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Hawkers and darters

Posted on August 17, 2011 | 3 comments

A lunchtime visit to Milton Country Park on the northern periphery of Cambridge this week to look for dragonflies turned out to be an hour well spent. For the first 10 minutes they were conspicuous by their absence but then a brown hawker appeared over a small pond, very distinctive with it’s rufous wings shimmering in the sunlight. I tried to photograph it in flight but it proved beyond my talents. Even though there were three or four whizzing around the pond at any one time I moved to a place under a tree which was overhanging the water and knelt down and waited. Within a couple of minutes a female brown hawker alighted on a log floating in the water and started ovipositing:

She seemed to be completely unfazed by my presence and busily worked her way along the log probing below and above the waterline for a suitable crevice to secrete her eggs.

She was there for around 10-15 minutes in total and every so often she took off but seemed to always to return either to her log or to a spot on the pond bank 5-6 feet from me. Even when my friend, Joe, came to see she still went about her business taking little notice of us.

She eventually departed along with all the other hawkers so we strolled along to a different location which was a wooden jetty protruding into another of the lakes. It’s surrounded all the way by dense rushes and has previously been a good place to see dragons. And that didn’t let us down either. We saw only two species, a pair of blue tailed damselfy and numerous common darters. All males. The males of this species are easily distinguished from the female as they are a glorious red and gold compared to the less vibrant green of the female.

The male of the species…

…and the lady:

There were several male common darters perched along the wooden jetty (the female above was snapped in Histon a couple of weeks ago), they were also happy to let us get up close and would occasionally dart away suddenly to chase a prey insect before returning to the same spot.

The compound eyes of the common darter – I like the depth of field in this picture rendering just the dragon in focus

When I was out for a stroll with my friend, David, who’s a bit of a dragonfly expert, last weekend, we found a dead migrant hawker dragonfly. We were marvelling at the complexity of the compound eye and he told me that a chap in the States had counted the facets in the eye of an American species, and apparently he got up to around 28000! He must have had a string of long winter evenings to fill. But dragonflies are amazing creatures and I can see how they inspired him to want to do it.

I also took a couple of nice bird photo’s on this trip but I’ll save those until next time.

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Posted in Anisoptera, Darter dragonflies, Dragonflies, Hawker dragonflies, Insects, Milton Country Park, UK wildlife, Walks around Cambridge and Histon

Tagged Anisoptera, blue tailed damselfly, Brown hawker, Cambridge, Common darter, dragonfly, Ischnura elegans, milton country park, ovipositing, Sympetrum striolatum

Dragonfly discoveries

Posted on October 19, 2010 | 1 comment

Dragonflies are amazing insects which it seems can be seen just about anywhere. In 2010 I’ve even spotted them over my garden in Histon. It occurred to me I should put some effort into photographing and learning about them as I sat with my family eating a picnic on a bench overlooking the fountain in the Cambridge University Botanical Gardens. Numerous small, magnificently bright red, individuals were whizzing around the pond, and a few big blue ones a short distance away from the pond (the small red ones were a species of darter, and the big blue ones were hawkers – probably migrant hawkers). I attempted to photograph these for subsequent identification but alas my photographic skills were’t up to the job on that occasion. Despite that I looked up some information and lots of great pictures at http://www.dragonflysoc.org.uk/home.html. This is a great website and started me off on a journey of discovery which is now pretty much finished this year but which I plan to resume as soon as possible next year.

Subsequent walks around Milton Country Park and the fields north of Histon provided lots of opportunities to see many species of dragonflies and damselflies:

Female brown hawker sunbathing on a birdbox at Milton Country Park

Migrant hawker male. The yellow ‘golf tee’ on segment S2 is diagnostic

Dragonflies originate from species which fossil records date to the Carboniferous period, around 300 million years ago, and could have a wingspan up to 70cm! They comprise the taxonomic sub-order known as ‘Anisoptera‘ and are part of the order ‘Odonata‘ which also includes damselflies (Zygoptera) and another group which is virtually extinct called ‘Anisozygoptera‘. There are now around 5300 species of dragonflies of which approximately 40 reside in the UK.

Dragonflies lay eggs underwater either loose, protected by a hygroscopic gel, or inside plants. The eggs of some species can lie dormant over winter and resume development when temperatures rise in springtime. Time to hatching is temperature dependent but is usually around 2-5 weeks and the larvae develop in water, all British species overwinter once as larvae but time to adulthood can be up to 5 years. Metamorphosis from larva to adult is triggered by increasing day length and temperature in spring, and includes the transition to breathing air and growth of their incredible compound eyes. Each eye is made up of around 29000 individual elements called ‘ommatidia‘. They also possess three addtional ‘simple eyes’ call ‘ocelli‘ which are sensitive to light intensity and assist in controlling flight via direct nerve connections to the flight muscles. Dragonflies have excellent colour vision and can also see ultra violet light. It makes me wonder what the neural activity behind all that optical prowess is capable of: are dragonflies merely responding directly to visual stimuli via reflex arcs, or is there more sophisticated image processing going on?

Common darter, immature male

Common darter, female. Note the compound eye.

Dragonfly larvae are ambush predators feeding on moving prey including other insect larvae, snails, small fish and tadpoles. Adults catch other insects in flight, holding their legs out in front like a bread basket to scoop prey from the air such as flies, flying ants, mosquitoes and even other, smaller, dragon flies.

Newly emergent dragonflies are powerful fliers and disperse widely. They are often seen a long way from water and can even cross seas. They reach maturity after a few days to a few weeks, a process also controlled by temperature, when the males will congregate at vantage points near water where females come to oviposit. The male will grasp the female around the head with his anal appendages and attempt to mate with the female by swinging her round to form the ‘copulatory wheel’ enabling him to pass a packet of sperm from the secondary male sexual organs located on his second abdominal segment to the female sex organs located on her eighth abdominal segment. After mating, sometimes under the guard of the male, to ensure his sperm is not displaced by another male, the female will oviposit underwater. And so the cycle begins again.

Broad bodied chaser – female