Separating Lasius niger and Lasius platythorax workers

By popular request! Well, one person requested it anyway.

Lasius niger and Lasius platythorax are undoubtedly hard to separate. This becomes particularly difficult when you find specimens that seem to be somewhere in between. However, it is always possible to assign them to one species or another, with a bit of work.

First of all, L. niger and L. platythorax are small dark brown Lasius with outstanding hairs on their antennal scapes. In the UK there are no other species that have these characteristics, though in Europe it gets a little more complicated¹.

Some people seem to base identification of the workers of these species on one or two characteristics. However, experience has taught me that it is usually more reliable to use all of the following:

• Density of pubescence on the clypeus
• Shape of propodeum in profile
• Setae length on the mesosoma
• Shape of clypeus in profile

These are all shown in the photomontages. I have, on occasion, also felt the need to use the length of the setae on the underside of the head, but I can usually identify these species reliably without this and the setae are difficult to measure.

Probably the most useful character is the clypeal pubescence. Quite often this can be visually assessed, with practice, though on occasions an entire nest series will have moderate clypeal pubescence and have to be assessed morphometrically. To do this you use a measuring line, as shown below. Measure the length of this line in µm (l) and then count the number of hairs that intersect it (n). Seifert (2007) gives the average distance between the hairs (l/n) for L. niger as <16µm and for L. platythorax as >19µm.

Clypeal pubescence in Lasius niger. Measuring line shown in red.

Clypeal pubescence in Lasius platythorax. Measuring line shown in red.

Whilst you're looking at the clypeus it's worth noting its shape in profile. Typically, L. niger has a rather curved clypeus, whilst in L. platythorax it is comparatively straight. The idea that they differ has been floating around in BWARS for a couple years now, though I must admit I'm one of the last to accept that this is probably a good character. I feel like I've now seen enough and have not been able to dispute it - plus Barry Bolton is apparently using it, so it must be good!

Check the shape of the propodeum in profile. In L. niger the propodeum tends to be a smoothly rounded dome, whilst in L. platythorax it tends to be more conical in shape.

Finally, look at the relative length of the setae on the mesosoma, especially the pronotum. In L. platythorax these setae tend to be distinctly longer. Seifert (2007) gives figures for the length of the longest setae on the pronotum divided by the length of the head as 0.119±0.009 in L. niger and 0.159±0.010 in L. platythorax.

The two species have distinct ecological preferences. L. niger prefers drier habitats and has synanthropic tendencies - this is the species that sometimes invades houses. L. platythorax prefers wetter conditions, and is usually the species found in woodland and wet grasslands.

I've also started noticing that the larvae in L. platythorax seem to be more slender than in L. niger. Of course, this is based wholly on my subjective observations, but might be a good indicative field character.

NB: The photomontages are of British specimens² that show generally typical characteristics. However, both specimens show setae on the propodeum that curve forward, which is not typical. In the Scottish L. platythorax, which were collected in atypical habitat at their most northern known location in the UK, the entire series had curved forward propodeal setae.

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¹ American L. niger are most likely a different species again, as the images I have seen do not have the dense clypeal pubescence of L. niger, but do not appear to be L. platythorax either.
² To be honest, I don't think that them being British matters - if anything I find the characteristics of these species to overlap more in the UK than in mainland Europe. However, I know some British myrmecologists who get funny about such things!

I ♥ morphometrics

Put simply, morphometrics is 'measuring shapes'. In the normal, biological context this refers to the shape of study organisms. It is used by the taxonomist as a way of determining species boundaries.

Not long after I first encountered morphometrics I found myself working on tricky Lasius sp., which meant that I had to try to make sense of Seifert (1992). Seifert seems to be the Master of Morphometrics, in Europe at least, and the measurements and indices he uses are much more complicated than I had encountered up until that point. At first I really hated them. I couldn't understand why identification of Lasius needed to be so complicated - couldn't the species be better described and thus made easier to identify?

Over time morphometrics have grown on me and I've come to really appreciate them. I now even understand most of what Seifert does, though I will admit to being lost by some of the discriminants that he uses!

The most recent and one of the cleverest morphometric tools that I've encountered is from Seifert (2007), for separating the Formica picea/gagatoides/gagates group from the F. fusca/lemani group. I had already decided that the specimen that I was working on was F. fusca and knew that it definitely wasn't from the picea/gagatoides/gagates group, as these are all much shinier. However, in the couplet for separating the two groups I spotted an index that I had not encountered before: sqPDF.

Translating as best I can from the German, sqPDF is the square root of the distance between individual pubescence hairs within the triangle formed by the ocelli. PDF = l/n; where n = number of the pubescence hairs that intersect three transverse measuring lines of overall length l. The position of these measuring lines is shown in red in the picture below.


The thing that I found so intriguing about this is the pubescence hairs are extremely small, so I decided to see if I could make sqPDF work. I measured the length of the three lines combined to be 663µm and counted 68 hairs intersecting the three lines. This gave a PDF of 9.75µm, so the sqPDF was 3.12µm. This is within the range given for F. fusca/lemani (sqPDF 2.4-3.5µm) and outside of the range given for F. picea/gagatoides/gagates (spPDF 3.6-10.8µm), so the calculation worked!

This might seem like a lot of work, but it's not. Morphometrics are indisputable. I know from experience that you can get to the end of a key based on descriptions alone and be less than convinced that you've got the right answer. You then spend hours checking the other 'possibilities' had you taken different routes through the key. Morphometrics eliminate much of the doubt, give you a nice, neat, believable answer and ultimately save time.

If and when I finally write my first key, expect to see lots of numbers in the couplets!

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?

Camponotus vestitus subsp. intuens

I originally identified this as Camponotus cosmicus and wrote a nice long blog defending my decision to call it that. I even provisionally synonomised it with a few other species and subspecies. I was wrong!

At the time, I sent photographs to Brian Taylor, who replied, 'Why do you not think this is Camponotus vestitus?' Perhaps I should have recognised that his far greater experience was likely to lead to a correct identification, but I like to learn from my own mistakes.

C. cosmicus was one of the species that I checked when I visited the London Natural History Museum, as the holotype is there. C. cosmicus is a large and fairly stocky ant, even the minors, unlike my specimens. I was also able to check C. vestitus subsp. pectitus, which has all the right features (divergent pubescence on the gaster, same number of setae on the mesosoma, same propodeum shape) and differs only slightly in the colour of the head.

Arnold (1924) stated that Smith's (1858) description of C. cosmicus is useless, and it appears as though he is right. However, none of the authors who have written about C. cosmicus picked up on what I believe is probably the most significant characteristic, as all specimens in the museum had many erect setae on the ventral surface of the head. I can also add that C. cosmicus is predominantly very dark red (not black), with the propodeum rounding smoothly from the metanotal groove, less divergent pubescence on the gaster and a greater number of setae on the mesosoma.

So, maybe it is C. vestitus subsp. pectitus. It gets complicated because Bolton (1995) states that Santschi (1926) suggested that C. vestitus subsp. pectitus and C. vestitus subsp. intuens are the same, but at no point do they appear to have been properly synonymised. Reading Santschi's statement, it seems pretty clear that he has no doubt, so I can only assume that Bolton did not synonymise it because the description of C. vestitus subsp. pectitus post dates this statement! Forel's (1909) description of C. vestitus subsp. intuens is just as ambiguous as the descriptions of C. cosmicus and C. vestitus subsp. pectitus.

Update Nov 2008: My specimens have now been compared with the type specimens from the Naturhistorisches Museum Basel. This allowed me to confirm that they are very clearly C. vestitus subsp. intuens - a completely identical match. Possibly the colour of the head is the key feature in discriminating these subspecies.

I also showed my specimens to Cedric Collingwood, who claimed them to be C. jizani. This is a species from the Middle East, though he admitted that the only reason he named it thus was because he didn't know what else to call it! Possibly, C. jizani is a junior synonym of C. vestitus subsp. intuens or C. vestitus.

This species was collected a number of times only in the grounds of the hotel we were staying at in Kololi, Gambia. Here it could be found nesting in turret nests, so named because there were steep 'craters' of sand piled around the nest entrances, up to about an inch high. These are described by Arnold (op. cit.) for C. vestitus subsp. pectitus, which he states is rare and found nesting in sandy soils. He then describes,

'nest-entrances[s] surrounded by a circular, high and sharp-edged crater... This form of crater is distinctive of this insect.'

Camponotus olivieri

Camponotus olivieri is a widespread species in Africa, ranging from South Africa north to Sudan and west to Ghana. However, it is made up of a great many subspecies that probably deserve species status, so the situation may be rather more complicated than it seems at first.

Two subspecies of C. olivieri were collected in The Gambia: lemma and delagoensis. They differ quite markedly in pubescence and also, to a lesser extent, morphology. What I think provides some evidence that they deserve separate status is that they were both found at one site.

C. olivieri subsp. lemma was found in two locations: Abuko National Park, Lamin and Bijilo Forest Park, Kololi. In both cases it was found only on the ground. It can be distinguished from other species of C. olivieri by the rust colour at the base of the antennal scapes and the gaster, which is uniformly shiny, sparsely pubescent but still more densely pubescent than the mesosoma and has erect hairs that are thicker than the pubescence.

C. olivieri subsp. delagoensis was found at Abuko National Park, on a tree. It is similar to subsp. lemma, but has denser, yellow pubescence, especially on the gaster.

Both ants have been recorded only rarely, possibly due to the fact that they are regarded as subspecies and so have been more frequently recorded at species level. C. olivieri subsp. lemma had been recorded in Angola, Tanzania and former Zaire. C. olivieri subsp. delagoensis was only known from the type location at Delagoa Bay in Mozambique. This makes these Gambian records an extension of the known range of the species and both subspecies, especially large in the case of subsp. delagoensis.

These have both been compared with the type specimens from the Muséum d'histoire naturelle, Genève and the Naturhistorisches Museum Basel. This allowed me to confirm C. olivieri subsp. delagoenisis.

The syntypes of C. olivieri subsp. lemma are rather variably hairy, so much so that I wondered if Forel had mounted specimens from different colonies on the same pins. The main difference between these and my specimens was that most of the types were much less hairy, though one was similar to mine. It could be that the specimens have lost some of their pilosity. Regardless, I feel I need to do some more work to confirm that there is nothing closer.

Unusual petiole shape in Lasius umbratus

In my last post I mentioned photographs of a queen ant that were posted to the BWARS discussion group. I asked the collector to send the specimen so that it could be identified properly and I've now had the opportunity to look at it.

It is a Lasius umbratus queen, which was correctly guessed by Andrew Jarman (my guess wasn't even in the right genus). The L. umbratus group is difficult, so I worked the specimen through a number of keys, but it was the head measurements in Seifert (1988) that gave the least ambiguous answer.

The petiole, the interesting thing about this specimen, is as unusual as it appeared in the original photographs taken by Claes-Göran Magnusson. So as to give a clearer impression I've taken some images of my own. The petiole in L. umbratus usually is emarginate, but it can be seen from these photographs that the emargination is much deeper, almost to the level of the anterior peduncle.

Oddities

The latter few months of 2007 seemed bring encounters with an unusual number of freak ant specimens, unfortunately none of which were collected by me.

First, an ant with a third eye, a Myrmica as I recall, was brought to the BWARS workshops in September. At least, it appeared to be like a third eye. It was a black area that was partially buried in the cuticle of the head, but where it came to the surface it did have a bobbly appearance like that of a compound eye.

The ant presumably used the eye for seeing into the future.

Then before Christmas I looked properly at a specimen lent to me by Cedric Collingwood some months ago. He knew that I was working on the ants of The Gambia and had a few specimens in his collection. This specimen had been collected by Nicolas Blacker and identified by Cedric as Camponotus vividus, but it was unlike any of the specimens I had collected. I finally got around to identifying all the Camponotus I collected in the Gambia and discovered that I did have C. vividus, so needed to check why Cedric's were different. To my surprise I discovered that the specimen is actually a pseudogyne (between a queen and a worker). It is larger, paler and more matt, with a more developed mesosoma and one pair of barely developed tiny 'wings'. It may be the only pseudogyne of that species in any collection.

Finally, right at the end of 2007, C-G Magnusson posted some photos of an ant for ID to the BWARS discussion group. It was agreed that it looked most like a queen of a Camponotus... or a Lasius, but it had a very unusual shaped petiole, being so deeply emarginate that it had two erect blunt spines there instead.

Tetraponera ambigua

My Tetraponera ambigua specimens have been the subject of some friendly debate, as Brian Taylor and I have been discussing them. I noted before that they have three ocelli, albeit reduced ones, but they have particularly wide postpetioles as well.

I've now conducted a very detailed investigation, including comparing the morphometrics of my specimens with Ward's (2006) measurements, and am convinced that they fall within Ward's broad definition. The ocelli number and postpetiole width (which Ward does not include in his paper) seems to add to the overall variability of this species.

There is probably still a chance that T. ambigua is a species complex, which is hinted at by Ward. Brian Taylor suggested 'Sima ambigua Em. r. erythraea v. occidentalis Stitz 1917' as a possible identity for my specimens, which has the broader postpetiole and similar overall appearance, but is more sparsely hairy and probably lacks ocelli. Nothing so far described seems to be a perfect match for what I've got.

I often find myself thinking that if organisms such as these occurred in Europe they would have been split into dozens of species, like Lasius, Myrmica, Formica, etc. Africa probably needs the attention of more taxonomists.