Caenis robusta

One of a group of species known as Angler's Curse.

This individual was narrowed down to either Caenis horaria, C rivulorum or C robusta by the presence of a small spine projecting from the second tergite. Out of these three, although robusta is supposed to be common I've previously only managed to identify horaria with any confidence. The Pictorial Guide by Macadam and Bennett only offers one character to separate the three species - the number of body segments with dark markings. In practice this isn't always easy to use - I've seen a few where there have been clear greyish markings on segments 1-5 or 1-6 (as in horaria) but very slight markings on the rest of the segments. I've not been sure how to interpret that - is it enough to call them robusta or not? I've suspected not but typically left the insect unidentified. On this individual the markings were clear on all segments, with no significant difference between segments 5, 6, 7 and 8. Other slightly smaller Caenis were caught with it which showed a clear contrast between the relatively solidly grey basal 5-6 segments and the yellower, but still lightly marked with grey, distal segments.

Having recently acquired The Mayflies of Europe by Bauernfeind and Soldán I turned to this to see if I could find any further differences to clarify the identifications. This is a hefty and comprehensive tome but isn't the easiest to use for identification - it doesn't spell out the differences but does include a detailed description for each species and within these it's possible to unearth some ID pointers. Size is one such difference with forewing lengths given as 3.8 to 5.6mm for robusta and 2.5 to 3.6mm for horaria. Well, my suspected robusta had a forewing length of about 6mm and my suspected horaria was arond 4.5mm, so I wasn't not sure what to make of that. Later on I acquired the Elliot & Humpesch key which I should have got years ago - this has a different take on the sizes distinguishing both horaria and robusta from rivulorum on size, with 2-3mm indicating rivulorum and 4-6mm indicating either horaria or robusta, but it doesn't break down these two species by size.

Bauernfeind and Soldán suggest a difference in the abdominal posterolateral processes, long and filiform in both speices but 1/3 to almost 1/2 segment length on segments 5-7 for horaria and longer than segment length on segments 4-5 for robusta. One of the challenges in identifying Caenis is that they stick their abdomen to the specimen pot with a secretion (egg mass I think) which makes it hard to see abdominal characters such as this. But the processes were clear on my suspected horaria, and short enough to fit horaria. They were a little harder to see on the suspected robusta, but they were indeed longer than the segment length, so confirming my suspicion that both species were involved.

female Caenis robusta showing forewing, abdominal spines (tips arrowed in red) and antenna, Wendling Beck Environment Project (Norfolk, UK), 11th June 2023

This one I wasn't sure about. It was much smaller than the one above, with a forewing length of around 4.3mm, but that's within the stated range for robusta and it was a male. It definitely didn't have the spine on tergite 2 so, given the form of the antennae, should be macrura - and based on Macadam & Bennett there's no reason not to call it macrura. The problem with calling it macrura is that Bauernfeind and Soldán show this species as having much longer forceps. The genitalia seem to rule out any of the T2-spineless species but seem to be a perfect match for robusta. Unfortunately I had already cleared the abdomen in potassium hydroxide to get a clearer view of the spines and genitalia before I realised that in the one photo I'd taken of the live insect, the insect was lying on its back. This meant I didn't know - and now couldn't know - how extensive the grey markings were on the tergites. I'm quite sure about the absence of a spine on T2 - this part of the abdomen was in good nick when I examined it carefully from all angles prior to maceration, and was even easier to view after maceration. If it had had a spine on T2 then it had been broken off before I examined the specimen. My confidence in this identication grew when I acquired Elliott & Humpesch as this has really clear diagrams of the male genitalia for the five common species and mine is a very clear match for robusta (and importantly given the absence of the spine on T2, very clearly not a match for either macrura or luctuosa).

male Caenis robusta (on its back) showing forewing, base of abdomen, antennal bases and forceps/penis, Wendling Beck Environment Project (Norfolk, UK), 12th June 2023