gorganus or britannicus?

[Essex Bertie]

Hi All,

We have had a second Swallowtail sighting in Essex, this time on the coast. There are good photos on the branch website: http://www.cambs-essex-butterflies.org. ... s_2012.php (bottom 2 rows)

At first sight, it seems the darker britannicus ssp. but there are other indications that it could be ssp. gorganus. i.e less tapered sub-margin on the upper forewing and more elongated cells adjacent. Also on the upper hind-wing, the darker medial vein. But I don't know the distinguishing features.

Either way, it seems worryingly fresh.

Can anyone help please?

Rob Smith

[David M]

That last one definitely looks like a britannicus. What happens in the event of a pairing between a gorganus and a britannicus? Does the resultant adult resemble the British species whilst also retaining its continental dispersal habits?

[Jack Harrison]

I would not have thought that without very close study and/or exceptional photos, it is not possible to make any deductions from the pictures seen so far

However, the location coupled with the (reportedly - and two attempt by me have failed) poor year for British Swallowtails strongly suggests Continental form gorganus.

Jack

[Padfield]

That last Essex swallowtail certainly looks like gorganus to me. My experience of the British subspecies is too limited to be aware of the limits of its variation but for the parameters I am aware of the butterfly is within the limits of gorganus and wrong for britannicus. Even if I saw that individual on the Norfolk Broads I think I would question its provenance.

Guy

[MikeOxon]

This seems to be tricky and, so far, we have opinions going either way. For once, there seems to be little information on the species pages of this website, although there are several excellent photos of bred specimens of both sub-species by Peter Eeles.

According to Heart of England Butterflies: "Gorganus has larger blue spots than our own native swallowtail and its black markings are perhaps less dense and compact."

I have taken the liberty of placing two of Peter's photos, from this site, side-by-side (and adjusting the orientations to be similar), as shown below:

both photos by Peter Eeles from this site

I believe it would be very helpful if someone could highlight the significant differences between these two photos and,in particular, perhaps Padfield could elaborate on his statement that "for the parameters I am aware of the butterfly is within the limits of gorganus and wrong for britannicus".

Mike

[David M]

For me it's the greater area of yellow in the lower central wing area that first hits me, followed by the elongated shape of the upper wings. I've certainly never seen a living gorganus which could even remotely be mistaken for a britannicus or vice versa, but there have been a couple of images lately that have deceived me so, like most, I tend to rely on the instincts of Guy, Roger, Pete, etc.

[Padfield]

Apologies for lack of clarity. I often post in haste (though seldom have the leisure to repent) for one reason or another - today it was because I'm looking after my parents on their Swiss holiday.

Firstly, of course, although britannicus represents a small, isolated population, gorganus covers a huge geographical area with great variation over its range. Some forms look closer to britannicus than others, though of course the only relevant forms for this discussion are those in sufficient proximity to the UK to have a chance of flying over.

The three 'measurable' features I look at (which are not really independent so should count as three aspects of one feature) are:

Constancy or otherwise of the width of the dark submarginal band on the forewing (in britannicus it tapers conspicuously towards the apex, or rather, it gets substantially broader towards the inner margin); ratio of yellow space to dark band in the region of s.2 (a little over 1:1 in britannicus, much more than 1:1 in gorganus); distance of dark end-of-cell mark on hindwing from the broad submarginal band (usually, but not always, rather close in britannicus - sometimes touching; usually rather distant in gorganus).

Russell Neave's photos show a ratio of yellow : black of about 3 : 2 - substantially more than usual in britannicus. The submarginal band is barely tapered - it appears parallel-sided apart from at the apex. The mark at the end of the hindwing cell is sufficiently far away from the dark submarginal band for gorganus, though not definitively too far for britannicus.

As I say, I have very little experience of britannicus in the flesh and will happily bow to greater wisdom.

Guy

[Jack Harrison]

Is there a Friesland (northern Netherlands) equivalent fenny race to our britannicus? I ask because it might have evolved when the North Sea was dry land but at the end of the last Ice Age, the populations became isolated.

If there is a Dutch fenland race, then conceivably vagrants of this race might occasionally reach eastern England.

Jack

[Cotswold Cockney]

To my eyes, those two examples appear to be one of each sex, which in my experience of both those from the Broads and Mainland Europe will further confuse the issue. Worn or faded wild examples will further cloud things from the 'sub' species viewpoint.

[Padfield]

To my eyes, those two examples appear to be one of each sex, which in my experience of both those from the Broads and Mainland Europe will further confuse the issue. Worn or faded wild examples will further cloud things from the 'sub' species viewpoint.

That's very true. I annotated that picture because Mike had already posted it but I didn't draw my original conclusions from it - I drew them from my books when I was a little boy, dreaming about things I had never seen... (I saw my first swallowtails on a sales trip with my Dad around Norfolk when I was about 12 - and have never seen a British one since).

I'd be interested in your experienced opinion on the butterfly in question, CC. To me it looks very strongly like a continental swallowtail. It also appears to be a male, so its relative openness is even more significant I would have thought.

Guy

[MikeOxon]

To my eyes, those two examples appear to be one of each sex, which in my experience of both those from the Broads and Mainland Europe will further confuse the issue.

Apologies if I inadvertently confused the issue - purely due to my ignorance! I was trying to tease out more information about the differences between these two sub-species.

Apologies for lack of clarity. I often post in haste (though seldom have the leisure to repent) for one reason or another - today it was because I'm looking after my parents on their Swiss holiday.

Thank you for taking the trouble to expand on your previous response. May I suggest that your observations could usefully be added to the species pages on this website.

Mike

[Cotswold Cockney]

Although I have not bred this species for what, over forty years now, I have a small series of bred specimens from Broads Stock, Mainland Europe and the Japanese race hippocrates, the latter which on average is slightly larger than the others. So the two extremes of this specie's worldwide range are represented.

I have looked at these and many other specimens and despite close scrutiny, I find it very difficult indeed to make any clear areas of consistent variation between the 'sub' species. I have seen specimens from the Continent which resemble those from the Broads and vice versa. Thus I simply cannot be definitive in response to the OP's request.
..

[JKT]

Firstly, of course, although britannicus represents a small, isolated population, gorganus covers a huge geographical area with great variation over its range. Some forms look closer to britannicus than others, though of course the only relevant forms for this discussion are those in sufficient proximity to the UK to have a chance of flying over.

Hmm. Would Crete be a place for such a similar form? At least to my eyes the sample below seems to pass all three tests.

[Essex Bertie]

Thank you everyone for all of your comments, particularly the detailed explanations from Padfield. I'm intrigued that the subspecies identification is less than straightforward.

As for location, we had another one in Essex in the first week of June, near the border with Hertfordshire, and there was another in Middlesex in the middle of the month. There have been no sightings in Norfolk away from the Broads, and none in Suffolk or Kent. This makes me a bit suspicious, although this last coastal Swallowtail arrived on a day of light winds when many parked car across the county received a light dusting of fine orangey sand. The observer has also pointed out that he found a link to a study of Dutch butterflies looking at densities in preferred habitat, Swallowtail was found only in dry semi natural grassland . . .interesting given this one’s habit of sitting on the path or perching in the grass. It has stayed at these old gravel workings for 3 days at least, so seems happy enough.

Thanks again.

Rob

[NickB]

Since this is in my patch (Cambs & Essex BC) I watched with interest as the reports and pics came in....
My immediate thought was either an immigrant or a release of a captive specimen (as the other odd sightings elsewhere might indicate)...
The debate here has helped to cement that first impression - thanks to Guy et al. for their detailed comments.
Given other circumstantial evidence (the fine Saharan dust that was deposited over our region at that time, indicating a southerly or south-easterly air-flow) and the fact that there have been no other breeding populations recorded outside the Broads, it does seem that it is probably a Continental immigrant.....
But, not having actually seen it, and the pictures themselves being inconclusive, one can never be 100% certain......

N
(Nature has a way of proving us all wrong at times - ask any birder about the unexpected birds that turn-up "in the wrong places"......)

[lee3764]

In my study at home I still have an enlarged print from a slide I took at Hickling Broad in 1994 of 'Britannicus' Swallowtail & comparing it to the photographs on the Cambridgeshire & Essex BC website I would 99% almost certain state that the Essex butterflies are the continental 'Gorganus' race! My experience also dictates that 'Britannicus' usually likes fen/marsh style nectar sources such as Ragged Robin or mearly basking on reed syems/leaves in the fens as mine did when I took my lovely photograph. It is just possible thay are captive reared releases but given more than one location I doubt it. Be happy for a migrant pretty much as rare as say a Queen of Spain Fritillary!! ie: only a very few every few years!!
Hope this helps,
Lee Slaughter (Cornwall).

[NickB]

I enclose an example above of how difficult it is to base judgements on one photograph of an individual out of context.
This specimen is as britannicus as they come, taken at Strumpshaw, but does not have the tapering black band on the fore-wing that Guy high-lighted as an indicator...

[David M]

This specimen is as britannicus as they come, taken at Strumpshaw, but does not have the tapering black band on the fore-wing that Guy high-lighted as an indicator...

No, and in addition, the black marking at the lower end of the hindwing cell spot is much further from the submarginal band than is usual.

[NickB]

Indeed, David!
And yet this is in Dr. George's garden, which he plants specially with flowers to bring Swallowtails to his garden to nectar.
I arrived one morning to find at least 8 or 10 Swallowtails in a feeding frenzy to nectar-up and get on with their serious business of the day.
Not only does he do that, he allows those who venture there to admire, photograph and share his passion for britannicus as a British sub-species.
He was worried that if gorganus gains a foothold in the UK, it may inter-breed with our own ssp.
Does anyone know if there is any evidence to support inter-breeding of ssp's?
Here are two specimens: britannicus from Strumpshaw and gorganus from Switzerland at 1800m on a trip with Guy and Roger and Paul.

[Jack Harrison]

Possible differences from looking at many photos.

britannicus CAN have has convex outer margins of forewings although these are often be more or less straight.

gorganus never has convex outer margins, and are sometimes quite markedly concave.

Just an idea.

Jack