I have paid some attention to English odonata again for the 2019 season, and one priority has been to re-experience Common Clubtail emerging from their nymph cases at the classic site of Goring railway bridge. A certain mystique always surrounds this dragonfly because though not rare it is so difficult to cross paths with, and a deal of luck is usually required to do so. I had been successful at the same site two years ago but not in 2018.
Newly emerged Common Clubtail drying off in reeds
Of Europe’s six Clubtail species, Common Clubtail (Gomphus vulgatissimus) is the only one that occurs in the British Isles and the Oxfordshire River Thames from Pangbourne upstream to the west of the county is a traditional stronghold. Last year I was invited to participate in a British Dragonfly Society survey of the population in this area, but declined because they are just so hard to come by. Numbers of exuvia (nymph cases) found on the river’s banks are often much greater than actual sightings of the relatively shy adults that either mature in nearby rough ground or relocate quickly to tree tops. So the best chance of observation is usually during or immediately after emergence.
At Goring they have for many years emerged on the abutment of a bridge carrying the main Reading to Birmingham railway across the Thames (SU606795). That (pictured below, left) was where I went to try my luck again on 19th. Since my last visit a boating club has appeared on the opposite bank (below right) so rather more was going on here than in previous years. But perhaps surprisingly no-one on that side of the river challenged me about what I was doing or assumed I must be pointing my camera at themselves. So no explanation that the Clubtails have been here for much longer was necessary.
Upon arriving with my chair at around 10:30 am, I found three exuviae on the abutment wall. My strategy is always to sit here and wait, checking the structure at around 15 minute intervals for signs of new activity. After an hour a fourth nymph had indeed appeared, with a pair of eyes sticking out of the top (below, left). But sadly this Clubtail failed to emerge fully and eventually expired. I retrieved it and placed in in the sun on a bank-side fir tree (below right) but there was no hope. Later, a little downstream I did see an adult that must have emerged in deep cover flying inland from the river, but these were the sum of my day’s results.
Five days later (24th) I made a second attempt. Arriving at about the same time another observer was there ahead of me, confirming there was nothing at one end of the abutment as I did about the other when we met in the middle. He soon moved on to be succeeded by a second person to whom I talked for a while before checking the concrete wall again. When I returned to his end he had found a newly emerged Clubtail in waterside reeds. A patient wait then ensued to see if this specimen would dry off successfully.
The second observer left after taking his pictures and I was next joined by the county odonata recorder for Essex, Neil Phillips. Being also a former education officer at Wat Tyler Country Park (of Blue-eyed or Southern Migrant Hawker fame) where I had met him previously, I took the opportunity to ask how such a large insect gets out of such a small nymph case. He explained how on emergence dragonflies pump out their abdomen and wings as part of their breathing function. This clarified why the failed emergence I recorded here two years ago was puny while the second Clubtail encountered then was so much larger and robust (see here).
There can be a 25 to 33% failure rate in this process and indeed all did not appear to be well with our particular Clubtail (pictured above). Despite flapping twice and altering position on its chosen perch, one wing appeared to be deformed and after two hours it had still not flown and was not moving at all. In such circumstances newly emerged dragonflies are vulnerable to attack by territorial males of other species, drowning from the wash of passing boats or being snapped up by passing wildfowl or other opportunistic birds. Alas, unable to fly this individual was most likely a goner and would remain where it was until starving or being eaten itself.
To quote a BDS guide, since dragonflies unlike other winged insects do not have a pupal stage transition occurs straight from a larva to an adult. This final larval moult takes place out of water being triggered by day length and temperature. Final-stage larvae sit in shallow water near the margins for several days, getting ready for their final moult and starting to breathe air. Most species leave the water during the morning, other than hawkers that do so under cover of darkness.
Larvae climb up emergent vegetation although some may walk several metres over dry land before finding somewhere suitable to emerge. After finding a secure support they redistribute their body fluids, pushing the thorax, head, legs and wings out of the larval skin. There is then a pause of about 30 minutes to allow their legs to harden enough for the next stage when the abdomen is withdrawn. The wings and then the abdomen are then expanded and start to harden. The whole process that leaves behind the cast skin of the exuvia may last for up to three hours and mortality during it unfortunately can be high.
A successful emergence, how things should be
For a beginner’s guide to finding emerging Clubtail dragonflies and exuviae along the River Thames in Oxfordshire see here. Of the four emergences or newly emerged specimens I have observed at Goring railway bridge in 2017 and again now only one (pictured above) has so far been healthy. So I left site on 24th considering I am due a change in that respect.
Ramblings and Scribblings 