Saturday, April 23, 2011

Common Darter dragonfly Sympetrum striolatum

I am an amateur naturalist trying to learn something about everything that lives in my garden.

Photo 1, taken back in Autumn shows one of Britain's forty-or-so dragonfly species. Referring to the magnificant colour photos in my copy of Britain's Dragonflies (Smallshire and Swash) I'm confident this is one of the commoner species, The Common Darter (Sympetrum striolatum). The yellow stripes down the legs indicate this one is a male. Adult Common Darters can be found on the wing from March through to earlier December if the weather is mild.

Two dragonflies were present the day I took photo 1. This is the first time I have seen them in my garden. About a kilometre from my house there is a large expanse of wetland however and there, at times, I have seen swarms of several hundreds.

To learn something about dragonflies I have been reading the book of the same name by Corbet and Brooks (New Naturalist Series). I particularly enjoy natural history writing that educates you about not only what is known, but also what isn't. The book is fully satisfying in this respect, concluding each chapter with a section 'Opportunities for Investigation', listing projects of genuine scientific value that any sufficiently motivated amateur might tackle.

The book opens with a discussion of habitat selection: Many dragonfly species are selective over the habitats they will adopt as breeding sites. Some demand flowing water, others still. Some require the presence of certain types of aquatic vegetation. Some will select a pond only if the trees on the bank are below a certain height. Etc. It's hypothesised that an adult arriving at a new location runs through a checklist of 'proximal cues': 'Is it water?' -> 'No', then fly on / 'Yes' then 'Is the water flowing?' -> 'No', fly on / 'Yes', then are there trees on the bank... What the cues are for many species isn't understood however. The fact that adults have been observed being fooled into, for example, trying to lay eggs into a wellington boot (!) suggests one line of enquiry might be careful experimental manipulation of environmental cues to try to work out those that appeal to different dragonfly species.

The topic of reproductive behaviour is a very rich one for study. The authors break things down into the four stages of Recogition (of a prospective mate); Sperm transfer; Guarding Behaviour; and Oviposition. Guarding behaviour is the habit amongts the males of many species (including my S. striolatum) of staying with a female even after she has been fertilised. The males of some species continue to grip the female by the head until she has finished depositing her eggs. Some even 'dunk' the female under water to assist her egg laying into the submerged stems of plants. An advantage of guarding from the male's perspective is that it prevents other males from coming along and displacing his sperm with their own. For the female there are presumed survival advantages: Two sets of eyes are better than one when it comes to spotting predators. Some species even go in for group oviposition, whereby half-a-dozen or more males/female pairs congregate in one spot to lay eggs.

There are a great many opportunities for amateur study concerning the larval stages of dragonflies. Dragonfly larvae go through a number of stages called 'stadia', moulting their exoskelton between each. The stadia for many species however, are simply not known. As the book puts it "there is an urgent need for keys to earlier stadia".

Some species go through all development stages in one year, others 'sit out' the winter in 'diapause', still others have the option to do either. Much remains unexplored concerning the factors that control the development rate of larvae and whether they overwinter or not.

Some dragonfly species lay their eggs several metres from the water's edge. How the first larval stadium (a minute limbless 'tadpole') makes its way from the egg-site to the water is again unknown.

Finally, there are many opportunities for studying the behaviour of larvae - their strategies for stalking different types of prey for example - that any suitably motivated amateur armed with a fishtank and plenty of patience could attack.

There is a great deal more that could be said about the study of dragonflies ('Odontology'). Part of me feels I ought to go on and write more here since I have not seen other dragonfly species in my garden and so may not get a chance to return to them on this blog in the future. On the other hand when I started logging my garden's visitors several years ago I never imagined I would find myself writing about peacocks and budgerigars. So perhaps I can afford to take the risk!

No comments: