Tuesday, 11 December 2007

Plagiolepis mediorufa

Plagiolepis mediorufa had only been recorded from the Congo and the Central African Republic, generally from myrmecophilous plants. My Gambian specimens were collected from plants, trees and a palm frond in Bijilo Forest Park, Kololi and Abuko National Park, Lamin, but nothing about the plants indicated that they were adapted to supporting ants. The Gambia also represents a significant range extension north and westwards.

The trouble is that I suspect there may be a few, as yet undescribed, small yellow and morphologically similar species of Plagiolepis in Africa as a whole. I have placed my specimens in P. mediorufa because it is currently the best fit, but would be interested to see where they are placed if the genus is ever reviewed.

The specimen that I photographed was sadly rather battered. Plagiolepis often tend to be rather soft and fragile, and so shrivel when they dry. Tapinoma does the same thing. (I think the funiculus of the antenna was missing when I collected it.) It was still the best of the three specimens that I had mounted.

Monday, 10 December 2007

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.

Sunday, 9 December 2007

Death of a monarch

It's quite sad really. The queen of my Leptothorax acervorum colony has died. She was rather unceremoniously dumped at the far end of the foraging arena by the workers. It is possible that they have another queen. I can't check, as I cut the colony out in a piece of turf and, because they have been doing so well in there, left them alone.

I collected the colony earlier this year at Chevin Forest Park near Otley, West Yorkshire. In 2004 I collected a single dealate queen of Formica picea there, well outside of its established range. Despite frequent visits, no more F. picea have been found there. I went this year with Cedric Collingwood, who has helped me look for F. picea before, as we decided to make one last (unsuccessful) visit. The only good thing to come out of the trip was the L. acervorum colony.

Shortly after I got the L. acervorum home they produced an abundance of alate queens, but no males. Later they went into a period of inactivity when I started to wonder if they had all died, but then became active again a few weeks ago. Now that the queen has gone I'm wondering if the workers will produce any males, if there isn't another queen present.

I'd strongly recommend L. acervorum for anyone interested in keeping ants. They may be small, but they're kind of cute and very easy to look after. All they seem to need is a confined space with reasonable humidity and a steady supply of slightly crushed houseflies, which they hollow out. They will also drink sugar solution, but seem to prefer freshly-killed flies.

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.

Monday, 3 December 2007

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.

Sunday, 2 December 2007

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.