Generally differences between species are more or less clear cut involving distinctions such as modified shapes or extra features to the body e.g. colour patterning, size and shape, etc. There are a number of species, for instance, Scaeva pyrastri, Epistrophe elegans, Episyrphus balteatus, Merodon equestris and the Volucella, where the differences are such that accurate identification can be made by comparison with colour pictures in a book.
Volucella bombylans
Then there is a whole suite of hoverfly species where the differences are very slight requiring the use of a magnifying lens or microscope to see them. For example, the distinguishing feature separating Platycheirus immarginatus and P. scambus is the latter lacks a small white bristle at the base of its front leg!
Among other groups of species, particularly the species-rich genera such as Cheilosia, Sphaerophoria and Pipizini, the differences in structure or colour patterns are either non-existent or too variable among individuals to enable accurate identification.
So, rather than being merely a perverted curiosity, the “hidden treasure of characteristics” (as eminent 19th century hoverfly expert C.L Metcalf put it) found in the structures of the genitalia can help resolve problems in distinguishing many apparently identical appearing species. Thus I have spent many a happy hour bent over a microscope getting to grips with these organs and making identifications that were hitherto impossible.
Armed with my net and various tubes of varying sizes I began recording spring hoverflies in the first warm, sunny days of March. Apart from a small number of species which I was able to accurately identify in the field (Eristalis tenax, E. pertinax, E. intricarius, Portevinia maculata, Helophilus pendulus and the males of Melanostoma scalare and M. mellinum), my practice has been to collect a small sample of specimens on each trip.
These I then took home and cooled down in a fridge, and then keyed out with the help of Stubbs & Falk’s Guide to British Hoverflies. For species such as male Platycheirus albimanus and Syrphus torvus this appears to be an adequate practice on most occasions. However, for other species such as Melangyna lasiophthalma, Cheilosia pagana, Parasyrphus punctulatus, Neoascia meticulosa and Eristalis arbostorum voucher specimens were taken for microscopic examination and comparison with preserved museum specimens to enable certain identification.
Helophilus pendulus
An interesting sideline from hoverfly work has been an attempt to identify the flies that have turned up in my lunch! With a batch of peppers and tomatoes freshly bought from Morrison’s I spotted, in the nick of time, a number of flies, mostly exotic Drosophila (Fruit Flies). It is fascinating what occurs in your salad and how cosmopolitan it all is with vegetables sourced from Spain, Italy and Israel. But, more common place species can occur also. One time I found a small puparium on the stalk of a Tesco’s Broccoli. Out of curiosity I placed the floret into a plastic tube and after a few days a lovely Episyrphus balteatus (a very ubiquitous species whose larvae feed on aphids) emerged!
Episyrphus balteatus