking diversity of caste morphologies.

Identification
Workers of the genus are easily recognised by the following features: elongated compound eyes; antennal scape passing below the eye; masticatory margin of mandible with more than four teeth (Bolton, 1994). The workers have 8-jointed antennae and soldiers of Gesomyrmex chaperi have 9-jointed antennae (Wheeler, 1916), whereas females have 10-jointed antennae Dubovikov (2004).

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

Biology
From Peeters et al. (2017) Gesomyrmex presents an intriguing division of labour: workers are the active hunters, with very distinct mandibles. Queens (and both 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.

Dubovikov (2004) - The genus Gesomyrmex was established by G. Mayr (1868) for a single species from Baltic amber (Lower Oligocene). The four extant species (Gesomyrmex chaperi, Gesomyrmex howardi, Gesomyrmex kalshoveni and Gesomyrmex spatulatus) are known only from workers. Gesomyrmex luzonensis and Gesomyrmex tobiasi were described from lone queens.

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.