Monomorium dakarense

I previously thought I had collected Monomorium dictator in the Gambia, as that is what this specimen keyed out as. However, I've been doing a lot of work on the salomonis-group recently and I no longer think that I was correct. I now think it is Monomorium dakarense.

It all boils down to couplet 36 of Bolton's (1987) key. I think it would be possible to debate for hours over whether the sculpture on the head of this specimen is 'uniformly densely reticulate-punctate' or 'with a silky, smeared or roughened appearance'. Initially I opted for the former because, well, it is reticulate-punctate, just more finely than the likes of Monomorium bicolor! However, I've since come to realise that this is what Bolton means by the latter, which leads to M. dakarense.

The difference between M. dakarense and M. dictator, other than the ambiguities of cephalic sculpture, seems to be size, number of ommatidia in the longest row and, importantly, scape index (SI). M. dakarense is overall the smaller of the two, with fewer ommatidia and a SI of 95-100, whereas M. dictator has a SI of 107-109. For those not familiar with SI, it is as follows:

Scape length × 100

Head width

The SI in this specimen is 92, putting it closest to the range given for M. dakarense. Given that Bolton only had the three syntype workers to measure it is likely that the true range of the SI includes 92. However, this is not the only difference between this specimen and the holotype, as the other measurements are (quoting the measurements for MJL143, followed by the range given by Bolton, in mm):

     Head length = 0.61 (0.57-0.59)
     Head width = 0.51 (0.44-0.47)
     Cephalic index (ratio of head width to height) = 84 (77-80)
     Scape length = 0.47 (0.44-0.45)
     Pronotal width = 0.31 (0.30-0.31)
     Mesosoma length = 0.72 (0.66-0.70)

Despite the larger size, the specimen is not big enough to be M. dictator and differs in the other features mentioned, so it is most likely M. dakarense.

It still remains a bit of a rediscovery. M. dakarense had been collected once in Senegal in 1914, so it's more likely than M. dictator, which had only been collected in Angola. The only specimens that I collected were found on the ground at Madiyana Camp on Jinack Island, and only at night. Since we stayed at Madiyana Camp for a few days it would be surprising if M. dakarense had been active during the day and I had not collected it then, which indicates pretty strongly that it is entirely nocturnal. Perhaps a combination of its nocturnal habits and its small size have meant that it has not been found again until now?

Monomorium bicolor

Monomorium bicolor is an abundant ant in The Gambia. It had been collected there before in around 1926 in Banjul (formerly Bathurst), and in 2007 in Kololi and on Jinack Island. It is distributed from Angola north to Sudan and from Somalia across to Senegal.

M. bicolor seems to be present in a variety of habitats, as it was found on trees, on the ground in open areas, on buildings and once scavenging from a dead fish, but was only found nesting in the ground.

M. bicolor is very similar to Monomorium dictator, but differs by being generally larger, though also variable in size, and lacking abundant setae on the anterior half of the first gastral tergite. It should have at least two erect setae on the anterior margin of the first gastral tergite, but these were lacking in all Gambian specimens. Five out of the nine specimens examined had distinct pits where the setae should have been, but all lacked setae. Nevertheless, there is no doubt that they are M. bicolor, so the hairs have perhaps rubbed off or Bolton (1987) defined the species too narrowly.

Monomorium osiridis

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.

Monomorium rosae

Monomorium rosae is a widely distributed species with a central African distribution that stretches from Kenya to Senegal. It was collected twice on Jinack Island in The Gambia, once on a tree and once in dry leaf litter at the base of a tree.

Bolton (1987) notes that this species varies in size and pilosity over its range, suggesting that M. rosae may contain more than one species. He distinguishes the species as currently defined as having 11-segmented antennae, dark colour, moderately long scapes and a distinctively shaped postpetiole.

It's interesting how many of the Monomorium now known from The Gambia have 11-segmented antennae. M. dolatu, M. exiguum, M. mictilis and M. rosae all have 11-segmented antennae, out of ten species recorded from The Gambia (40%). This contrasts with 12 species out of 149 listed by Bolton for Africa as a whole (8%). Maybe Monomorium with 11-segmented antennae are more common in this part of west Africa?

These have been compared with specimens checked by Bolton at the Natural History Museum, London, and are confirmed as M. rosae.

Cataglyphis oasium

The size of an ant and how easy it is to identify are not necessarily related. Whilst Monomorium mictile, at just over 1 mm, was relatively easy to identify, I'd been pondering this very large species of Cataglyphis for weeks.

Cataglyphis is a genus that is taxonomically muddled. I've finally concluded that this species is probably Cataglyphis oasium. It was the keys in Santschi (1929) and Radchenko (1998) that eventually provided the most likely identification.

C. oasium was originally described by Santschi (1929), though the name he gave it (Cataglyphis bicolor st. nodus v. oasium) was invalid, so the authority for this species is Menozzi, 1932. Menozzi only gave two lines of text to the species, changing the name to Cataglyphis bicolor var. oasium, without noting that it was a new combination or providing a fresh description, and naming Santschi as the authority. The variety oasium was later given subspecies status and then species status by Radchenko (1997). Wehner, Wehner & Agosti (1994) suggested that C. bicolor subsp. oasium was synonymous with Cataglyphis savignyi, though they didn't actually synonomise the two names. Thus, the valid name is C. oasium. It's very confusing, so it took a little while just to sort out this history.

Information on the distribution of C. oasium is very sparse. Santschi mentions records from Tunisia and Algeria, whilst the most recent record that I'm aware of is Libya in 1931, which is reported by Menozzi. Radchenko (1998) states that it is found in North Africa and the Middle East.

This gives the impression that C. oasium is rare, but this seems unlikely. It is probably under-recorded due to the taxonomic difficulties. It is possible that the species collected by Lenoir and mentioned in The Ants of Africa is also this species. If C. oasium is found in oasis in the Sahara and in the Middle East it seems reasonable that it could also occur in The Gambia and Burkina Faso.

The Gambian specimens were collected from Kololi and Madiyana Camp on Jinack Island, mainly from sandy ground in scrub savannah. In this sort of hot scrub savannah it could be quite common, so it may be abundant further inland, away from the coastal breeze.

Extreme myrmecology: Monomorium mictile

Monomorium mictile has to be the smallest ant I have ever found. It is very inconspicuous.

I started off with four specimens, but misplaced two before I could look at them properly (I still don't know how). I selected one of the two specimens to be mounted and, whilst I was getting everything ready, I brushed a bit of dust off my microscope stage. I knew what I had done as soon as I had done it - the bit of dust was the ant. It was no good looking for it on the floor, as it was gone.

This left me with just one, which had not died in a position that was conducive to mounting and I felt that trying to fix this would be too risky on such a small specimen. This is why the specimen in the photograph is not as well mounted as most others.

Bolton (1987) comments that material of this species is relatively scarce; it's not hard to see why!

M. mictile is widespread in Africa, found from South Africa north to Mali and Sudan. It as collected once in The Gambia from a tree in the coastal scrub on Jinack Island.

Pachycondyla sennaarensis

Pachycondyla sennaarensis is probably one of the most common ant species in the coastal region of The Gambia, as I collected it 10 times. I also got to know it over the week that I was there and had soon started to ignore the many lone workers that I frequently saw.

Remarkably, given how common it is in the area that other myrmecologists were most likely to visit, it hadn't been recorded from The Gambia before. I'd be tempted to suggest that it might have spread into the country since the early 20th century, when the last myrmecologists visited, but it was recorded in Senegal at about that time. Perhaps it's just been more successful than other ants as the area has been developed.

It seemed equally happy around buildings and in natural areas, where it nested directly in the ground, often in cracks in concrete. It was described by Dejean & Lachaud (1994) as being at least partially seed eating, which is extremely unusual for a ponerine ant. My observations agree with this, as the refuse piles around nest entrances often contained seed husks and other vegetable matter.

They also showed the kind of wear on their teeth that other seed eaters, such as Messor, tend to develop on older workers. Rather typically, it proved impossible to get a decent photograph of the one specimen that showed really distinctive wear. I hope that the photos below give some indication of what I mean, despite the fact that they both chose to die chewing on their own legs.

The first one shows little wear:

Whilst the second shows slightly more (note particularly the tooth on the right just above the leg):

Perhaps you'll just have to take my word for it.

I also found a queen. This was washed up on the shore on the part of Jinack Island that is within Senegal, though it probably washed down the river Gambia to get there, as it was already dead. Despite the fact that it was not found in a colony, it has been possible to identify it as P. sennaarensis with some confidence, as none of the defining features are on the mesosoma and so are presumably the same on the queen as the worker.

This species also stings. I discovered this on my first day in The Gambia when, in my rush to get out and explore, I left my forceps and pooter in my case in the hotel. Picking up these with your fingers is perhaps not to be recommended! In terms pain, it wasn't actually too bad initially (I've been stung by worse), perhaps rating as a 2.5 on the Schmitt Sting Pain Index (just above a European honey bee). However, after 20 or so my thumb, in particular, was starting to feel a bit numb, so I decided to stop. For the next 3 days the skin on my thumb and forefinger, which had taken the bulk of the punishment, slowly peeled away.

All good fun.

Nesomyrmex angulatus

Nesomyrmex angulatus is another widespread species, found from South Africa to Saudi Arabia. The closest it has previously been found to The Gambia is Ghana, so this represents another western range extension (along with Cataulacus traegaordhi). The key thing about this species that separates it from other Nesomyrmex is the very smooth profile of the mesosoma, coupled with the yellow colour.

Though I only found it twice, it's probably fairly common in more wooded parts of The Gambia. It tends to be pretty inconspicuous. I collected it from trees in Bijilo Forest Park and on Jinack Island, from coastal forest and scrub savannah respectively.

Tetramorium sericeiventre

Tetramorium sericeiventre is a very widespread species, occurring from South Africa north to Mali. A single worker had been collected in The Gambia in 1926, recorded as subsp. arenarium.

This species is not as common as some ants in The Gambia, but can be found in Kololi and on Jinack Island, where it can be quite conspicuous. It was collected from savannah, where it nested in the ground.

I was lucky enough to retrieve a queen from one of the nests, which is shown in one of the photographs.