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Most users ever online was 248 on Wed Dec 12, 2007 11:53 pm


Melangyna species ID Please
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Roger K.A. Morris



Joined: 06 Nov 2005
Posts: 1583

PostPosted: Mon Sep 15, 2008 9:20 pm    Post subject: Reply with quote

Hello Rob

I understand the dilemma and hope you will take the following as that of someone who has done the most they can to assist ID by photos. I'm not trying to be awkward or a miserable old bugger (even though many would say that I am!).

If there was a practical approach to ID of specimens by photos it would have been found by now. I don't know how many times the keys have been modified to take account of complexities of dealing with highly variable specimens. This is the problem - the ID from picture brigade don't spend their time puzzling over awkward specimens under the microscope. This may sound provocative but the sad fact is that we always have problems if we cannot check specimens - e.g. can you ID Cheilosia ranunculi by its geitalia from a photo? Likewise Sphaerophoria, Pipiza and numerous others are simply un-doable without specimens.

I know this is upsetting, but having spent numerous weekends running training courses to get people close to accurate ID I think I have a handle on the problem. It is quite common for me to check my ID with others and for them to check with me. I frequently find myself getting challenged that I was wrong over the ID of a Syrphus because the colours of the photo were the same as those in the illustration in Stubbs & Falk (despite the fact that colour is not a determining factor and microtrichia are far more important. Alternatively I have been challenged on why photos purporting to be Helophilus groenlandicus from southern England were not this species (not checking that this is a v. rare northern sp.). If people used the keys they would understand! We don't make a fuss just because we want to anihilate hoverflies! I am seriously worried where websites publish records of species such as Eristalis cryptarum from photos that are quite obviously not that species (e.g. from the Midlands when the only locations are from soutrhern Dartmoor and many absolute experts have spent weeks of searching to find the beast. This has very serious implications for efforts to conserve inverts and others and can be highly deleterious to biological recording.

Maybe this is me getting too engaged - perhaps I should jack it in and let people create maps according to photos. In ten years time who would know what the score was and how would they have the data to back up acertions about status?

Much depressed yet again.

Regards

Roger
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Roger K.A. Morris



Joined: 06 Nov 2005
Posts: 1583

PostPosted: Mon Sep 15, 2008 9:22 pm    Post subject: Reply with quote

PS Rob

I'm not sure I understand the com,ments about M. umbellatarum. Your photo looks good for this spp. BUT I would still want to see a specimen.

Regards

Roger
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robinsects



Joined: 25 Aug 2008
Posts: 18

PostPosted: Tue Sep 16, 2008 2:20 pm    Post subject: Reply with quote

Nice set of hoverfly photos Nigel

Spot on focusing.

Interesting to see such a range of Cheilosia species.

Regards

Rob
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robinsects



Joined: 25 Aug 2008
Posts: 18

PostPosted: Fri Sep 19, 2008 12:08 am    Post subject: Reply with quote

Hi Roger

Sorry I did not see your reply(s) as I did not realise the comments had overrun onto page 2.

Dont get depressed we are just grinding two different axes - both need sharpening.

My point about using this Melangyna Id. as an example of the value of photos in narrowing down possibilities was that the M. umbellatarum photo shows the female as markedly black and white, in particular having the black shiney thorax described in Stubbs. whereas my photo of the female shows a dull brown thorax more like for instance this photo of M. labiatarum taken by F Kohler shown on the insektenbox website



Regards

Rob
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Roger K.A. Morris



Joined: 06 Nov 2005
Posts: 1583

PostPosted: Sat Sep 20, 2008 9:07 pm    Post subject: Reply with quote

Not sure where my post went (thanks to the person who deleted it) but important to note that we have a definite problem with changes in recorder practice - common species that are easy to ID are on the up, whilst species that are hard to ID are on the decline - my field experience does not corroborate these trends and so I conclude that we have a problem.

In the long term the last thing we want is an alert on a common species just because no-one can ID it! We need more people that will take specimens and will make sure that the data are robust. Conservation policy and management rely on robust data.

Roger
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robinsects



Joined: 25 Aug 2008
Posts: 18

PostPosted: Sun Sep 21, 2008 5:19 am    Post subject: ID Reply with quote

Hi Roger

Your reply was there it just went onto page 2 so it wasnt visible.

Perhaps the answer is to have two different levels of proof for some species, so that sightings could be rated as confirmed or not confirmed according to certain specified criteria. You could map these then using different symbols- and unconfirmed sightings could then be followed up by collecting specimens from the same site - (even if they were supposedly common species) just to make sure they were as reported.
I must admit the tendency is to take into account the reported probability of seeing a species even when the evidence is not conclusive or even in favour of another species which is reportedly much rarer

Regards

Rob
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stuart
Site Admin


Joined: 11 May 2005
Posts: 737
Location: Peterborough, UK

PostPosted: Sun Sep 21, 2008 8:40 pm    Post subject: Rarity vs difficult of identification Reply with quote

Quote:
Accept what you are saying but find it perverse that the rarer a species is (and therefore the more unlikely its sighting), the more pressure there is to kill it to provide evidence of its (past) existence.

There is a basic fallacy in this argument - there isn't a correlation between rarity and ease/difficulty of identification! Some very common species are difficult to identify. For example, three Syrphus species are very common but require very careful examination under magnification to tell apart and there is currently no way to be certain about the extremely common S. vitripennis in the male (can only be distinguished from S. rectus in the female). You cannot do these species beyond genus from photos. On the other hand there are plenty of rare species that can be identified easily. For example, the two most critically endangered species we have in GB - Blera fallax and Hammerschmidtia ferrugunea - are immediately recognisable and a would present no identification problems even from a poor photo.

Quote:
I think there is a role for field photos of difficult-to-identify flies which are subsequently caught and positively identified.

I would agree with that, but the key point here is "caught and positively identified".

The trouble with trying to do things entirely by photos is that it is usually pot luck as to whether you get the right bits in view. For example, the common smaller Eristalis species require examination of the front legs (orange or dark feet), face (central stripe or not), wings (extent of dark clouding and stigma), back legs (colour of hind metatarsus and whether or not it is swollen) - a pretty tall order for a photo! It also means you have to have a very good idea what species is at the time you take the photos so that you know which shots you must get to be able to confirm identification. The frustrating question in the field is do I catch it or try to take its picture first? - usually leading to the beasty flying off when you try to get close so you end up with neither!

I would suggest to anyone who seriously wants to get into photographing hoverflies:
  • Start by collecting a good range specimens
  • Have a go at naming them yourself
  • Get them checked. Roger and I are always willing to put you in touch with somebody or do it ourselves
  • Once you have got some experience and some reliably named specimens to fall back on, then you can concentrate more on the photos
  • If you are pretty sure what it is, then take photos knowing what characters you have got to get in the images
  • If it is something you don't know then try and make sure you get the specimen as as well as the photo - its up to you given the time, place and situation to decide which is the priority - photo or specimen
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