|The Ants of Africa
Chapter 1 - Fascinating ants
Much has been written in recent years about ants but I would like to convey something of my own fascination with these widespread and, in the tropics, ubiquitous insects.
Although most ants appear to be brown or black, one of the most notable variations is bicolouration. Typical of this is Monomorium bicolor, with an orange head and thorax but a dark gaster; a reverse pattern is shown by Crematogaster clariventris, with a distinctive orange gaster. More vivid is the gold and silver pubescence and long hairs of some e.g. Polyrhachis militaris, which is made more attractive by their remarkable sculpturation and large spines.
Other remarkable forms are - the slender bodied Tetraponera with large eyes, obvious high visual acuity and rapid evasion; the sculptured, arboreal e.g. Cataulacus guineensis, again with large eyes well able to see attackers but able to roll into a ball with nothing exposed, and, if still attacked, dropping off the tree; and, the furry little "teddy bears", of Meranoplus inermis, Tetramorium brevispinosum (formerly Triglyphothrix, with bizarre divided hairs), and the species which I labelled Acantholepis T²´³ now Lepisiota cacozela .
No-one can fail to be impressed by the overwhelming power of the rivers of Army Ants, genus Dorylus subgenus Anomma, which for me was most vividly illustrated by a column perhaps 20-30 ants wide emerging from a teak plantation and flowing along the path beside my Onipe 1/1 study area for some 200 m before re-entering the secondary forest. Much of the trail was covered with soil and vegetable debris, but open spaces were bridged by the larger ants especially the soldiers. The column was moving and continuous when I arrived to check the cocoa plots and was no different some two hours or more later when I came back to the path. When they are not migrating, one learns of their activities by sudden sharp pains to the nether regions. For me, working in cocoa meant looking upwards, so blundering into a Doryline foraging area not infrequently resulted in a pressing desire to dash some distance, down trousers (I gave up shorts in tropical field work sometime before) and rapidly remove all the ants which were biting with their needle-like mandibles.
The well-developed stings of ponerines and myrmicines are rarely experienced but once, when climbing a step ladder in the canopy, I brushed my face against a leaf with a Tetramorium aculeatum nest, resulting in a combination of pain and numbness in my right cheek for several minutes. The attack pose of Crematogaster species is to turn their tails over their head. In contrast, their cousins the Myrmicaria always keep their tails held tucked under the hind legs - one wonders, being soil-dwelling, do they expect an attack from below? Other posers are the Odontomachus with their mandibles outspread like the moustaches of RAF pilots from World War II, but able to snap the moustache shut against the substrate and propel themselves backwards.
Well-known are the Oecophylla, or Weaver Ants, which use larval silk to bind live leaves into a ball to form their nest. They are very sharp-eyed watching anything that approaches their territory and confronting the intruder with their tail above their head ready to spray the defensive formic acid.
Polymorphism is at its most extreme in Dorylusfrom the small (3-4 mm) relatively inoffensive workers through several castes to the large and very vicious soldiers (10-12 mm), with needle-like mandibles, e.g. Dorylus wilverthi. Among the myrmicines examples include Atopomyrmex species, with the largest morph being bulkier and much more coarsely sculptured, and Crematogaster (Atopogyne) species, for instance Crematogaster africana, with a range of sizes but not of shape. Then there are the dramatic soldiers of many Pheidole species, with almost absurd big-heads; the overall size being up to twice that of the workers but the heads perhaps three times bigger and bulkier. Formicines display no more than dimorphism, such as Camponotus species with large headed majors, perhaps one and a half times bigger than the minors. Also dimorphic but with the smaller caste rarely seen away from the nests, is Oecophylla longinoda.
The ability of ants to build nests and other structures also reveals remarkable variations in form and sites. The latter range from inside little dead twig ends, like the apparently highly successful tiny (1.3 mm) Plagiolepis brunni which despite, or perhaps because of, its small size, could be found on nearly all the cocoa farms in Nigeria and on some 8% of the trees; to the massive structures of carton, the millions of years old forerunner of our chipboard, built high on forest trees by Crematogaster africana and Crematogaster depressa. Silk is used to bind leaves by Oecophylla longinoda and to bind various vegetable fragments by Polyrhachis species. Those ants which live in the soil reveal little of their nest structure but the tents built over Homopteran "cattle" by Myrmicaria fumata show strong cementing of the soil particles, this is revealed also in their semi-subterranean trails from the nest in open ground to the trees. Ponerines clearly move soil in large crumbs, seen in the pyramidal nest entrances of Pachycondyla tarsata and Pachycondyla analis, and the crude tents built by Odontomachus troglodytes. Also revealed by the nest entrances of Pachycondyla tarsata is their diet of insects and other arthropods, seen as skeletal remains. Food remains appear also in the debris nests in tree crevices of some semi-arboreal species of Pheidole.
Diet related modifications abound. The tent-builders clearly use Homopterans (aphids, mealybugs, hard scales, psyllids, etc.) as sources of sugar. General carnivores such as many ponerines have simple but powerful mandibles. Many species, however, have elongated mouthparts used for seizing and manipulating smaller soft-bodied prey, such as the Collembolans (springtails) and Oriobatid mites known to be the food of the Strumigenys species. Others with long mandibles are Leptogenys, Plectroctena and Psalidomyrmex.
Ants are well known for their well-developed systems of communication. Most important among those systems is the use of chemicals and a useful summary of the state of knowledge in the early 1980s was provided by Bradshaw & Howse (1984). West African ants included in their review are Camponotus sericeus, Myrmicaria natalensis (as Myrmicaria eumenoides) and Myrmicaria striata, Odontomachus troglodytes, Oecophylla longinoda, Pachycondyla analis (as Megaponera foetens), Pachycondyla soror (as Bothroponera soror), and Pachycondyla tarsata (as Paltothyreus tarsatus).
©1998-2000, 2001, 2002, 2003, 2006, 2008, 2012 - Brian
Taylor CBiol FSB FRES
11, Grazingfield, Wilford, Nottingham, NG11 7FN, U.K.