|" Ow pet, do yer dee owt ? "|
" Aye, but not with you ! "
Sid the Sexist - Viz
Well, not one to be stumped for an answer on my favourite subject I had a look in my copy of Dragonflies by Corbet and Brooks, an excellent publication explaining all aspects of dragonfly behaviour, though can be a bit hard-work at times for a thicko like me :)
Anyhow a brief summary of the relevant paragraphs and a bit of general information from my own subconscious :
It is reckoned that dragonflies recognise potential partners of the same species by purely visual means, based on flight style, size, colour and pattern, and ultraviolet reflection.
It is the males who do the 'hunting' and will respond first from a great distance to a potential mate, then zoom in for a closer inspection. Receptive females often just stay put, hidden in marginal vegetation and will rise up to greet a male of the same species she recognises flying over.
Interestingly enough, experiments have been carried out using tethered female Southern Hawkers which have attracted six different species of male (including Common Hawker) and it is noted that those six species (Common, Southern and Brown hawker, Downy and Brilliant Emerald, and Four-spotted Chaser) have been noted over the years as being the most likely to 'try it on' with other species. This may either be down to a flaw in their recognition abilities or a character trait which means they'll have a go with anything as long as it's female (ha'way we all know someone like that, usually aided by beer-goggles:)
A closer inspection by the male should reveal enough about body and wing patterns to put off most non-compatible suitors but some individuals will still try to form the tandem link with the claspers around the neck, though as both claspers and neck shapes have evolved differently in the species this is usually impossible, or at best in similar species a loose connection can be formed (which is what I witnessed yesterday explaining why the females were able to escape with comparative ease)
And even if an inter-species mating wheel is formed the genitalia are different again so it's like going home drunk and trying to fit your car key in the front door lock (you just ain't getting in there).
I'm no expert but I think it's a fact that where species have reached their evolutionary peak, they are no longer able to copulate (successfully that is ;) with any other species, though as Wallace and that other bloke discovered, geographically separated species start to evolve differently, forming sub-species and races which may look different but are still capable of producing offspring where they overlap. This is thought to be true of Eastern and Western Pondhawk in the US, where intermediary individuals in more central areas are believed to be hybrids of the two.
Over here we have the Highland Darter which isn't recognised as a full species but a race of Common Darter evolving in harsher climates, but that's as close as we get; I can't speak for the rest of the world but certainly (to my knowledge) there has never been any suggestion that hawkers have interbred in Britain.
Species also try to ensure they keep to themselves in other ways, by synchronising emergence, choice of habitat, time of day visiting the breeding sites to name but a few; basically there's a reason for all aspects of behaviour and it mostly revolves around the breeding process.
And all this reminds me about something I forgot to mention yesterday; one individual at TWPond had me baffled for a while on ID but turned out it was a Southern Hawker (male) with all blue markings along the abdomen (instead of green turning to blue at the tail). Unfortunately I couldn't get a photo and he soon disappeared up into the trees with one of the females. It's an unusual rogue colouring but not unheard of, and unfortunately has nowt to do with interbreeding of the species. Certainly the first time I've encountered it in a mature adult.
Anyhow I'm off for a rest, my brain hurts. If anybody knows anything different or additional, I'd be interested in hearing, and cheers John for the question.