Tuesday, May 27, 2014

Dragonfles at Woodwalton

I spent a couple of afternoons at Woodwalton Fen last week photographing dragonflies - especially Libellula fulva (Scarce Chaser). I think there must have been a big emergence recently. When I went on Wednesday there were quite a loy of immature males about which either had not yet developed their blue pruinosity or only had it partially developed. This one has the blue colouring only partially developed down his mid line, with the yellow base colour of the abdomen still showing through down the sides. I think they look rather attractive at this stage.

Male Libellula fulva with partially developed blue pruinosity
Since it has been rather warm and sunny, there was a tremendous amount of activity with males chasing each other around and mating pairs also being quite numerous. Male dragonflies (like most insects) produce a packet of sperm called a "spermatophore" which is passed to the female during copulation. She then stores it internally and uses sperm from it to fertilise her eggs before they are laid. The male's genitalia open near the end of the abdomen, but he has "secondary genitalia" located near the base of his abdomen. A spermatophore is loaded into the secondary genitalia before he goes looking for a mate. He will try and grasp any female he encounters. If she is not receptive she will try and evade him and fly off. Otherwise, he grasps her using "anal appendages" at the end of his abdomen which lock into grooves in her pronotum - just behind her neck. These are complex structures with grooves and teeth in both sexes which act like a lock and key, so they will only connect up properly if they are the same species. Male dragonflies are not very discriminating and will try and grapple anything vaguely female-dragonfly like - including other males and individuals of other species! Once they are connected up, the female curls the tip of her abdomen round and inserts it into his secondary genitalia and the spermatophore can be passed across.

Pair of Libellula fulva in cop.
This takes some time in L. fulva and they remain coupled up like this for 15 minutes or more. They usually settle on a reed stem or something similar, but if disturbed they will fly around still coupled up. (I assume only the male uses his wings when they fly in this position).

You can tell a male who has mated because the female's front legs, grasping the males abdomen during copulation, leave a "mating scar" - a dark patch where the blue pruinosity has been scuffed up.

Mature male L. fulva showing "mating scars"
The other species that are common at Woodwalton at the moment are L. quadrimaculata (Four-spotted Chaser) and Brachytron pratense (Hairy Dragonfly). I saw females of both of these ovipositing, so I suspect they emerged a bit earlier and are a bit further on in their season.

L. quadrimaculata male


These shots were taken using a 60D with a Canon 70-200 f2.8L lens at the 200mm end of the zoom range and at an aperture of f8. The closest focussing distance of this lens is 1.2m at which it gives a reproduction ratio of 1:5 (one fifth life size - 0.21x according to Canon's specs). These shots were taken a bit closer than that using an extension tube on the lens. All were taken using a tripod at a shutter speed around 1/160 - 1/200s.