Birch sawfly

Another day, another new garden discovery.

This wonderful beast is a birch sawfly, Cimbex femoratus, which at around 25mm is one of the largest and most spectacular sawflies we have in the UK. Our other two Cimbex species, C. connatus and C. luteus, are about the same size, but they're both extensively yellow and brown, and bear a superficial resemblance to queen wasps or hornets. In its usual dark form C. femoratus doesn't look like anything else: it has a shiny black body, with variable amounts of red on the abdomen, smoky wings with a dark margin, and orange-tipped legs and antennae. All the Cimbex species have the same pale membranous 'window' within the first tergite of the abdomen; I've been able to find no explanation for this strange feature, which you'd think was an inbuilt vulnerability, but it does make for easy and definite identification.

I rescued this specimen from a patch of muddy ground in the wild garden, where he was crawling about, occasionally trying and failing to take off. I couldn't see anything obviously wrong with his wings, but he was clearly struggling and was precariously placed there to avoid human feet and hungry birds, so I offered him a dock leaf, which he promptly climbed aboard, and moved him onto one of my grandmother's peonies. After a bit of bumbling around he eventually managed to take off and flew very slowly for about 20 metres, but then dropped onto the lawn, from where he couldn't get himself airborne again. So back onto the dock leaf and then back onto the peony he went. My working theory was that he was newly emerged and hadn't yet strengthened up enough to fly, but my hope of seeing him get safely away later in the afternoon was dashed: even after the sun had come out and warmed him up, he stayed put - sometimes sleeping, and sometimes alert and wandering around, but not making any obvious efforts to take off.

He may not have been interested in exercising his wings, but he spent quite a lot of time waving around what I assume to be his genitalia (I'm good on the dongly bits of Odonata, but sawfly bits and pieces are mysterious to me, so this is just an educated guess). If you look at this image full-screen, you'll see what I mean: at rest, those protruberances are withdrawn so far into the abdomen that they're barely visible, but he repeatedly extended them, flared them outwards, and - forgive me - wiggled them around each other. At points the whole thing put me in mind of one of those claw weeders you twizzle in the soil to grub up roots, or maybe the blades of a hand blender - either way, I found myself thinking that if he wasn't careful, he could do someone a nasty injury with them.

On the subject of which, the male birch sawfly has a pretty spectacular pair of mandibles, which I tried without success to photograph today. Reading up on the species, I learned that males are very territorial, and use their mandibles to fight each other - which makes me rather glad that I offered today's new friend a dock leaf to climb onto, rather than my finger.