When I first attempted to identify this ant I decided that it was Monomorium exiguum, though I wasn't exactly convinced. I was using Bolton (1987), though I find the key fairly vague at times, which isn't helped by the fact that the descriptions aren't exactly consistent and many species lack diagrams.
M. exiguum is the last species in the key - okay, so there aren't that many Afrotropical Monomorium with 11 antennal segments, but even so I feel that if you've got to the end of the key and your not convinced, then chances are you are wrong.
Then I discovered that I had collected specimens that were a perfect match for M. exiguum and very different from this species, so it was clear that I was wrong. I went back to the key and decided that the next best fit was Monomorium dolatu, though I was rather less than convinced - the petiole shape and Cephalic Index appeared to be correct, but there were slight differences in the structure of the clypeus.
My trip to the Natural History Museum, London has thrown the latest spanner in the works, as I've now been able to examine specimens that Bolton identified as M. dolatu closely. It's pretty clear that this isn't M. dolatu, as the petiole shape is not near as conical as on that species and it's generally wrong. I was also able to check and eliminate M. pulchrum, M. taedium, M. fastidium and M. vaguum. I have in my collection and was able to confirm M. rosae, M. exiguum and M. mictilis. The only other species that it could remotely be is M. bequaerti, which was not present in the museum collection, but this should have a postpetiole the same size as the petiole, as in M. pulchrum, which my specimen does not have.
Since I've eliminated all Afrotropical species of Monomorium with 11 antennal segments I'm having to call it 'species A' for the moment, at least until I've checked those species occurring elsewhere. This is the second potentially new species of Monomorium I've encountered in as many months - the first I've already established is definitely new.
M. sp. A was collected once, nesting within an Acacia pod, in Kololi. Because this was within our last few hours in Gambia and I had packed everything to leave, they were collected alive. I had to carry the pod, with the ants, back to the hotel in my hand. This involved dodging the bumsters (notorious in The Gambia) and trying to keep the pod safe whilst I had a chat with a charming young Gambian woman, who invited me to a 'club' and I suspected to be a prostitute. Once I made it back to my room I put the pod and the ants in a bag and in my case, where they remained until I got home.
I kept them alive for a few days before realising all I had were a few workers - not worth the effort of keeping long-term. They were also a pain; they proved to have an incredible ability to walk upside down on a smooth surface and vertically across fluon. This, combined with the fact that they tended to wedge themselves into the small gap between container and lid made them very difficult to contain. Given these recorded habits it seems that this species is well adapted to an arboreal lifestyle.
Incidentally, the droplets of liquid are grease. I will have to do something about this soon, whereupon I may upload some fresh photographs.