Identifying Sphaerophoria in the UK from Photos
Continuing this little series on the neediest hoverfly genera, we come to Sphaerophoria. Another genus that is frequently observed but often unidentifiable.
Only 'two and a half' of the UK's 11 species are typically identifiable in photos: S loewi, S rueppellii and male S scripta. Thankfully S scripta is by far the commonest species, so a lot of males can be identified.
There is also an observation field called 'Sphaerophoria identifiable group', which Caleb Scholtens produced to help categorise the North American species. I asked him to add some values useful for European species, which he kindly did. You might want to use this because it makes it possible to search for the rarer species in iNat even if we can't identify them: but unfortunately it will not feed through to the HRS. There are 5 values that are relevant to UK (and European) species: Complete lateral scutum stripes, Partial lateral scutum stripes, "cleoae" pattern, interrupta-group, and cf. S scripta. 'Partial lateral scutum stripe' isn't very useful in the UK because the two species it includes are both identifiable to species (S. loewi and S rueppellii).
Distinguishing Sphaerophoria, and differences between the sexes.
Sphaerophoria are very elongate black and yellow hoverflies. They have a clear yellow stripe down the sides of at least the front half of the scutum. Males have a massive round genital capsule on the underside of the tip of their abdomen (hence the name 'Globetails'), which is diagnostic for the genus. Most males are very straight-sided, although some have the abdomen constricted near the front. Female abdomens have more convex sides, and the abdomen tip is rather pointed; also the frons usually has a fairly broad black stripe descending from the vertex (male frons is all yellow). Like most hoverflies (and all Syrphini), the eyes of males are connected, but the eyes of females are clearly separated (though the area between the eyes is black, which can sometimes make it hard to see).
Is the scutum stripe 'complete' or 'partial'?
One useful marker in identifying Sphaerophoria is whether the stripe at the side of the scutum stretches the full length of the scutum ('complete'), or whether it stops at the wing base ('partial'). Care needs to be taken with this because the stripe can be much fainter beyond the wing base - this still counts as 'complete'. When 'partial' it is truly shining black behind the the wing bases. A partial scutum stripe is a feature that separates both species in which both sexes are identifiable (S rueppellii and S loewi) from all the others.
Identifying S rueppellii from Photos
This is quite a distinctive species with an incomplete scutum stripe. The abdomen of both sexes is unusually bulbous - constricted near the front. The abdomen markings, especially in the female, are quite curved. Curved bars from the side of the abdomen meet or nearly meet in the middle, but on T2 the spots are usually well separated and blunt-tipped. The antennae are pale but may sometimes be darkened above - they are never truly black though. This species is widespread but only crops up on iNat occasionally.
Identifying S loewi from Photos
A very distinctive species - the only one with jet black antennae. The abdomen is very dark, with a unique pattern of oblique spots becoming less well separated toward the tip. The male abdomen tapers toward a constriction at the front. The female abdomen seems to be even more constricted, and T5-7 are clearly visible. The species is associated with reedbeds, especially those containing club-rushes, and most often coastal ones. It is extremely rare - there are two on iNat (females from Poland and France), the images of both sexes from Steven Falk are excellent
Identifying S scripta from Photos
Males are quite easily identifiable because the abdomen is remarkably long compared to the wings. When the wings are folded almost the whole of T5 is beyond the wing tips. In other species the wings almost entirely cover the abdomen. It is of course harder to judge this when the wings are open, and you have to be very careful to take into account the angle at which the photo is taken, and whether the abdomen is being held straight or bent down in order to judge whether the abdomen is long.
HRS will generally not accept a female S scripta ID from photos. But it is useful to add the 'cf S scripta' value in the 'Sphaerophoria Identifiable Group' observation field so that it is possible to separate out the many probable scriptas from the more unusual species. The females of S scripta are also a little longer than other species owing partly to a longer T5 - the effect is much more subtle. The images below show a female that is probably S scripta compared to one that is definitely not S scripta.
Only one feature is generally regarded as diagnostic for S. scripta females: there is a broad glabrous line on the underside of the hind femora (that is to say a smooth shiny line without black bristles). In other species this line is narrow, but in scripta it is almost as wide as the femur itself. This is extremely difficult to capture in a photograph. I have tried! The best way in the field is to get the fly in a spi-pot. Otherwise obviously a microscope does the trick!
So when should we make it Research Grade at genus?
When the lateral scutum stripe is complete AND EITHER
a) it is male with a short abdomen OR
b) it is female without an exceptional view of the underside of the hind femur showing the tiny black bristles and the glabrous strip between them.
The remaining sections are only relevant for using the 'Sphaerophoria identifiable group' observation field.
What is "cleoae" pattern?
The species "Sphaerophoria cleoae" was described by Metcalfe in 1917 from North American females, however it was later found to represent genetic abnormalities that can occur in the females of several species - including European species. (Some other elongate genera show analogous phenomena). Such females may lack ovaries/spermathecae - in terms of appearence the tergites and sternites toward the end of the abdomen are fused together into a ring shape, and therefore the abdomens are less pointy and more male-looking (so do check the head to determine male sex!). They are usually darker and the particular abdomen pattern is distinctive - with the bands broken (or weakened) in the lateral third.
interrupta-group
The rest of the species with a complete scutum stripe fall into two general categories - those with banded abdomens and those with spotted abdomens. Care must be taken however because dark individuals of banded species may look spotted and vice versa. The interrupta-group is for the usually distinctly spotted species, primarily interrupta and fatarum. (See examples - Some other species are sometimes described as 'usually spotted' but many images and diagrams contradict this - including Falk's - philanthus is the closest to being spotted, and often is so). In these two species the markings are usually very clearly separated, and the females have a very distinctive sort of pattern - I'm not sure about fatarum but interrupta also has a distinct black face stripe. Females are probably easier to determine because their spots are quite well formed with a distinctive shape. Using this will enable people to filter for spotted specimens (it doesn't matter enormously if we accidentally catch some dark normally-banded species.)
So how can we use the 'Sphaerophoria identifiable group' observation field?
The main value of this field is to separate the more unusual species from the things that are probably S scripta and make them searchable. This way they don't just get lost in 'Genus Sphaerophoria' RG observations. If S. loewi and rueppellii can be excluded then I suggest using 'cf S scripta' for the females that match the indications of scripta given above, using interrupta-group for those that seem clearly to be spotted species, and 'cleoae pattern' as described above; and then using 'Complete scutum stripe' as the fallback for things that don't fall clearly into those three.
Anything else to be aware of in Europe?
Since some of you do identifying in continental Europe too it is worth mentioning that there are two other species in the 'Partial lateral scutum stripe' group - and both are identifiable from photos, S shirchan is distinguished by the distinct black face stripe, and S estebani lives in the mountains of central Europe and is distinguished by not having the distinguishing features of the others! (i.e. antennae not jet black (cf loewi), not with a face stripe (cf shirchan), and not having a constricted abdomen (cf rueppellii) - according to van Veen it has elongate, straight, lemon-yellow bands of constant width which are sometimes interrupted in the middle, on shiny black tergites. There is also one species with no scutum stripe at all - Sphaerophoria nigra, which is endemic to the Azores - completely black apart from the bright yellow scutellum, and parts of the legs in females, so rather distinctive!
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