Identifying Syrphus in the UK from photos

Observations of Syrphus build up very quickly on iNat - they are common and usually cannot be identified to species. It is also a genus where it is helpful to ID males and females separately because there are different things to look for - so it's particularly helpful to add the sex to these. I made a key to the European species a while ago and I won't repeat too much of the information there. Everywhere in Europe the main species are S ribesii, vitripennis and torvus, other species are always rare. The aim here is to focus on these three species to help us identify them, and know when an observation should be made Research Grade at Genus level. S nitidifrons has also been recorded in the UK, but you won't confuse it with these three - I'm just ignoring it here, the statements below may not apply to S nitidifrons.

Syrphus usually have:
--a golden dusty coloured scutum (but sometimes can be greyer or shinier)
--Front margin of abdominal bands quite straight and horizontal
--Hind margin of abdominal bands pinched forwards at the sides (and usually in the middle)
--Male frons mostly yellow but with a black mark above the lunule in the middle
--Female frons the same but the black mark is extended in a triangle with its apex close to the black part of the vertex.
--Scutellum is distinctly yellow
--T4 and T5 have narrow yellow hind margins, but T5 itself is not distinctly yellow.
--The face is entirely yellow, without a stripe.
--Dark antennae (not necessarily completely black though)

Distinguishing Syrphus from Parasyrphus

Parasyrphus is the genus most easily confused with Syrphus. Their scutum is usually darker (typically dusted a golden colour in Syrphus), Syrphus usually have a bushier fringe of yellow hair around the scutum too. The legs of Parasyrphus -especially the hind legs - are generally much blacker and the body is typically more slender. The face of Parasyrphus usually has a vertical black stripe down the middle and the frons is typically much darker. Familiarise yourself with the pictures of Parasyrphuson Steven Falk's Flickr page to see these features.

Distinguishing Syrphus from banded Eupeodes

The bands on banded species of Eupeodes are wavier than Syrphus (especially the front edge - having either a 'moustache' appearance or a 'sunglasses' appearance. The scutum is dark and shiny in Eupeodes (typically dusted a golden colour in Syrphus), and the fringe of hairs around the sides of the abdomen (except T2) is black rather than yellow.

Distinguishing Syrphus from banded Epistrophe

Epistrophe also usually have a face stripe and a dark shiny scutum. The bands on most banded Epistrophe are particularly straight - they may have a dink in the middle of the hind margin, but they meet the edge of the abdomen almost at full width. Several species have distinctly orange antennae and the frons is usually different. The most Syrphus-like species is E grossulariae, where the female also has a dark triangle on the fron - but the antennae are deep black and the frons triangle reaches all the way to the antennae - not just the top of the lunule as in Syrphus.

Identifying UK Syrphus species

A general scheme for identifying UK Syrphus looks like this:

(Coverage of microtrichia on the 2nd basal cell of the wing (incomplete in vitripennis, complete in the others) and the colour of bristles/hairs on the knees (yellow hairs in vitripennis, black bristles in the others) can also be used for ID but are basically never visible in photos).

This seems fairly simple but it's worth giving a few more pointers:

Identifying Syrphus ribesii in photos

Male S ribesii are not identifiable from normal photographs, because they cannot be distinguished reliably from S vitripennis. However typically S ribesii males have the hind femur 1/3 - 1/2 yellow, whereas S vitripennis have it only 1/5-1/4 yellow. If the hind femur is very yellow, you may rarely find some people (especially from overseas) identifying the species on that basis. HRS will not accept the record at species, but personally I am not inclined to disagree on iNat where this happens. Leave it be - the recording scheme can decide what they want to accept.

For female S. ribesii you need to be confident that you can see at least half of the hind femur to be sure it doesn't go black. Beware: the abdomen is fringed with yellow hairs, so if you are looking down from above through these hairs, they can obscure the colour change making you think that the femur remains yellow for longer than it does. Colour can also be hard to judge through the wing if there is glare.

Identifying Syrphus torvus in photos

The key thing to realise about S torvus is that the eye hairs are much more obvious in males than females. In males you can often see the eye hairs as a 'halo' around the eye, even in pictures that are not especially sharp. Females are a different ball game - it takes a very clear picture to be sure that there are hairs - they are both shorter and sparser than the males' eye hairs.

S torvus, especially females, also tend to have a bit of a different 'jizz' to the others - typically a bit darker than other Syrphus with narrower bands and a darker wing stigma. That alone is not a basis for an ID though.

Identifying Syrphus vitripennis in photos

For male S vitripennis see comments on male S ribesii above. Some people, especially from continental Europe, will ID this species if the hind femur is only very narrowly yellow - but HRS will not accept such records.

Identifying female S vitripennis is also extremely difficult because you have to be sure that eye hairs are absent. Given how hard these are to see on a female torvus, it is even harder to prove the negative! Some workers will ID a female S vitripennis if the eyes are visible sufficiently clearly to see the ommatidia over a significant area of the eye but HRS will still not accept such records - for their own database they will record it as 'Genus Syrphus'.

So when is it safe to make an observation research grade at genus?

It should be safe to make an observation research grade at genus if microscopic features of the knee hairs/bristles or the wing microtrichia are not visible and

1) It's a male and hairs are not visible on the eyes

2) It's a female, no hairs are visible on the eyes but it's not an exceptionally clear picture of the eyes, and either the hind femur is only visible for less than the apical half or the hind femur is partly black.

If you're not sure of your judgement, there is never any harm in leaving it to someone else!

Posted on September 8, 2023 12:30 PM by matthewvosper