Dorylinae

Belongs within: Formicidae.

The Dorylinae, driver ants, are a group of ants that have only temporary nests, with the colony moving as a whole as it forages for food. They are found in the tropics of the Old World. The Dorylinae include some of the largest ants, with females of Dorylus species being up to 5 cm in length. The elongate, wasp-like males are commonly referred to as ‘sausage flies’.

Ants on the move

Published 14 September 2010

The animal in the image above is a male driver ant of the genus Dorylus, commonly known as a sausage fly due to the appearance of the elongate abdomen. African Dorylus species include the largest of all ants: the male in the photo is about 3 cm long while females can be up to 5 cm.

Army ants and driver ants are a clade of tropical ants characterised by a number of distinct behavioural and morphological characteristics, including the construction of temporary nests with at least occasional colony migrations to new nest sites and obligatory foraging in large groups. Queens are permanently wingless; both queens and workers are eyeless or possess very rudimentary eyes and often exhibit a large variation in size within a colony. Males possess fully-developed eyes and wings and appear so different to the queens that Linnaeus originally described one in 1764 as a species of wasp, and the relationship between males and females was not established until Savage collected them in association in 1849 (earlier authors, noting the common proximity of males to workers, had suggested that the former might be parasitoids of the latter!) During the 1800s, army ants were commonly recognised as a separate family Dorylidae, but since then they have been included in the family Formicidae with all other ants and ‘Dorylidae’ has not been used as a valid taxon. The former Dorylidae is now referred to as the ‘doryline’ or ‘dorylomorph’ group (also including the related subfamily Cerapachyinae, the members of which do not show the full range of army ant characteristics) with the army ants divided between four subfamilies. This subdivision was established when it was thought for a large part of the 1900s that the subfamilies might be convergent rather than related, but in recent years both morphological and molecular studies have supported a single origin for the army ant lifestyle (Brady 2003; Brady & Ward 2005). Within the army ant clade, the basal division has been consistently identified as between the South American Ecitoninae and an Old World (African and Asian) clade including the genera Dorylus, Aenictus and Aenictogiton*, each currently placed in a separate monogeneric subfamily. The biology of army ants was reviewed by Gotwald (1982); I’ve drawn heavily on that publication for this post.

*Aenictogiton is a rather enigmatic genus, known only from the rarely collected males. Whether the species of this genus exhibit army ant lifestyles, as suggested by its apparent phylogenetic position, requires the identification of females to establish.

Army ants can be roughly divided between species that forage above ground (epigaeic forms) and those that forage subterraneously or beneath leaf litter (hypogaeic forms), though some hypogaeic species will forage epigaeically if conditions permit (usually at night). Similarly, epigaeic foragers may in turn be either epigaeic or hypogaeic nesters. For obvious reasons, the epigaeic species are the best studied with little being known about the sociobiology of most hypogaeic species; however, it is the hypogaeic forms that account for the majority of army ant species. In all army ants studied to date, foraging is done by advancing columns or networks of groups of workers, without individual exploratory scouts searching ahead for food sources. Location of food sources is therefore largely fortuitous, though advancing columns are attracted to movement (potential prey animals have been observed avoiding predation by the simple expedient of not moving). In the subterranean forager Dorylus laevigatus, smaller workers produce small exploratory tunnels, often by muscling into pre-existing cracks in the ground; once a suitable food source is discovered, the tunnels are enlarged to allow the passage of larger workers (Berghoff et al. 2002). Digging is done by workers wither carrying away soil pieces or passing them beneath their body to on-following workers (Berghoff et al. 2003). In epigaeic forms, all sizes of worker participate equally in exploration, though the largest of workers (often referred to as ‘soldiers’; however, different worker sizes exist as points on a spectrum rather than as discrete classes) usually only perform defensive rather than food-gathering roles for the simple reason that their hypertrophied mandibles (which increase allometrically with body size) are largely incapable of handling food items. Smaller workers will also handle smaller food particles or may transport liquid rather than solid food sources. Most food items are small animals such as earthworms or arthropods though some species will also take plant foods such as oily fruits; despite the somewhat macabre stories sometimes told about army ants, larger animals are rarely at risk unless injury or confinement restricts their ability to move away from the foraging column. Army ants have functional stings that most use in addition to their mandibles for attacking prey or other animals disturbing the column; however, Dorylus species appear to use their mandibles only. Many African tribes have supposedly exploited the aggressiveness of driver ant soldiers for suturing cuts; the ant was held to the open wound and its head broken off after its mandibles had clamped into the patient’s skin.

In many species, a regular transition occurs between nomadic and stationary phases in the life cycle; in others, the transition is more irregular and less defined. In those species with separate phases, the queen of a colony lays eggs during the stationary phase, her abdomen becoming massively swollen while laying. An army ant queen may produce hundreds of thousands of eggs over the course of a year (which will of course include a number of nomadic phases in those species that have them); the queen of Dorylus wilverthi has been estimated to produce from 3 to 4 million eggs per month. When the young reach the larval stage, the ants become nomadic and the queen’s abdomen telescopes down in size so that she is able to move along with the rest of the colony (Dorylus queens, which do not have such clearly separated life phases, do not undergo this regular expansion and contraction). When the larvae moult into pupae, the colony returns to the stationary phase and produces a new nest. Hypogaeic nesters dig a nest into the ground; epigaeic nesters produce a bivouac between plant branches by means of larger workers linking themselves together into a living net with the queen safely held in the centre.

Because army ant queens do not possess wings at any point in their lives, they do not perform dispersive nuptial flights in the way of many other ants. The winged males do still function to disperse genes between colonies; mature males leave their parent colony to find a suitable mate, dying after a period of a few days to a few weeks. New colonies are formed by colonial fission; if a brood contains developing queens, some workers in the colony become attached to the new queens instead of the old. Usually only the first one or two young queens to emerge from the pupa establish new colonies; once they reach maturity, they will leave their parent colony, taking those workers that have transferred their allegiances with them. The workers are unable to survive long without a queen, but if a colony should happen to lose its matriarch all may not be lost for the individual workers. If they should find another colony before they perish, they may be adopted into that colony and transfer their loyalty to a new queen. Of course, if they are carrying any larvae or eggs produced by their old queen, those will soon be consumed by the new colony.

Systematics of Dorylinae

Characters (from Wheeler & Wheeler 1990): Eyes absent or vestigial; pedicel usually of two segments in worker (one in female and male); clypeus short; frontal carinae short and vertical, not covering antennal insertions; antennae usually short; epinotum usually unarmed.

<==Dorylinae [Dorylida, Dorylidae]
    |--Curticorna Hong 2002H02
    |    `--*C. leptogastrosa Hong 2002H02
    |--Eoaenictites Hong 2002H02
    |    `--*E. castanifurvus Hong 2002H02
    `--Dorylus Fabricius 1793B03 (see below for synonymy)
         |--*D. vespahelvola Fabricius 1793Z01
         |--D. brevinodosa [=D. (Typhlopone) brevinodosa]E89
         |--D. fuscus Emery 1889E89
         |--D. helvolaG20
         |--D. juvenculusL49
         |--D. labiatus Shuck. 1840 (see below for synonymy)B03
         |--D. laevigatus (Smith 1857) (see below for synonymy)B03
         |--D. nigricansI92
         |--D. oraniensis (Lucas 1848)E89, E12 [=Typhlopona oraniensisE12, [=D. (Typhlopone) oraniensisE89]
         |--D. orientalis Westwood 1835B03 (see below for synonymy)
         `--D. punctata [=D. (Typhlopone) punctata]E89

Dorylus Fabricius 1793^B03 [incl. Alaopone Emery 1881^B03, Anomma Shuckard 1840^Z01, Cosmaecetes Spinola 1851^Z01, Dichthadia Gerstacker 1863^B03, Rhogmus Shuckard 1840^Z01, Shuckardia Emery 1895^Z01, Sphegomyrmex Imhoff 1852^Z01, Typhlopone Westwood 1839^Z01]

Dorylus labiatus Shuck. 1840 [=D. fulvus var. labiatus; incl. D. hindostanus Smith 1859, D. (Typhlopone) laeviceps]^B03

Dorylus laevigatus (Smith 1857) [=Typhlopone laevigata; incl. Do. (Typhlopone) breviceps Emery 1889, Dichthadia glaberrima Gerst. 1863, Do. klugi Emery 1887]^B03

Dorylus orientalis Westwood 1835^B03 [incl. Labidus (Typhlopone) curtisii Shuckard 1840^Z01, D. fuscus Emery 1889^B03, D. longicornis Shuckard 1840^B03, Alaopone oberthuri Emery 1881^B03]

*Type species of generic name indicated

References

Berghoff, S. M., J. Gadau, T. Winter, K. E. Linsenmair & U. Maschwitz. 2003. Sociobiology of hypogaeic army ants: characterization of two sympatric Dorylus species on Borneo and their colony conflicts. Insectes Sociaux 50 (2): 139–147.

Berghoff, S. M., A. Weissflog, K. E. Linsenmair, R. Hashim & U. Maschwitz. 2002. Foraging of a hypogaeic army ant: a long neglected majority. Insectes Sociaux 49 (2): 133–141.

[B03] Bingham, C. T. 1903. The Fauna of British India, including Ceylon and Burma. Hymenoptera vol. 2. Ants and Cuckoo-Wasps. Taylor and Francis: London.

Brady, S. G. 2003. Evolution of the army ant syndrome: the origin and long-term evolutionary stasis of a complex of behavioral and reproductive adaptations. Proceedings of the National Academy of Sciences 100 (11): 6575–6579.

Brady, S. G., & P. S. Ward. 2005. Morphological phylogeny of army ants and other dorylomorphs (Hymenoptera: Formicidae). Systematic Entomology 30: 593–618.

[E89] Emery, C. 1889. Viaggio di Leonardo Fea in Birmania e regioni vicine. XX.—Formiche di Birmania e del Tenasserim raccolte da Leonardo Fea (1885–87). Annali del Museo Civico di Storia Naturale di Genova, Serie 2^a, 7: 485–520.

[E12] Evenhuis, N. L. 2012. Publication and dating of the Exploration Scientifique de l’Algérie: Histoire Naturelle des Animaux Articulés (1846–1849) by Pierre Hippolyte Lucas. Zootaxa 3448: 1–61.

[G20] Goldfuss, G. A. 1820. Handbuch der Naturgeschichte vol. 3. Handbuch der Zoologie pt 1. Johann Leonhard Schrag: Nürnberg.

Gotwald, W. H., Jr. 1982. Army ants. In: Hermann, H. R. (ed) Social Insects vol. 4 pp. 157–254. Academic Press: New York.

[H02] Hong Y. 2002. Amber Insect of China. Beijing Scientific and Technological Publishing House.

[I92] Imes, R. 1992. The Practical Entomologist. Aurum Press: London.

[L49] Lucas, H. 1849. Exploration Scientifique de l’Algérie pendant les années 1840, 1841, 1842 publiée par ordre du gouvernement et avec le concours d’une commission académique. Sciences physiques. Zoologie. II. Histoire naturelle des animaux articulés. Troisième partie. Insectes (suite). Imprimerie Nationale: Paris.

[Z01] Zhou, S. 2001. Ants of Guangxi. Guangxi Normal University Press: Guilin (China).