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!