Wednesday, 2 April 2014

Getting to grips with hoverfly identification

Learning to identify any group of organisms is a challenge that at some stages can seem to be insurmountable. I'm not sure there is a 'right' or 'wrong' way as everybody learns in different ways. What works for some may not work for others; but there are some simple tips that may help.

  • Identification is dependent upon recognition of features that are constant within the species concerned. In the case of hoverflies it needs to be borne in mind that they can be hugely variable in size and colouration in response to temperatures during larval and pupal development as well as sexual dimorphism and brood dimorphism. I know Alan Stubbs spends goodly periods of time checking a range of characters for consistent features when he develops keys - and the message that comes out is that relatively few are reliably the same.
  • This complication means that whilst illustrations are useful, they only form part of the process. Understanding how to recognise the different Tribes and genera is crucial. This is where access to complete keys is very helpful - the keys will provide a series of markers that help to direct observations. It is also important to bear in mind that keys work on the process of elimination; so, the earlier they appear in the key the more genera or species may have this feature in common. In addition, once two different pathways are followed, it is quite possible that a very similar character may be used to identify two very different species. Understanding these relationships is important. This is one critical reason why we have been resistant to producing a guide to all hovers without detailed keys. The current WILDGuide is a best effort to introduce the family but cannot be regarded as perfect. The 'key' to tribes is critical and really should be used in the first instance; at least until you have seen most of them or recognise obvious ones. If in doubt, go back to first principles.
  • Working with keys does not mean that one should not use the photographs/illustrations to help arrive at a conclusion, but the crucial point is not to force the identity to fit the picture. There are several obvious areas where this can (and does happen): hive-bee mimics and bumblebee mimics often cause problems. It is important to recognise that whilst the markings on the abdomens of hoverflies often appear to be distinct, they are frequently no guide to the species. Confusion within Syrphus is an obvious example where the critical characters are microscopic. And, I have regularly been asked 'so why is this a Syrphus' - answer - you need to examine the upper surface of the squamae to be sure - but these are not visible on photographs or on specimens with the wings folded.
  • If one sets aside the keys and simply tries to match specimens to photographs there will always be challenges because the introductory guides cannot ever provide all the necessary corroborative information (bear in mind that Stubbs & Falk is well over 500 pages of keys and descriptive text plus illustrations of male terminalia). It may not always be possible to reach a firm conclusion for a substantial sub-sample without taking specimens and keying through using a microscope and detailed monographs. So, we must work with what we have to hand, and use a certain level of judgment. Corroborating evidence can be helpful but is not foolproof - e.g., time of year, location, habitat, size (difficult to be sure with photos). It is distressing but not surprising that the NBN often has maps with dots well out of range - those recorders ought to have challenged their own IDs if they had read the distribution information.
  • We have tried to illustrate critical features in the WILDGuide, and where these are shown they should help to separate species involved. This is not perfect, however, and making judgments is often dependent upon experience. So, the novice should not be disappointed by problems in making early identifications: until you have a mental picture of critical features the process will always be slow and frustrating. But, it does get easier.
  • In my experience the majority of 'novices' actually come across quite a limited range of species. Most are 'common': Eristalis, Syrphus, Helophilus, Volucella form the majority of the assemblage. Tricky genera such as Cheilosia, Pipiza and Platycheirus are rarely seen over and above some very obvious animals such as C. illustrata, C. variabilis and P. albimanus. Recognising the genera is a good first base. Once you can readily say that this is an x or y, then you can move on to separate the species.
  • One way of getting to know what the animals themselves look like is to make a habit of working through the guide and reading descriptions and species accounts so that you have a feel for range and variation. As a small boy I spent hours engrossed in whichever book caught my attention. I had a pretty good idea what I was looking for even without seeing the animals themselves. What is more, I knew what to look for! So, as an example, I recall reading about puss moth eggs and that they were found in pairs on the underside of poplar leaves. At the right time of year I looked for the eggs and the second leaf I turned over yielded a pair of beautiful little eggs - I was hooked! I shared this haul with my pal John Stevens - each of us rearing a wonderful caterpillar and enjoyed watching them make their cocoon. A beautiful fluffy white moth emerged the following year!
  • Identification is a process of elimination. It is as important to understand what the subject matter is not, as much as what it is. So, one can say - I know with confidence that this is not an x or y - and hence the numbers of possibilities is reduced to those that actually fit the what it might be section. The critical markers generated by previous experience are part of this process. Remember: getting identifications right first time does not necessarily mean you have learned anything - you learn as much from getting it wrong and of course when this happens you actually have a mental hook to use to help in future. I use this process all the time. I cannot remember everything, but if I come across certain cues, I am reminded of past mistakes (we all make them).
  • Those entomologists who maintain a collection have the advantage of referring back to voucher specimens, but you can do something similar with photos - creating albums in taxonomic order. These may help - although you might also usefully look at Steve Falk's excellent Flickr library.

Following these processes, I think the best way is to think in the following manner:

1. Can we establish the Tribe to which the specimen belongs?

2. If so, aim to the relevant part of the book. In many tribes the animals are very different, and so you may find that there are several very similar animals involved.

3. For many of the Tribes, a simple key to genera is provided - follow this and you should get to the genus.

4. When at the generic level, individual photos should help, but there are also supplementary photos that should help - use these where they are available.

5. If you come to a conclusion about a species, check the descriptive information and note whether there are similar species. If similar species are listed, check against these before making a judgment.

6. If working from a photo - post it on a suitable platform (UK Hoverflies Facebook group, iSpot, Wild About Britain) and see what others think.

7. Don't be put off by mistakes - simply look upon them as a learning experience and have another go with other examples. Remember, we all start at the beginning and progress; and even the 'experts' make mistakes!


  1. Good article Roger. It is indeed daunting, but for me the challenge is in the long game and learning, and I'm grateful to people like yourself who inspire us to give it a go

  2. Respect for your dedicated efforts Roger. Gratitude for your patience and help in teaching us learner Dipterists.