Sunday, 31 July 2016

A Thorny Subject

We're in Thorn season, so I thought I'd write a post about my experiences with the potentially tricky August/September differentiation.  In my garden, August Thorn is by far the commonest Thorn, so I have got very used to seeing them and their variations, whilst September is very infrequent.  Below is a photo of some of the 47 August Thorns that arrived at the trap on July 10th 2009:

You can see that some of the outer wings are quite dark and indeed some are as dark as Dusky Thorn, but the cross lines are key.  The outer line always has a double kink at the leading edge where it bends towards the wing tip and then sharply upwards, often ending in a broadened blob.  The inner line also tends to have an almost 90 degree bend upwards as it nears the leading edge as well.  Both the lines tend to start quite well spaced on the trailing edge.  This moth also tends to hold its wings the flattest of the thorns.  The picture below from last night illustrates these points quite well:

For me, September Thorn is very infrequent, so when one appears it tends to stand out.  It tends to hold its wings more vertically.  It is also a much cleaner looking moth than August, which is often veiny, a bit spotty and also often shows dark areas beyond the outer cross line.  The cross lines start much closer together on the trailing edge and they both sweep round in a nice smooth arc.  The bottom line continues in a smooth arc as it joins the leading edge and its direction tends to point towards the wing tip, it usually ends in a diffuse blob.  The upper line also joins the leading edge in a smooth arc and also usually ends in a diffuse blob.  The picture below was from last night, my first September Thorn of the year, and helps to illustrate the point:

 I suppose that Dusky Thorn and the darkest forms of August Thorn could be confused, but the cross lines of Dusky are very different to August.  They start very close together on the trailing edge and continue in a relatively smooth arc to the leading edge, the outer line hitting the leading edge much closer to the trailing corner than August.  They also tend to hold their wings more vertically than August.  I managed to catch 5 Dusky Thorns last night, which is nice to see.  I used to catch in the high teens of this species about 10 years ago, but it really tailed off after that and these are my first Dusky Thorns in the garden since 2011.  The picture below illustrates my point:

A dark form of August Thorn for comparison taken on 11th July 2004.  This one's cross lines do start slightly closer together than some, but note the shape of the lines and how flat it holds its wings:

Just to round off the Thorns, I also caught a Canary-shouldered last night (plus Early and last week Purple, which completes my garden list of Thorns).  It always surprises me that certain moths categorised as common are either never, or rarely seen in my garden.  Canary-shouldered Thorn is one of these species and last night's record represents only the second ever for me.  Not much confusion with this one!

And finally, this rather splendid Beautiful China Mark was a new garden tick.

Adam Bassett
Marlow Bottom


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