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.