Monomorium afrum is the largest species in its genus found in the Gambia so far. It is very widespread in Africa, being from from South Africa to Sudan. The closest it has been found to The Gambia is the Ivory Coast, so this find represents an extension of its known range westwards.
It was found only once as a worker, running on the ground in the coastal resort of Kololi. Other species of Monomorium are much more common in The Gambia.
The sculpturation of the head and mesosoma is interesting, as it makes even in focus pictures appear out of focus. This reticulate-punctate sculpturation can be seen on the close up photograph of the head.
On the same day and in the same location as the M. afrum worker was found I collected a lone dealate queen. Queens away from colonies present problems for identification, as most have not been described. For this reason, I left the Monomorium queens that I collected until last (within the Monomorium at least). Fortunately, the queen of M. afrum has been described by Arnold (1926).
What's more, it is incredibly distinctive. In fact, when I first looked at it, I assumed that it was a Tetramorium, at least until I realised that it had the single unpaired seta projecting from the midpoint of the anterior clypeus typical of Monomorium and lacked the raised clypeal ridges of Tetramorium. It is the most peculiar Monomoroium that I have ever encountered.
My specimen seems to differ slightly from those described by Arnold. He describes the second and third abdominal segments of M. afrum queens being anteriorly smooth and shining. This presents a problem, as the second and third abdominal segment is the petiole and postpetiole. However, authors are sometimes unclear about this, so Arnold could have been referring to the second and third gastral segments (abdominal segments five and six). Despite this uncertainty, I can find no smooth and shining areas on any part of my specimen.
The queen of M. afrum is obviously very different from the worker. Arnold states that had he collected a specimen without the context of workers and other queens he would have considered it to be the type of an entirely new genus, and it's easy to understand why. Bolton (1987) comments that it shows modifications characteristic of socially parasitic species of Monomorium and may prove to be a temporary social parasite.