Monomorium osiridis had only been collected twice, in Kenya, before I picked up these specimens in The Gambia. It seems quite surprising to find M. osiridis on the other side of the continent, but I don't know what else these specimens could be.
M. osiridis and the southern African species Monomorium zulu and Monomorium rabirium are separated from other Monomorium by having 12-segmented antennae, smooth mandibles, conspicuous eyes, sculptured propodeum and no standing hairs on the dorsal mesosoma and gaster. M. osiridis differs from the other two by having a sculptured head, except for a median strip (visible in the bottom photograph).
I only collected M. osiridis once, on the ground at Madiyana Camp on Jinack Island. They were collected only at night, strongly indicating that they are entirely nocturnal, as may be the case with Monomorium dictator. This could be another instance of a species being under-recorded partly because it is nocturnal.
These have been compared with specimens checked by Bolton at the Natural History Museum, London, and are confirmed as M. osiridis.