Dlussky, G. M., Wappler, T. & Wedmann, S., 2009, Fossil ants of the genus Gesomyrmex Mayr (Hymenoptera, Formicidae) from the Eocene of Europe and remarks on the evolution of arboreal ant communities., Zootaxa 2031, pp. 1-20: 5-6

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Genus Gesomyrmex  HNS  Mayr, 1868

Type species. Gesomyrmex hoernesi Mayr  HNS  , 1868, by monotypy.

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 Mayr  HNS  , 1868 (Baltic amber, late Eocene), G. expectans Theobald  HNS  , 1937 (Kleinkembs, France, Oligocene) and G. miegi Theobald  HNS  , 1937 (Haut-Rhin, France, Oligocene). The last two fossil species must be excluded from Gesomyrmex  HNS  (vide infra). Five new species from middle Eocene deposits of Germany are described below.

Comments. Mayr (1868) described the genus Gesomyrmex  HNS  with the unique species G hoernesi  HNS  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. hoernesi  HNS  , and he described it as Gesomyrmex chaperi  HNS  . The second species differed by its large size, a more elongate head and smaller eyes, and he described it as Dimorphomyrmex janeti  HNS  . Emery (1905) found in Baltic amber a specimen similar to D. janeti  HNS  and described it as Dimorphomyrmex theryi  HNS  . Finally Wheeler (1915) re-described G. hoernesi  HNS  and D. theryi  HNS  and described two new species from Baltic amber: Gesomyrmex annectens  HNS  and Dimorphomyrmex mayri  HNS  .

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 Wheeler  HNS  . 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 Dimorphomyrmex  HNS  , and small (minor) and medium workers those of Gesomyrmex  HNS  . As a result Wheeler designated Dimorphomyrmex  HNS  as junior synonym of Gesomyrmex  HNS  , and concluded that both species of Dimorphomyrmex  HNS  and both species of Gesomyrmex  HNS  , described from Baltic amber, really belong to one polymorphic species Gesomyrmex hoernesi Mayr  HNS  .

Most extant species of Gesomyrmex  HNS  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 luzonensis  HNS  (Wheeler 1916, 1930) and a wingless gyne of G. tobiasi  HNS  (Dubovikoff 2004). The last species is known only from this gyne.