Monday, 12 September 2016

Hoverfly identification: getting started



CHOOSING AN IDENTIFICATION GUIDE TO HOVERFLIES

If you have not got a specialist guide book, the chances are that you are either trying to match photographs to others posted on the internet, such as Steven Falk's comprehensive pages. The big drawback with this approach is that you will probably end up trying to match your photograph using colour patterns. It will work with some of the very distinctive species, but in many cases there are several species within the same genus that look very similar. Rather more unhelpfully, hoverfly patterns can vary between males and females (sexual dimorphism), between spring and summer broods (generational dimorphism) and depending on the temperatures that they developed in larval and puparial stages. Gut contents can also affect the intesity of colouration, and they may also darken with age!

The characters used to separate them are therefore often confined to structures that you will not think of without having gained a more detailed knowledge of hoverfly taxonomy. A guide book is therefore an essential part of the process: it will help you to understand how to arrive at a reliable identification and what many of the pitfalls are.

At the moment, there are two readily available guides to UK Hoverflies, both of which have strengths and weaknesses:

British Hoverflies: an illustrated identification guide
(2nd edition) 2002 by Alan E. Stubbs & Steven J. Falk (revised and updated by Ball S.G., Stubbs A.E., McLean, I.F.G, Morris, R.K.A. & Falk, S.J.. Published by the British Entomological and Natural History Society. Price £20.00 +p&p to members from the BENHS or around £30 +p&p from online retailers.

Strengths: This is the most comprehensive guide to Britain's hoverflies and contains keys to all genera. It also contains a comprehensive array of line drawings of relevant features including detailed illustrations of the genital capsules of Cheilosia and Sphaerophoria. The plates are arranged in taxonomic order and allow readers to get a general feel for some of the range and variation.

Weaknesses: This book was revised in 2002, since when about a dozen species have been added to the British list. A further revision is really needed and we have developed much of the material needed to produce a supplement. Thus, the serious student of hoverflies really needs to use this guide in conjunction with European literature.

Britain's Hoverflies: a field guide
 (2nd Edition) 2015 by Stuart G. Ball & Roger K.A. Morris. Published in the WILDGuides series by Princeton University Press. RRP £24.95 but available online for considerably less.
Strengths: This guide uses photographs throughout and is very richly illustrated with detailed shots of critical features. It was designed as a companion to Stubbs & Falk and contains illustrations of many of the features that are difficult for the novice to understand. It is the 'entry level' guide that will resolve many of the problems that the novice encounters. It also contains the most up-todate checklist for Britain's hoverflies (apart from the UK Diptera checklist on the Dipterists Forum website).

Weaknesses: This guide does not cover the entire British fauna but focusses instead on illustrating all of the genera and those species that are most likely to be encountered by the non-specialist.

In addition, it is worth drawing attention to the most useful European guide:

Hoverflies of Northwest Europe: identification keys to the Syrphidae
(2nd Edition) 2010 by Mark P van Veen. Published by KNNV Uitgeverij.


Strengths: This is the most comprehensive set of keys to the hoverflies of NW Europe and is a must have for those people who take a detailed interest in hoverflies.

Weaknesses: This guide lacks colour plates and although well-illustrated is not suitable for the novice because it depends upon a strong grasp of hoverfly morphology.

There are also several less comprehensive guide books that may be useful to those who have a very limited interest in hoverflies, but if using them be aware that many of the identifications that you may arrive at would be questionable without validation by a specialist.


SOME TIPS ON MAKING YOUR DIAGNOSIS

Many people simply try to match a photograph or a specimen in the field with a picture in the book (i.e. the way many birders work). This approach is unlikely to yield reliable records for many genera as I have noted in my introduction.

You can help to improve your analysis by checking:
  1. Is the flight time right? If you decide upon an animal that flies in April but you have seen in September, the chances are that your determination is wrong!
  2. Is it within geographic range? It is amazing how often people submit records for northern or upland species from southern or coastal locations. With a few noteworthy exceptions (such as Callicera rufa), these are also likely to be wrong.
  3. If the guide book points towards the use of genitalia characters or greatly magnified microscopic features to separate species then the chances of a correct identification are no better than guesswork. 
  4. If the animal is listed as rare or highly localised, the chances are that you have not found it. The vast majority of reliable records are for about 50 species. Start with expecting the commonest species but of course exceptions do happen very occasionally.
  5. Treat your identifications with scepticism.
Over the years I have seen an awful lot of cases where  people have asserted that they were certain about their diagnosis, only to discover that they had not even reached the correct genus! Identification skills grow with time and there is no disgrace in arriving at the wrong diagnosis. Providing you then get help to see what has gone wrong, this forms the central plank of learning: we don't really learn from our successes but mistakes coupled with re-analysis embed skills.

Finally, it is probably worth reflecting that there are relatively few people who make the transition to photographic/live animal identification from the use of a microscope and preserved specimens. They will often be far more cautious about making a firm diagnosis of live animals because they have seen a lot more of the range and variation and are aware of many of the pitfalls that can catch out the unwary.

1 comment:

  1. Great information especially on making your own diagnosis, thank you.
    Steve

    ReplyDelete