Crematogaster sp: Cocktail ants

MYRMICINAE

The Genus Crematogaster (Cocktail ants)

One of the largest ant genera on the planet, with 500+ recorded species and about 300 subspecies, the Cocktail ants occur on every continent except Antarctica, and on most islands with temperate climates. They are always immediately recognizable for their ability to ‘cock their tails’, ie raise their gasters almost over their heads when alarmed. They achieve this because the postpetiole is uniquely attached to the upper or dorsal surface of the first gaster segment, allowing the ant to turn her gaster forward over her mesosoma. If a nest is disturbed most species will swarm out with gasters erect. More difficult to see is that at the same time their tiny stinger is extruded. Crematogaster do not actually sting their prey or enemies, but instead exude volatile defensive (and offensive!) chemicals at enemies and any perceived threats.

Many Crematogaster are of some economic importance, as distributors and ‘farmers’ of plant pests such as aphids, mealybugs, etc. Many species build elaborate ‘carton’ nests in bushes and trees, constructed of chewed vegetable matter and resembling brown papier maché, and they use the same material to build sun and rain shelters over their aphid farms.

Typical ‘carton’ nest built by Crematogaster ants; this one in the hedgerows near Albertinia

Most species are omnivores, very active by both day and night, and generally able to resist attack by other ants: their victorious resistance to swarms of the nomadic army and driver ants (Dorylus species) is well-documented. Despite that, in our region they are curiously unable to defend themselves against Anoplolepis species, the latter being able to reduce a carton nest of tens of thousands of Crematogaster to a complete ruin in a matter of minutes – a gruesome sight. Presumably for this reason the two genera are always mutually exclusive in any habitat.
The building of paper nests in bushes in fire-adapted vegetation such as fynbos or grassveld may seem counter-intuitive, but carton-building species also nest in the ground, under stones, etc., and frequently abandon their carton nests in hot, dry summer weather. It can be reasonably assumed that hundreds of thousands of years of adaptation to the threat of fire has left these ants well-able to care for themselves when the first whiff of smoke is on the wind. In any event they are always present in burnt areas within a few weeks of a veld fire.
Apart from the carton nests already mentioned, and tunnels built under stones, etc in the ground, some Cocktail ant species also nest in the large thorns or galls of many African trees, in rotten wood, under bark, or in the abandoned burrows of other insects. Nevertheless all species are largely arboreal, most run in trails, and many have well-developed tarsal claws that give them a better-than-average ability to hang on to vertical or even upside-down surfaces.
There are 48 recorded species and 44 subspecies in our region, divided between two subgenera: the species described here are all subgenus Crematogaster unless specifically labelled as Orthocrema. Many species are extremely difficult to tell apart; the entire genus is in fact in a fair old taxonomic mess. This is not least because of the frequent individual variation in size, colour, length of spines, distribution of hairs etc etc not only in the same species, but even in the same nest. Don’t be too concerned if you cannot identify a Crematogaster very precisely – in most instances nor can the ‘experts’. I’ve tried to get the species I have included here as correct as possible, but I am quite sure that there might be a few bloopers, too. Forgive me.
Diagnostic features:  Post-petiole a node, often with a median longitudinal groove, attached to the dorsal surface of the first gastral segment. Head usually broader than long. Mandibles four or five dentate. Antennae 10- or 11-segmented. Eyes present, usually well developed, set at or just behind the midline of head. Promesonotal suture usually a weak impression. Metanotal groove impressed, often deeply so. Propodeum usually armed with a pair of spines or teeth, sometimes reduced to tubercles or absent. Petiole dorsoventrally flattened, without a node, the dorsal surface weakly convex to concave.  Gaster heart-shaped or triangular in dorsal view. Sting spatulate. Tarsal claws simple, often large.

Crematogaster peringueyi (Emery, 1895):
Black cocktail ant

Crematogaster melanogaster (Emery, 1895):
Brown cocktail ant

Crematogaster castanea (F. Smith, 1858):
Red cocktail ant

Black cocktails are commonly found in strandveld and moister fynbos, while their reddish-brown black-gastered cousins (C. melanogaster) prefer more arid habitats. The Red cocktail ant (C. castanea) prefers forested or moister habitats. Note that C. castanea are massively varied, with several subspecies and more ‘varieties’ than we can count.

Size: Some confusion exists around the size of the ants. Some of the species are quite variable; C. peringueyi can vary [but not in the same nest, please note – these variations are in separate colonies] from 3.5 to 5 mm in length.

All three species are diurnal and swarm aggressively when disturbed. Their bite can be painful, but their mandibles are too small and weak to draw blood. Despite this none of these species is able to resist invasive ants, especially Argentine ants (Linepithema humile) and, interestingly, they never seem to share a habitat with indigenous Pugnacious ants (Anoplolepis sp.), and are always fiercely antagonistic towards them.

Some useful photos of C. peringueyi can be found at these links:

www.ispotnature.org/node/600786

www.ispotnature.org/node/524985

www.ispotnature.org/node/641417

www.ispotnature.org/node/695810

www.ispotnature.org/node/707932

A couple of good photos of C. castanea are at these:

www.ispotnature.org/node/587881

www.ispotnature.org/node/598371

Crematogaster transvaalensis (Forel, 1894):
Small black cocktail ant

This is the only Southern African Crematogaster that belongs to the sub-genus Orthocrema. Based on characteristics not visible to the naked eye, the classification essentially places this small species amongst the most ‘primitive’ of the genus. Usually found under stones in subterranean nests, the species does not build the carton nests associated with the genus.

The workers are all the same size and measure about 2.5 to 2.8 mm; the colonies are small. The bodies are black with limited pilosity, with brown to reddish mandibles and brown legs.

The name means ‘from the Transvaal’, a complete misnomer as, although never common, the ants are widespread from the Western Cape fynbos to Limpopo.


Crematogaster orobia (Santschi, 1919)
Matroosberg cocktail ant

A small species (workers from 2.6 to 3.2 mm), the ants are black with dark brown mandibles, clypeus and tarsi [lower legs]. Fairly hairless; gaster smooth and shiny. At a microscopic level the petiole joint is more complex than in C. transvaalensis, but otherwise C. orobia is hard to tell apart from its distant cousin. However, the ant is rare and has only been found in the Western Cape at altitudes above 1500 m, so if you find black Crematogaster under stones anywhere above the Western Cape snowline they will with 99% certainty be C. orobia, not C. transvaalensis.



Survive invasive species:
Linepithema humile       : Argentine ants       : No
Pheidole megacephala  : House ants            : unknown
Lepisiota capensis         : Small Black ants    : unknown