A question-mark after Syrphus torvus (?)

One of my principles relating to wildlife has been to try where possible to avoid disturbance - where possible being the get-out, I suppose. If I were to stick to the principle, for example, I would never get involved in any practical work. Two big downsides of such a principle are that it has inhibited me in my early years from learning about the nesting habits of birds, and in latter years has prohibited the identification of some similar or closely related species.

The former is quite well known. Many of our best birders have been people who actually started out by collecting bird's eggs - thankfully not so common a pastime, these days! But when identifying species such as the hoverfly that I saw the other day, the only way to be reasonably certain of its identity would have been to capture it and put it under close examination. For one thing I am not prone to running about with a net, and for another it was busy doing its own thing and doubtless would have been less than happy to have been put in a plastic tube.

The hoverfly - on 28th March - was the first that I've seen this year and was settled on a willow catkin in the City of London Cemetery . As I so often do these days, I took some photos for examination later. It looked like one of the Syrphus species, but that was only hazarding a guess. At home, I found that the photographs were reasonably clear, showing some detail of the insect viewed from above and some viewed partly side-on. I am really not familiar with all the scientific terminology relating to the structure of a hoverfly, so it is difficult for me to follow a key through without referring back to the meanings of the descriptive terms. In addition, of course, I just had my photographs to go by.

Anyway, I deduced just from its general appearance (birders "jiz") that it was probably a member of the genus Syrphus. Wikipedia say of this genus: "Syrphus is a genus of hoverflies. It can be distinguished from other genera of the tribe Syrphini because it is the only genus that has long hairs on the upper surface of the lower lobe of the calypter. (as well as hairs on the rear margin of the calypter as in most Syrphini)." which - unless you are already an expert - shows you what I'm up against. There are six species of Syrphus in Britain. To start with, I could see that the specimen was a male; that's easy enough as in males the eyes meet on top of the head, in females the eyes are widely separated. I use a book called British Hoverflies, by Stubbs and Falk* as my guide, and the colour of the hind femur (part of the leg) is an important factor. Mine appeared to be slightly more than half-yellow, the rest black. Well, according to the book, that could mean that it was either S. ribesii or S. torvus. S. ribesii is a very common species, and may be seen from March to November, and might be expected in this area. S. torvus was described as being "easily overlooked amongst other Syrphus species, but should be found in most districts, especially about woodland." Another important distinction of species separation is the fact that S. torvus has hairy eyes, but that this can be inconspicuous. Looking closely at my photographs, the eyes do appear hairy, so that might help. One aspect of the description of torvus states "... often one of the common species at sallow catkins before other Syrphus have emerged." As I said, it's the first hoverfly I've seen, and it was on a sallow catkin! That is not confirmation, nor are my other indicators, but I do tend to think it may be the first specimen of Syrphus torvus that I have recognised in this area. I'd appreciate an expert comment.

Paul Ferris, 30th March 2011

* Reference: British Hoverflies, Alan E. Stubbs and Steven J. Falk, British Entomological and Natural History Society, November 2002