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Extant: 6 valid species, 1 valid subspecies
Fossil: 11 valid species
Diagnosis. Worker caste polymorphic. Eyes enormously large in workers and males. Antennae geniculate, 8-segmented in worker, 10-segmented in gyne and 8-11-segmented in male. Mandible in worker and gyne with 5-10 acute teeth; mandibles in male reduced, not opposable. Posterolateral corners of the head and propodeum without spines. Fore wings with closed cells 1+2r, 3r and mcu.
Species numbers and distribution. Six extant species are known which are distributed in the Oriental tropics (Fig. 1). Three fossil species were described: Gesomyrmex hoernesi MayrHNS, 1868 (Baltic amber, late Eocene), G. expectans TheobaldHNS, 1937 (Kleinkembs, France, Oligocene) and G. miegi TheobaldHNS, 1937 (Haut-Rhin, France, Oligocene). The last two fossil species must be excluded from GesomyrmexHNS (vide infra). Five new species from middle Eocene deposits of Germany are described below.
Comments. Mayr (1868) described the genus GesomyrmexHNS with the unique species G hoernesiHNS from Baltic amber from 19 workers and one male. A quarter of a century later Andre (1892) described two new extant species from Borneo. One of them was similar to fossil G. hoernesiHNS, and he described it as Gesomyrmex chaperiHNS. The second species differed by its large size, a more elongate head and smaller eyes, and he described it as Dimorphomyrmex janetiHNS. Emery (1905) found in Baltic amber a specimen similar to D. janetiHNS and described it as Dimorphomyrmex theryiHNS. Finally Wheeler (1915) re-described G. hoernesiHNS and D. theryiHNS and described two new species from Baltic amber: Gesomyrmex annectensHNS and Dimorphomyrmex mayriHNS.
Some years later Wheeler (1929) received 18 workers, collected by L.G.K. Kalshoven in Java. All these ants were collected from the same nest, so, naturally, they belonged to the same species described as Gesomyrmex kalshoveni WheelerHNS. Workers from the same colony were very polymorphic. Large, medium and small workers differed by the form of the head, eye size, and mandible form. Moreover large (major) workers had characters of DimorphomyrmexHNS, and small (minor) and medium workers those of GesomyrmexHNS. As a result Wheeler designated DimorphomyrmexHNS as junior synonym of GesomyrmexHNS, and concluded that both species of DimorphomyrmexHNS and both species of GesomyrmexHNS, described from Baltic amber, really belong to one polymorphic species Gesomyrmex hoernesi MayrHNS.
Most extant species of GesomyrmexHNS are known only from the worker caste. A revision of the genus and key for determination of workers was published by Cole (1949). Only three sexuals are known: a winged gyne and a male of G luzonensisHNS (Wheeler 1916, 1930) and a wingless gyne of G. tobiasiHNS (Dubovikoff 2004). The last species is known only from this gyne.