Common Whirligig Gyrinus substriatus

Common Whirligig Gyrinus substriatus, North Elmham (Norfolk, UK), 4th July 2018

male Common Whirligig Gyrinus substriatus showig scutellum, side of elytron, apex of elytron, side of pronotum and aedeagus, Wendling Beck Environment Project (Norfolk, UK), 21st September 2024

Common Whirligig Gyrinus substriatus, North Elmham Cathedral Meadows (Norfolk, UK), 30th June 2019

Common Whirligig Gyrinus substriatus, North Elmham (Norfolk, UK), 16th July 2019

I don't recall having too much trouble keying the above beetles but this one was more troublesome. I used Duff at first and got stuck at couplet 7 as I couldn't see any micropunctures between the rows of punctures at all. Duff says you need 40x magnification but I zoomed up to 63x. The elytra were a bit grubby so perhaps that was obsucring them, so I soaked one in potassium hydroxide solution and gave it a good clean. Still nothing. Clearly sparse micropunctures 1-3 abreast between each puncture row was closer than close micropunctures about 8 abreast, and taking that option led to substriatus, but they weren't 1-3 abreast, they were 0 abreast. I then looked at Hackston and there substriatus seemed to be ruled out by the colour of the elytral epipleura. Looking at it from below the epipleura appeared to be oily brown, at most vaguely yellow and certainly not "very pale orange-brown" as required for substriatus and illustrated - "black or brown with a metallic shine" seemed to fit better. That route led to marinus if I took "two inner striae" to mean one each side rather than two on each elytron - and if I took the claws to be black. I had another look at the claws recalling that on the only marinus I had examined previously had mostly black claws but with yellow bases, and observing that where Duff says to look at the mid and hind legs for this Hackston points to the front and mid legs. The mid leg claws were darker than the rest of the tarsi, though certainly not black.

I was now starting to think my beetle might be marinus (it was 6.3mm long by the way). I thought it might help to look at the gonocoxae. Initially I didn't spot that Duff shows these for all species and could only find pictures for substriatus and natator (and one or two other species that weren't relevant), not marinus. My beetle's seemed a good match for substriatus, but that didn't help rule out marinus. Then I realised I hadn't taken a photo of the epipleura so did that. Then as I picked up the elytron I suddenly saw a flash of yellow - it turns out that when I angle it so I'm looking at it directly from the side it suddenly becomes bright yellow! So in that case it keys to substriatus using Hackston, so surely that's what it must be. Other than not being able to see any micropunctures I couldn't find any reason not to call it substriatus now.

There is one final (so far) twist to this saga as while I was writing this up I had a quick look back at Duff to check some terminology and in doing so I realised Duff had diagrams for all the Gyrinus ovipositors including marinus. The gonocoxae of marinus do apparently look a lot like mine, so neither a help nor a hinderance to my ID. But then I noticed that Duff's diagrams show substriatus and natator the opposite way round to coleonet.de and Hackston - mine doesn't look like Duff's diagram of substriatus! Hackston and coleonet.de (both are based on Freude) show natator as the one with concave bases to the gonocoxae and teeth at the inner corners, whereas Duff's diagram suggests it's substriatus that looks like this. So if Duff is right, I'm back to marinus (as natator is ruled out for other reasons)! However, Duff's text has natator as the one with teeth at the inner corners of the gonocoxae so I think he must have got the diagrams mixed up?

female Common Whirligig Gyrinus substriatus showing scutellum, elytra (5 views), hind and mid tarsi (hind above, mid below), ovipositor and elytral epipleuron (from underneath and from side), Wendling Beck Environment Project (Norfolk, UK), 29th June 2025

The above beetles either came to light or were netted in flight, but I see Whiligigs more often on the surfacfe of the water. Unfortunately when they're doing this I can't see sufficient detail to identify them. I imagine they're most likely substriatus, but can't assume that's the case.

2x Gyrinus sp., North Elmham Cathedral Meadows (Norfolk, UK), 24th April 2020