Tuesday, 19 January 2016

Baffling Blue-tails

Going through my photos from the 'summer' now and tying any loose ends up. Which brings me to this pic of a female form of blue-tailed damselfly with a rusty orange thorax I hadn't seen before and didn't seem to appear in the field guides :

The mystery damsel
A striking individual with a rusty orange thorax and faint antehumeral stripe
Appears not to have a seg 8 tail light of any description
Not a great photo but I was straining over the fence just to get a shot
So needs to be sorted out properly. My Dragon ID 'bible', namely this one (published 1997) . . . .

. . . . states as we know, the female immature starts in two forms, either rufescens, with a pinkish thorax with faint antehumeral stripe and blue (seg. 8) tail-light, or violacea, with violet thorax and black antehumeral stripes and blue (seg. 8) tail-light. Fair enough, seem plenty of both of these forms :

immature form violecea

immature form rufescens

Both of those immature forms are little beauties, shame they have to change really. The development of the violacea can go one of two ways, maturing into either the infuscans form (green thorax with black antehumeral stripe and dull brownish tail light) or the typica form which resembles the male (bright blue thorax with black stripe, blue tail light). Here's a few I took earlier :

The only photo I can find of a mature typical form female
is this mating pair, where you can see quite clearly the female (left)
is exactly the same shade of bright blue as the male (right)

This photo shows an individual half-way between the immature violet form
and the mature bright blue form, she will eventually have a thorax
the same colour as the tail light.

This individual is again at a stage of development between
violet and mature but note the dulling of the tail light, I reckon
this individual will morph into the infuscans form
How to age the colours changes during development is something I don't know, but I have also encountered different shades of the infuscans form, it isn't easy but I'll have a go :

This dull form may be an early stage of development from the violacea

This individual seems to be getting close to the green of the
mature female infuscans shown in the books

This green individual photographed in shade is nearer the mark
Note the combination of the well defined black antehumeral stripe
and brownish tail-light, diagnostic of the infuscans form in whichever
stage of maturity 
It's the black antehumeral stripe which certainly means my mystery female is not a development of the violocea form as it has faint antehumeral stripes just slightly darker than the rest of the thorax, which leaves the rufescans form to look at.   
This form on the other hand, matures into only one type, the infuscans-obsoleta, with yellowish brown thorax, faint antehumeral stripe, and yellow-brown tail-light.
Or does it? because now there is a bit of a twist.

Fast forward to my European field guide  (published 2006) :

Which uses the same wonderful Lewington illustrations but the text by Odonata leading expert Klaas Douwe B Dijkstra refers to the different female forms as simply A, B, and C (where A=Typica, B=infuscans and C=infuscans-obsoleta, noting that B and C type females can become very dark when over-mature (a clue to my mystery girl perhaps).
Let's have a look :

This individual is on the turn from a rufescans, still showing
the pink thorax but the tail-light browning off nicely

This (apologies for crap photo) is an ovipositing female of the
mature infuscans-obsoleta form (type C) according to the books

Mystery girl is obviously of this form as told by the lack of
antehumeral stripes. The bolder colouring I conclude is due to ageing,
this must be quite an old individual and was indeed quite late in the season
when I found her. The tail light I presume also darkens with age which
is why it appears to be uniform with the rest of the abdomen.   

I think I've nailed it for definite, but the story keeps on throwing more confusion at me. Fast forward again to my latest acquisition (published 2014) :

This guide contains photographs of all the colour forms but the mature form of rufescens is referred to as rufescens-obsoleta rather than infuscans-obsoleta and shows a more orangey form of mature individual, now I'm more confused than ever, what's going on?
I look at the BDS website for clarity. Lo and behold, the home page has an item on the blue-tailed damsel which again refers to a rufescans-obsoleta form, so I look at the species account pages and find no mention of rufescans-obsoleta, but photographs labelled infuscans-obsoleta. (Hope you're keeping up with this :-O)
I then decide to use good old Google and I came across a scientific paper from 1999 which was aiming to standardize the names of the female colour forms and letting it be known that the form infuscans-obsoleta was to be known as rufescans-obsoleta, as simple as that :-)

So it seems that change has come into being and it looks like the BDS is complying with this but has yet to change the names on the species account.

So I conclude that my mystery individual will go down in the records as an over-mature form of rufescens-obsoleta (and certainly not as a boring old type-C female).
And what of infuscans-obsoleta? you may still see it being bandied about but I'm afraid it is now most definitely obsoleter ;-)

PS - By the way, the typica form can also be referred to as the andromorph form, just in case we need a bit more confusion.

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