Saturday, 21 June 2008

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!

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