At the time, I sent photographs to Brian Taylor, who replied, 'Why do you not think this is Camponotus vestitus?' Perhaps I should have recognised that his far greater experience was likely to lead to a correct identification, but I like to learn from my own mistakes.
C. cosmicus was one of the species that I checked when I visited the London Natural History Museum, as the holotype is there. C. cosmicus is a large and fairly stocky ant, even the minors, unlike my specimens. I was also able to check C. vestitus subsp. pectitus, which has all the right features (divergent pubescence on the gaster, same number of setae on the mesosoma, same propodeum shape) and differs only slightly in the colour of the head.
Arnold (1924) stated that Smith's (1858) description of C. cosmicus is useless, and it appears as though he is right. However, none of the authors who have written about C. cosmicus picked up on what I believe is probably the most significant characteristic, as all specimens in the museum had many erect setae on the ventral surface of the head. I can also add that C. cosmicus is predominantly very dark red (not black), with the propodeum rounding smoothly from the metanotal groove, less divergent pubescence on the gaster and a greater number of setae on the mesosoma.
So, maybe it is C. vestitus subsp. pectitus. It gets complicated because Bolton (1995) states that Santschi (1926) suggested that C. vestitus subsp. pectitus and C. vestitus subsp. intuens are the same, but at no point do they appear to have been properly synonymised. Reading Santschi's statement, it seems pretty clear that he has no doubt, so I can only assume that Bolton did not synonymise it because the description of C. vestitus subsp. pectitus post dates this statement! Forel's (1909) description of C. vestitus subsp. intuens is just as ambiguous as the descriptions of C. cosmicus and C. vestitus subsp. pectitus.
Update Nov 2008: My specimens have now been compared with the type specimens from the Naturhistorisches Museum Basel. This allowed me to confirm that they are very clearly C. vestitus subsp. intuens - a completely identical match. Possibly the colour of the head is the key feature in discriminating these subspecies.
I also showed my specimens to Cedric Collingwood, who claimed them to be C. jizani. This is a species from the Middle East, though he admitted that the only reason he named it thus was because he didn't know what else to call it! Possibly, C. jizani is a junior synonym of C. vestitus subsp. intuens or C. vestitus.
This species was collected a number of times only in the grounds of the hotel we were staying at in Kololi, Gambia. Here it could be found nesting in turret nests, so named because there were steep 'craters' of sand piled around the nest entrances, up to about an inch high. These are described by Arnold (op. cit.) for C. vestitus subsp. pectitus, which he states is rare and found nesting in sandy soils. He then describes,
'nest-entrances[s] surrounded by a circular, high and sharp-edged crater... This form of crater is distinctive of this insect.'