I find mayflies fascinating and regularly encounter them in my moth trap or elsewhere. I use the FSC AIDGAP guide "A Pictorial Guide to British Ephemeroptera" but although the FSC AIDGAP guides are mostly excellent I find this one rather inadequate. Despite the title there are few pictures (especially of adults) and the key is lacking in clarity and detail. Nevertheless, it does seem to be the best identification guide available for mayflies and it does enable at least the majority to be identified. I've also recently acquired the older Naturalists' Handbook on mayflies which has a few important insights that are missing from the Pictorial Guide but is generally more limited in scope (e.g. only males are keyed to species level) and detail. The lack of material online (even just reliably identified photographs) is really surprising. They are extraordinary and attractive insects that deserve far more attention than they appear to get at the moment. I'm pretty sure that a good clear guide would enable most mayflies to be identified fairly straightforwardly (with a few exceptions that will always be hard) so I very much hope someone rises to the challenge soon.
I have noticed that if I catch a sub-imago mayfly (in most cases recognised by its smoky wings) and keep it in a pot on my desk there's a very good chance that it will moult into an imago the following day, typcially around early afternoon. This doubles my chances of getting the ID right as both sub-imago and imago are described in the guide.
Except where indicated otherwise, I'm pretty confident about the IDs below. However, if you know better do please get in touch - I always welcome correction.
Family BAETIDAE
Pale Watery Baetis fuscatus - Male imagos recorded in my garden moth trap on 24th July and 25th September 2019 and on 3 dates between 4th July and 29th September 2020. Elsewhere a male imago and a male sub-imago (which moulted to imago) caught at light at the Cathedral Meadows on 15th September 2019 and a male imago there on 7th August 2020. Also a male sub-imago that moulted to imago found in Laura's trap in Hellesdon on 14th July 2019. There was at least one other female Baetis sp. which I didn't feel able to put a name to with any degree of confidence in 2019 and 2 that were either fuscatus or scambus at the meadows in 2020.
male Pale Watery Baetis fuscatus, North Elmham Cathedral Meadows, 15th September 2019 (both photos showing the same individual, as sub-imago in the top photo and as imago after moulting the next day in the second photo)
male imago Pale Watery Baetis fuscatus, North Elmham, 4th July 2020
female imago Pale Watery Baetis fuscatus or Small Dark Olive Baetis scambus, North Elmham Cathedral Meadows, 7th August 2020
male imago Pale Watery Baetis fuscatus, North Elmham, 27th September 2020
Large Dark Olive Baetis rhodani - None. I believe I caught this species here in 2018 but not since. According to the maps on NBN Atlas this is the one of the two most widespread Baetis in Norfolk.
Small Dark Olive Baetis scambus - None confirmed. According to the Pictorial Guide females of this species are inseparable from Pale Watery Baetis fuscatus but the description of female imagos of the two species differs. Presumably on the strength of those descriptions I tentatively identified females here in 2016 and 2017 but in hindsight I don't suppose those IDs were safe. The guide implies that this is probably the commoner of the two species and the number of dots on the maps in NBN Atlas support that statement, but all the males I've caught here have had entirely yellow eyes which apparently makes them fuscatus.
Medium Olive Baetis vernus - None. According to the maps on NBN Atlas this is the one of the two most widespread Baetis in Norfolk but I've not yet found anything that's given me any reason to suspect this species was involved.
Small Spurwing Centroptilum luteolum - 3 recorded in the garden moth trap between 19th June and 27th August 2019 and 6 between 8th August and 20th September 2020. Also at least 2 to light at the meadows on 15th September 2019 and 3 there on 7th August 2020.
male imago Small Spurwing Centroptilum luteolum, North Elmham, 9th August 2019
female imago Small Spurwing Centroptilum luteolum, North Elmham Cathedral Meadows, 15th September 2019
male Small Spurwing Centroptilum luteolum, North Elmham Cathedral Meadows, 7th August 2020 (both photos showing the same individual, as sub-imago in the top photo and as imago after moulting the next day in the second photo)
female Small Spurwing Centroptilum luteolum, North Elmham Cathedral Meadows, 7th August 2020 (both photos showing the same individual, as sub-imago in the top photo and as imago after moulting the next day in the second photo)
female imago Small Spurwing Centroptilum luteolum, North Elmham, 8th August 2020
male imago Small Spurwing Centroptilum luteolum, North Elmham, 12th August 2020
female imago Small Spurwing Centroptilum luteolum, North Elmham, 20th September 2020
Pond Olive Cloeon dipterum - Probably the second commonest mayfly in my garden with a total of 40 recorded on 25 nights between 29th May and 22nd September 2019. This was a lower total than in 2017 and 2018 but 2020 was worse with just 24 between 12th June and 9th September. Also 3 recorded on different nights at the Cathedral Meadows and 1-2 at 4 other sites, all in 2019. The Pictorial Guide refers to differences in wing venation between some of the Cloeon/Procloeon genera but doesn't describe these differences very clearly or illustrate them, so I've started photographing the forewing venation for a selection of insects. Some of these forewing photos are shown below although I haven't had to rely on this for this particular species.
female Pond Olive Cloeon dipterum, North Elmham Cathedral Meadows, 25th May 2019 (both photos showing the same individual, as sub-imago in the top photo and as imago after moulting the next day in the second photo)
male Pond Olive Cloeon dipterum, North Elmham, 1st August 2019 (both photos showing the same individual, as sub-imago in the top photo and as imago during moult the next day in the second photo)
male imago Pond Olive Cloeon dipterum, North Elmham, 29th June 2020 - showing forewing
male sub-imago Pond Olive Cloeon dipterum, North Elmham, 13th July 2020 - showing forewing
female imago Pond Olive Cloeon dipterum, North Elmham, 15th July 2020
male sub-imago Pond Olive Cloeon dipterum, North Elmham, 23rd July 2020
female sub-imago Pond Olive Cloeon dipterum, North Elmham, 26th July 2020
male imago Pond Olive Cloeon dipterum, North Elmham, 5th August 2020 - showing forewing
male sub-imago Pond Olive Cloeon dipterum, North Elmham, 27th August 2020
Lake Olive Cloeon simile - Separation of this species from Pale Evening Dun Procloeon bifidum is an example of where the Pictorial Guide could have been so much better with just a bit more clarity, and in hindsight I think the absence of that detail has led to some errors in my past attempts to separate the two species. Differences in wing venation aren't well desribed - a simple diagram could have so easily improved this, but even now I'm still not sure which cross veins are supposed to line up (or not) or how precisely they need to line up to qualify as lined up. Hopefully I'll figure that out as I start to take more photos of the forewings and, I think, am finally getting to grips with the tarsal characters.
And therein lies another problem with the guide, but one which getting the old Naturalists' Handbook has helped with. Part of my problem in the past has been that on some individuals the tarsal segments haven't been clearly defined so it's been really hard to gauge their relative length, but part of the problem has also been that it hasn't always been easy to to differentiate between the base of the tarsus and the tibia. The Naturalists' Handbook does two things that the Pictorial Guide misses - it specifies the hind leg (I don't know if that's important or not, but at least I'm not left wondering if I'm looking at the wrong legs) and it shows a diagram pointing to the relevant tarsal segments, and that was a bit of a revelation! If the first and second segments are those arrowed then the whole tarsus must have four visible segments excluding the claws, and anything basal to the first segment must be the tibia. But if this is so then the tibia (at least on some of the mayflies I've been looking at) must closely resemble the first tarsal segment in structure (except longer) and (unlike most insects) it appears not to be obviously jointed from the tarsus. That probably shouldn't have confused me as the placement of the femora should have made it clear where the tibia started, except that sometimes I've tried to judge the relative distances of segments from photos (as it's easier to measure ratios on a photo) that don't show the whole leg.
There's a further problem with judging the ratio of segment length, namely the strongly slanting ends of each segment. If you measure the length of one segment from the the most proximal point visible on one side of the leg to the most distal point visible on the opposite side of the leg you get a much longer length than if you measure along the axis of the leg. As the degree of slant varies between segments the two approaches will give different ratios. Anyway, sometimes I've judged the relative lengths of the first and second tarsal segments to be about 1:2.5 and thus indeterminate between the two species (either 1:2 or 1:3) and now I'm wondering whether I was looking at the wrong legs and/or at the tibia and first tarsus segment instead of the first two tarsal segments. Potentially even where the ratio wasn't indeterminate it might have been the wrong ratio so could have led to the wrong identification.
The long shot is I don't have much confidence in the accuracy of the identifications I've made of Cloeon simile and Procloeon bifidum up until a little before the end of the 2020 season. I suspect most were right and it was just a few where I got muddled up, and maybe once I've really got to grips with this pair I'll be able to look back at the photos and be more confident about some of the past records but for now I think it will be safest to regard all pre 2021 records as tentative (and therefore won't be submitted) unless there is a photo that clearly shows the tarsal segments. I've got photos of the forewings for a few more individuals and I anticipate that once I've ironed out the problems understanding the differences in wing venation these will allow more of them to be named with confidence.
So, for now, the only record of this species that I'm confident about was a male caught in my garden moth trap on 1st August 2020. I have tentative records of others here in 2019 and spring 2020 plus 2 at the meadows in 2019.
male Lake Olive Cloeon simile, North Elmham, 1st August 2020 - showing hind tarsus and wing venation (both photos of the whole insect show the same individual, as sub-imago in the top photo and as imago after moult the next day in the second photo)
Pale Evening Dun Procloeon bifidum - As with the above species I don't have confidence in my identifications of this species until some of the latest ones where I checked and photographed the hind tarsus, namely females in the garden trap on 2nd and 12th August 2020. If my original/tentative IDs were correct then this species appears here more often than the previous species.
female Pale Evening Dun Procloeon bifidum, North Elmham, 2nd August 2020 - showing hind tarsus and wing venation (both photos of the whole insect show the same individual, as sub-imago in the top photo and as imago after moult the next day in the second photo)
female imago Pale Evening Dun Procloeon bifidum, North Elmham, 2nd August 2020 - showing hind tarsus and wing venation
Large Spurwing Procloeon pennulatum - The first examples of this species that I have identified were caught in my garden moth trap on 30th September 2019 and 30th July and 9th August 2020.
male imago Large Spurwing Procloeon pennulatum, North Elmham, 30th September 2019
female imago Large Spurwing Procloeon pennulatum, North Elmham, 30th July 2020 (found dead in moth trap) - showing hindwings and forewing
male imago Large Spurwing Procloeon pennulatum, North Elmham, 9th August 2020
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Family CAENIDAE
Angler's Curse Caenis sp. - 2-3 species identified:
Caenis horaria - I retained 2 of at least 6 Caenis that came to light at the Cathedral Meadows on 25th May 2019. Unfortunately Caenis are very short-lived (usually not surviving the whole night) and seem to have a habit of exuding goo from their abdomens on death causing them to get stuck to the pot and making it difficult to see the finer details required to identify them. I was pretty happy that I could see enough on at least one of them to nail it as horaria, though I'm not 100% sure that the individual shown below was the same individual that I confirmed as horaria. Also one in my garden moth trap on 30th May 2020.
Angler's Curse Caenis horaria, North Elmham Cathedral Meadows, 25th May 2019
dead Angler's Curse Caenis horaria, North Elmham, 30th May 2020
Caenis luctuosa - 5 in the garden moth trap on 20th May 2019, all dead by first light. Another probable on 22nd and another dead one on 1st August. Strictly speaking I didn't completely eliminate Caenis pusilla for the first ones but apparently that doesn't occur round here. In 2020 singles caught here on 19th and 20th May.
dead Angler's Curse Caenis luctuosa, North Elmham Cathedral Meadows, 20th May 2019
dead Angler's Curse Caenis luctuosa, North Elmham, 19th May 2020
Angler's Curse Caenis luctuosa, North Elmham, 20th May 2020
Caenis rivulorum - One tentatively identified from the garden moth trap on 19th July 2020 but not with enough certainty to call it a record. The dark dusting on tergites 1-3 was clearly darker than on the other tergites but there was a light dusting on tergites 4-6 too (the spine on T2 was present). This species is apparently considerably scarcer than the previous 2 species in Norfolk but there are dots on the NBN Atlas maps along the Upper Wensum and in the Norwich area so is probably quite feasible round here. As with most of the other Caenis caught this year it had expired before I checked the trap at first light and I couldn't get it to lie in position where I could photograph it properly.
possible dead Angler's Curse Caenis rivulorum, North Elmham, 19th July 2020
Caenis robusta - None. Going on the NBN Atlas maps this is the next most likely species of Caenis here, but I've not identified any yet.
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Family EPHEMERIDAE
Green Drake Ephemera danica - One resting on my shed in the afternoon of 25th May 2019 then 23 recorded in my moth trap on 14 dates between 24th June and 26th August. Fewer in 2020 with 12 between 31st July and 20th August. Although much larger and more robustly built than the previous species, this is another species that doesn't survive for long and many of the individuals caught in the moth trap failed to survive until first light. Also 2 next door in August 2019 and recorded at the Cathedral Meadows on 4 dates from 21st May 2019 (including several engaged in their characteristic yo-yo dancing flight on 29th June) and 2 there on 13th June 2020.
Green Drake Ephemera danica, North Elmham Cathedral Meadows, 29th June 2019
Green Drake Ephemera danica, North Elmham, 20th August 2020
Drake Mackerel Ephemera vulgata - None. This looks like it should be the next most likely member of the family to find in Norfolk but I'm not sure it's especially likely round here.
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Family EPHEMERELLIDAE
Blue-winged Olive Serratella ignita - I think this is the commonest mayfly in my garden with totals of 73 recorded on 25 nights between 5th July and 31st August 2019 (max. 20 on 26th June) and 90 on 17 nights between 20th June and 17th August 2020. These compare with 105 in 2018. A total of 5 records at the Cathedral Meadows including a later one on 15th September 2019. Females of this species vastly outnumber males at my moth trap.
male Blue-winged Olive Serratella ignita, North Elmham, 15th September 2019 (both photos showing the same individual, as sub-imago in the top photo and as imago after moulting the next day in the second photo)
female imago Blue-winged Olive Serratella ignita, North Elmham, 20th June 2020 (I assume that's a batch of eggs under the abdomen - this species often has this)
female imago Blue-winged Olive Serratella ignita, North Elmham Cathedral Meadows, 18th July 2020
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Family HEPTAGENIIDAE
Yellow May Dun Heptagenia sulphurea - None. I'm yet to find this species but judging from the dots on the map in the NBN Atlas it occurs in the Norwich end of the Wensum as well as various sites in north-central Norfolk so it should be one I bump into one day
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Family LEPTOPHLEBIIDAE
Ditch Dun Heblophlebia fusca - None. It looks like this must be one of the commonest mayflies round here that I'm yet to encounter, with plenty of dots on the map in the NBN Atlas. Maybe it's one that doesn't tend to come to light?
Sepia Dun Leptophlebia marginata - None. This doesn't look like it's anywhere near as common as the previous species but widespread so there could be a possibility of finding one locally. Claret Dun L vespertina looks like its also in range but even scarcer.
Turkey Brown Paraleptoplebia submarginata - None. This looks like it's another reasonably common mayfly round here but I haven't come across it yet.
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