Gesomyrmex

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Gesomyrmex
Temporal range: 48.6–0 Ma Eocene – Recent
Gesomyrmex chaperi
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Subfamily: Formicinae
Tribe: Gesomyrmecini
Genus: Gesomyrmex
Mayr, 1868
Type species
Gesomyrmex hoernesi
Diversity
7 species
12 fossil species
(Species Checklist, Species by Country)

Gesomyrmex-luzonensisLM4.jpg

Gesomyrmex chaperi

Gesomyrmex luzonensis, worker, head.jpg

Specimen Label

Synonyms

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).

Photo Gallery

  • Gesomyrmex chaperi worker from Vietnam
  • Supersoldier, soldier, workers and brood of Gesomyrmex sp. From Danum, Sabah. Photo © Christopher Wilson.
  • Two size classes of Gesomyrmex sp. soldiers (above), and workers (Sabah colony). Pupae are naked (Wheeler 1923a), thus cocoons have been lost as in Oecophylla. Photo by Christopher Wilson.
  • Gesomyrmex sp. from Baltic amber. Photo by Levon Biss.
  • A Baltic amber fossil.

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.

AntWeb icon 02.png See images of species within this genus

 

Distribution

Distribution and Richness based on AntMaps

Species by Region

Number of species within biogeographic regions, along with the total number of species for each region.

Afrotropical Region Australasian Region Indo-Australian Region Malagasy Region Nearctic Region Neotropical Region Oriental Region Palaearctic Region
Species 0 0 5 0 0 0 3 2
Total Species 2841 1736 3045 932 835 4379 1741 2862

Fossils

Fossils are known from: Baltic amber (Bartonian, Middle to Late Eocene), Bitterfeld amber (Bartonian, Middle to Late Eocene), Bolshaya Svetlovodnaya, Sikhote-Alin, Russia (Priabonian, Late Eocene), Danish-Scandinavian amber (Bartonian, Middle to Late Eocene), Eckfeld, Germany (Lutetian, Middle Eocene), Messel, Germany (Lutetian, Middle Eocene), Oise amber, France (Ypresian, Early Eocene), Radoboj, Croatia (Burdigalian, Early Miocene), Rovno amber (Priabonian, Late Eocene).

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).

  • Changes in the Eocene biome palaeomap in light of the distribution of Gesomyrmex species. (a) General Eocene biome zoning modelled by Harold et al. (2014); (b) expected gross biome coverage at the heart of the PETM based on the distribution of Gesomyrmex. Areas outside Eurasia are not considered here and are greyed out. (Aria et al., 2023, Fig. 5)

Biology

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.

Gesomyrmex nest entrance. Photo by Decha Wiwatwitaya
Gesomyrmex nest in the very center of the heartwood in living branches of small diameter. Photo by Decha Wiwatwitaya
To access this secure innermost pith, the founding queen needs to chew an entrance tunnel. Photo by Decha Wiwatwitaya
Supersoldier head blocking the entrance. Photo by Decha Wiwatwitaya

Aria et al. (2023) Although our understanding of the ecology and biology of Gesomyrmex remains scarce, observations have been made by some authors pointing to a diurnal arboreal lifestyle (reviews in Dlussky, Wappler and Wedmann 2009; Peeters, Ito, Wiwatwitaya, Keller, Hashim and Molet 2017). Gesomyrmex typically nests in both live and dead stems or branches, co-opting tunnels burrowed by other insects or digging their own (Peeters, Ito, Wiwatwitaya, Keller, Hashim and Molet 2017). Colonies were initially reported to have a single queen and relatively few individuals (150 in one of the nests studied), whereas a more recent study found them to be polydomous (Peeters, Ito, Wiwatwitaya, Keller, Hashim and Molet 2017). Likewise, the worker population has initially been considered to encompass a broad polymorphism with morphological overlaps, but would instead be characterized by a subdivision into three distinct asexual castes including supersoldiers (Peeters, Ito, Wiwatwitaya, Keller, Hashim and Molet 2017). Soldier morphs had been already recognized in G. hoernesi and in some modern species in which asexual individuals closely resemble their gynes (Bolton1994; Dubivikoff 2004; Dlussky, Wappler and Wedmann 2009). These recent findings (Peeters, Ito, Wiwatwitaya, Keller, Hashim and Molet 2017) would be in accordance with recent phylogenomic analyses placing Gesomyrmex closer to other polymorphic lineages among Formicinae (Blaimer, Brady, Schultz, Lloyd, Fisher and Ward 2015), and therefore suggesting more complex eusocial habits, even if geographical or specific variations cannot be excluded. Interestingly, Gesomyrmex is found to be sister genus to Oecophylla (Blaimer, Brady, Schultz, Lloyd, Fisher and Ward 2015), also typically arboreal, but with a much broader contemporaneous distribution spanning SE Asia, India, Oceania and Africa (http://antmaps.org).

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).

Life History Traits

  • Mean colony size: ~150 (Greer et al., 2021)
  • Compound colony type: not parasitic (Greer et al., 2021)
  • Nest site: arboreal (Greer et al., 2021)
  • Diet class: omnivore (Greer et al., 2021)
  • Foraging stratum: arboreal (Greer et al., 2021)

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.


Heads of all four castes of Gesomyrmex sp. shown at different scales (A: worker, B: soldier, C: supersoldier (all from Sabah colony); D: queen from Ulu Gombak). Note differences in ocelli, clypeus, mandibles and frontal lobes (covering the antennal sockets). Photo by Mônica Antunes Ulysséa

Morphology

Worker Morphology

Explore-icon.png Explore: Show all Worker Morphology data or Search these data. See also a list of all data tables or learn how data is managed.

• Antennal segment count: 8 in worker, 10 in queen • Antennal club: gradual • Palp formula: 6,4 • Total dental count: 6-10 • Spur formula: 1 simple, 1 simple; 0,0 • Eyes: >100 ommatidia • Scrobes: absent • Pronotal Spines: absent • Mesonotal Spines: absent • Propodeal Spines: absent • Petiolar Spines: absent • Caste: polymorphic • Sting: absent • Metaplural Gland: present • Cocoon: absent

Male Morphology

Explore-icon.png Explore: Show all Male Morphology data or Search these data. See also a list of all data tables or learn how data is managed.

 • Antennal segment count 11 • Antennal club 0 • Palp formula 6,4 • Total dental count 1 • Notes: from literature

Phylogeny

Formicinae
Myrmelachistini
Lasiini
Melophorini
Formicini
Gesomyrmecini

Gesomyrmex  (7 species, 12 fossil species)

Oecophyllini

Oecophylla  (15 species, 16 fossil species)

Plagiolepidini
Gigantiopini

Gigantiops  (1 species, 0 fossil species)

Santschiellini

Santschiella  (1 species, 0 fossil species)

Myrmoteratini

Myrmoteras  (41 species, 0 fossil species)

Camponotini

See Phylogeny of Formicinae for details.

Nomenclature

The following information is derived from Barry Bolton's Online Catalogue of the Ants of the World.

  • 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.

References