Sunday, 5 March 2017

Superfly

One of my jobs today was to sort out the mess that is the store shed, it's where things get dumped. About 60 feet beyond the shed is a small, babbling burn (that's a stream to any non-Scots out there). I spent a while moving, chucking and sweeping when I was physically halted in my tracks by a *thing* scurrying across the floor, looking for all the world like a cross between a cricket and a giant earwig but with long trailing cerci. The heck was that then??? I threw down a bit of debris, it scuttled underneath and I ran for my camera and a pot. One whole minute later and I was back, was it still there though...yes it was, phew! I grabbed the camera and tried to grab a half-decent pic as it once again began to charge across the bare floor

Mystery 'thing' (on a beautifully cleanswept floor...)
Safely potted up I had a proper look at the beast. Hmmm...could it be a stonefly, I wondered. The short wings and large size were confusing me. Every stonefly I've ever seen has been a long-winged, rather dainty-looking thing. Not like this bugger at all. 

After work I checked the beast again, grabbed a few freshwater books and jumped on the internet. And I soon found a likely culprit - male Diura bicaudata with it's reduced wings. Except the wing veination didn't look right for that. Reaching for my 1958 (!) copy of the FBA Key to Adults and Nymphs of the British Stoneflies I soon discovered where I was going wrong. Right family, wrong genus. Firstly the tip of the forewing showed an irregular network of cross-veins, as you can kinda see here

So you are looking at the lower right corner of the shorter of the two wings on the right hand side
As you can (hopefully) see there are several small cells all next to each other, almost looking like a small piece of honeycomb, at the end of the forewing (bottom right corner at this orientation). That conclusively rules out Diura bicaudata which lacks these small cells. So what is it? 

Second thing to check are the shape and size of the paraprocts, the little spiky things that hang out of its arse. Despite a quick bit of Googling, I am none the wiser as to what function paraprocts provide. I did learn what the word means: (Greek, para beside; proktos anus) and indeed they are described as being 'paired plate-like appendages either side of the anus in certain insect families, notably dragonflies and damselflies'. Cool, I still don't know what they're for though! So what did this beast's bum spikes look like? They looked like this

The paraprocts are the two little thorn-like spurs at the end of the last abdominal section
In real life the stonefly was constantly flexing, twisting and relaxing these 'prongs', they were moving all the time but only ever so slightly - you'd never notice unless watching through a microscope. Maybe it was clenching it's bum muscles in fear...

Anyway, checking the Stonefly Key couplets - are they 'elongate, each shaped like a half-cylinder and extending beyond the 10th tergum by more than half its length' (Diura bicaudata - er, nope) or are they 'short and triangular, each extending beyond the 10th tergum by less than half its length' - yep, I think I'll go for that one please! 

So, what have I got after all that detective work? Definitely not Diura bicaudata as I first assumed (which flies in Apr-Jun anyway, so a tad early realistically) but Perlodes microcephalus, my first identified stonefly! The larva live in flowing freshwater streams (or burns as we call 'em up here) with stony substrate and fly Mar-July, mainly Mar-May. This individual, with its short wings, is a male. The males apparently emerge a few days ahead of the females and generally loiter with intent on the ground somewhere near the burn...er stream...until the females emerge and then they get all jiggy with it before dying shortly afterwards. This species' status in my 1958 key says it is a common and frequent species. Not sure how relevant that is 60 years down the line, but the water courses here are unpolluted and good enough to drink from (though we filter it anyway here at the hotel) so there's no reason to assume that I won't be encountering Perlodes microcephalus again in the near future. Though probably not on the floor of the store shed. Nice! I released him back outside by the shed, hopefully he'll find his female in the next day or so.

As an aside, older literature names it Perlodes microcephala and more recently it has been called Perlodes mortoni as it apparently differs from European specimens. I have no idea, P.microcephalus seems to be the name in widest usage, I'll stick with that for the moment until I figure out the true picture of British forms versus European forms. It's an ├╝ber cool beast whichever way you look at it.  

EDIT: Ok, seems as though British specimens are indeed of a different endemic species. All British males have reduced wings (variable length on the Continent) and the microscopic sclerites on the egg anchor differ. Think I'll stick with reduced wing-length as the feature to look out for! There's a nice paper here that explains in some detail why British specimens are Perlodes mortoni. Just need to quickly amend my list...
 

2 comments:

  1. Nice. A National Trust place I did some moth trapping for used to be great for them. They would crawl out onto a wooden bridge over the burn at night. Not sure why it is that they actually feel like they've been around for millions of years.

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  2. Is it because they are the size of a small dinosaur and have a terrifying roar?

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