The first and most significant challenge is that we don't actually know how many species there are in Britain! Some years ago, Syrphus rectus was added to the UK fauna. This is a North American species that may well be holarctic, but we don't know. Males are almost identical to Syrphus vitripennis, especially in the distribution of microtrichia on the wings. Females are an intermediate between S. vitripennis and S. ribesii, in so far that they have the microtrichia of vitripennis, but the hind femur has a dark ring on it. So, maybe the females are doable from photographs? Field photographs never generate the necessary definition to pick up the microtrichia so unless one can detect eye hairs (S. torvus) or the wholly yellow femur of S. ribesii there is considerable uncertainty about the identity of most animals. In theory, there are additional characters of hair colour and extent of yellow on the legs that can help in some circumstances, but these are highly variable features and can be influenced by the angle one views the animal. So, Syrphus is already a problem and that is compounded by uncertainty. Further difficulties arise because some very high quality photographs have shown that even S. ribesii posesses minute hairs on the eyes, and that in some situations these can be mistaken for the hairs of torvus (demonstrated in females with yellow hind femur).
The following photos may help to explain. These are high resolution stackshots taken by John Bridges (North East Wildlife northeastwildlife.co.uk/gallery/index.php). John has been doing some really interesting work this year compiling detailed shots of hovers and here are some of the results (many thanks for permission to use John).
|Syrphus torvus female, showing the eye hairs. The hairs of female eyes are far less obvious than those of males, but this shot nicely shows how small they are and how fine the resolution of photos needs to be to resolve identification.|
|Syrphus ribesii male with the second basal cell showing complete coverage by microtrichia. These minute hairs are only seen when the animal is carefully orientated to the light.|
|Syrphus vitripennis second basal cell with area devoid of microtrichia highlighted (area above the green line)|
So, what can we do? Well, the problem does partially resolve itself if the photos are high quality macro shots such as those taken by Brian Valentine (LordV on Flickr). Even so, getting the right angles is tricky and one has to work with what is presented. The other otion is perhaps to retain specimens, anaethetise them and use stackshot to take more detailed photos. That is quite an undertaking and is likely to be beyond the average photographer. So, we must accept that at least some species are rarely likely to be taken to a full ID unless one captures all the salient features. This is demonstrated by the table I have attached later in this account where the problem areas are highlighted.
Challenges such as this can be found in many genera, but even so, there are plenty of hoverflies that can be identified from photographs providing the photo is of sufficiently good resolution and is sharp. The smaller the animal is in the frame when photographed, the less likely it is that it will be possible to achieve a firm identification. This is illustrated in the table below. I have compared the data for 2015 and 2016 so that as a big a sample as possible is considered. Clearly there are year on year differences in success rates. In many cases there are very minor differences between years, but for some the differences are quite substantial - something I will have to look at when I have time.
|Success rate for identification of photographs in 2015 and 2015 at generic level. Genera where major problems arise are highlighted in orange.|