Here be dragons!

This year I'm taking a special interest on dragonflies. They are a relatively small group, species are easy to identify in the field and they offer a range behaviours relatively easy to watch. In July I atended an FSC course on identification of aquatic macroinvertebrates and I immediately wanted to learn to identify dragonfly and damselfly larvae. A few days after the course I visited my local wildlife garden and went to the pond to check invertebrates. Something on the leaves of an aquatic plant called my attention. It was a dragonfly exuvia, the moulted skin of a nymph as the adult dragonfly emerges! 

My first dragonfly exuviae.

Dragonflies have incomplete metamorphosis, which means they lack a pupal stage. The dragonfly eggs hatch into nymphs which grow though several stages as aquatic predators. Nymphs lack functional wings or reproductive organs. The final nymphal stage will creep out of water and moult into an adult. There wasn't just a single exuvia though, there appeared to have been a mass emergence of Common Darter dragonflies and I counted over 50 of them. 

Several visible exuviae on iris leaves (13/07/17).

These are all the the exuviae I could reach from one end of the pond.

A close up of one of the exuvia looking quite intact. The adult emerges though the thorax.

The following week I had the chance to pop in the wildlife garden one morning. I checked the leaves for more exuviae and I was surprised to find two emerging dragonflies!

A Common Darter just emerging, head down, showing how the old nymph skin holds it to the leaf. On the top shot, one already drying its wings, folded back and still lacking pigmentation (both photos on 21/07/17).

This fresh adult male Common Darter, perhaps from the first emerging batch, was sunbathing by the pond.

Common Darters

There were at least two male Common Darters, Sympetrum striolatum, around the wildlife garden pond today. If you move slowly, they are great posers and allow very close approximation. Their faces are locked into a perpetual smiling grimace, adding to their charm. If you watch them from a close distance,  you'll notice that they are constantly flicking their heads around, looking for insects flying overhead. If a suitable one is detected, they dart off, returning to their perch to eat their prey.

The ever smiling darter

A short clip of the darter watching for prey.

Migrant Common Darters

At this time of the year, my garden - which hasn't got a pond - hosts occasional dragonflies as they migrate to warmer climates. The most regular one is the Migrant Hawker, which often hunts in small parties 2-3 m high, occasionally stopping to rest hanging from branches. Rarer species are the Darters. In the last few days, I have watched female Common Darters, Sympetrum striolatum, resting and hunting. The first day it banked atop my parasol and I couldn't get good shots. Today I was luckier and the darter sat on the fence. With the help of my new Canon Powershot G12 flip out LCD display, and perched on a chair, I could hold the camera high enough to get its portrait.

This species, as other darters, is known to migrate regularly. David and Elizabeth Lack reported their pass south in the Pyrenees together with migratory birds, butterflies and hoverflies in October 1950 and reviewed the information available on dragonfly migration until then. Today, although the morning was warm with sunny spells, a large front with thunderstorms passed through all the afternoon, so it could be that the dragonflies were moving away from the coming rain. 

Common Darters are known to form occasional irruptive, massive migratory swarms, although the reason for these are not well known. A large irruptive swarm of this species was reported in Ireland in August and September 1947 by Cynthia Longfield from the British Museum. She compiled the letters of many observers amazed by the sudden, unprecedented invasion, including interviews with schoolchildren, which helped document the spatial extent of the invasion. As, she was visiting Ireland at the time, she identified the dragonflies and further documented the invasion with her observations. She concluded that this mass invasion event probably came from Spain and Portugal, as no invasion was reported in France or England at the time. She calculated that the dragonflies must have flown over the sea overnight. It is hard to imagine what being there at such massive influx event would have been:

'They were flying low in compact formation. . . . The flight was orderly, in a northern direction, as if they were intent on reaching some known destination.'
'They seemed more to be drifting with the breeze than flying, but were keeping to a straight course, without zigzagging or undulating, coming in from-the sea to the south and going north. What was so remarkable was the witnessing of the actual arrival and passing of the swarm.'

'They seemed to be coming in from the sea and flying about two feet above the ground, though some were higher, to about eight feet up. All were flying in the same direction, northwards. It seemed a never-ending stream, but we had to move on, and passed out of their track, so I could not tell the duration of the flight. They flew considerably faster than we were walking. The density was about one foot, or rather less, apart.'

For the next day, 3rd September, Mr. Allen writes: 'Tuesday's invasion was nothing to Wednesday. Wind south-west. Before noon they commenced and continued until evening. They not only came singly, drifting as on Tuesday, but in great, thick black masses, like dark clouds. They really frightened the people.'

Here he found 'the people looking out to sea, and I saw a sight which gave me a fright at first. I heard a hum like bees and yet not quite like bees. The next thing the whole sky filled with large flies that resembled aeroplanes, millions of them'

 Cynthia hypothesizes that the reason these dragonflies were emigrating was the drying up of the marshes where they regularly bred. That these dragonflies somehow were prevented from completing their reproductive cycle in the region of origin is illustrated by the fact that many of them paired on arrival, and some were even observed laying eggs on the sea. Although these spectacular irruptions appear to be extremely rare occurrences, they illustrate that the more regular, seasonal migratory behaviour of some dragonflies, can possibly be exacerbated by unusual weather conditions in their region of origin - in a similar way that occurs with Painted Lady irruption years.

More information

Cynthia Longfield (1948). A vast immigration of dragonflies into the South coast of Co. Cork The Irish Naturalists' Journal, 9 (6).
David and Elizabeth Lack (1951). Migration of Insects and Birds Through a Pyrenean Pass. Journal of Animal Ecology, 20, 63-67 DOI: 10.2307/1644

Hovering high

Today I read a very interesting paper by David Lack (of fame as an ecologist and evolutionary biologist for his bird studies and his books, of which 'The Life of the Robin' is my favourite). He described his observations on a day of October on top of a high Pyrenean pass. In addition to various flock of migrating birds, he accounts in detail how large numbers of butterflies (mainly Red Admirals and Clouded Yellows) and Common Darter dragonflies (Sympetrum striolatum) steadily crossed the pass towards the Spanish side. That birds migrated had already been well established, but insect migration was then a much more recent, still controversial phenomenon.Lack felt provileged to have witnessed such extraordinary phenomenon, and the feeling was probably enhanced by the beauty of the surrounding mountains, at 2227 m altitude. He wrote "Once in a lifetime perhaps, the ecologist is translated back into a naturalist, through chancing on a spectacle which combines grandeur with novelty. Such was our fortune at the Port de Gavarnie on 13 October 1950."

Cirque du Gavarnie (Photo by Marando-fr, Wikipedia Commons)

Lack's was the first account of hoverflies migrating. In his own words:

Not until we had been at the pass for over an hour did we realize that another insect was migrating, so we do not know whether it was passing from the start. This was a hoverfly, the two collected specimens being males of Episyrphus balteatus. All the other individuals looked the same, and we have no reason to think that any other species was present. The syrphid flies, like the dragonflies, flew steadily forward WSW. against the wind, keeping extremely close to the ground, where the wind was, of course, weaker. Very occasionally one settled for a moment, but otherwise they travelled steadily onward. An estimate of numbers was impossible, but at a guess they were at least twenty times, and perhaps a hundred times, as numerous as the dragonflies. They were the most remarkable migrant of all.

Today there were many E. balteatus in the garden. To think that such tiny, flimsy looking hoverfly, is able of such powerflul climbs and long voyages! It is just plainly amazing.

Episyrphus balteatus on Cat's ear (summer 2006)

A close up. Today, on an ice plant.

Reference
Lack and Lack (1951) Migration of insects and birds through a Pyrenean pass. Journal of Animal Ecology 20:63-67.