Island Conservation-Longest insect migration route may include Seychelles |20 July 2009

“Pat, have you seen the dragonflies?” I turned to see where the SBC’s Sports Editor, Patrick Hoareau, was pointing. A good twenty or so large, masala-coloured dragonflies were flying around slightly to our left. At first it seemed like a confused “milling around in the air” sort of flight. But as we watched more closely we could see that the sigal, to use their Creole name, were actually gliding to and fro for all the world as if they were patrolling that specific bit of air space. This is what biologists call “hawking” flight – skimming back and forth to catch the flies and other small insects that dragonflies feed on.

I recognized the orange-yellow sigal that Patrick had pointed out to be the species known as the “wandering glider” or “globe skimmer”, found throughout tropical, subtropical and even temperate regions around the world – indeed, it is the only dragonfly species with such a wide geographic range. For several minutes we marveled at their absolute manoeuvrability as they hovered – “on stairways of air”, to quote one poet - appearing to remain stationery in mid-air for some time, before darting away and back once again.  I remembered reading that, for this, they beat their wings at 39 times a second!

Indian Ocean route

All this came back to me this week as I leafed through a paper by biologist Charles Anderson in the Journal of Tropical Ecology. Based in the Maldives, he has been studying the movements of globe skimmers for the past thirteen years. He now believes that there is enough evidence suggesting that every year millions of these dragonflies cross the Indian Ocean from southern India to the Maldives and westward to Africa, at least some of them transiting via Seychelles along the way – then presumably back again, a round trip of 14,000 to 15,000 kilometres, which would make this the longest migration of any insect.

Anderson says the records he has collected of when the dragonflies appear in different places along the southerly route show a definite progression of arrival dates: October in the capital of the Maldives, Male; November in the granitic islands of Seychelles; December in Aldabra; December and January in Tanzania and Mozambique.

Even before the latest findings the globe skimmer was renowned for its ability to fly long distances. Moreover, it can fly up to over 6,000 metres high (seven times the height of Morne Seychellois), the highest that any dragonfly species can go. Anderson believes the dragonflies can cross the western Indian Ocean southward by flying, at an altitude of over 1,000 metres, on the winds that accompany the weather system called the Inter-tropical Convergence Zone as it moves towards the south.

The globe skimmer is known to breed in temporary rainwater pools (you may have seen them in Victoria and elsewhere mistaking car windscreens for pools of water, an example of an ecological trap). Presumably, then, it follows the rains, making use of the north-west monsoon (vandnor) rains in Seychelles, for example. It would survive its long trans-oceanic flights by riding on the winds and feeding on small insects travelling in the same manner. The dragonflies themselves are probably preyed upon by insect-eating birds, such as the blue-cheeked bee eater, that follow the same migratory path – indeed, this striking bright green bird is often seen catching dragonflies during its stopovers in Seychelles in November-December and March-May.

Link

The evidence for dragonfly migration across the western Indian Ocean has been there all along. But no-one had picked up the clues before Charles Anderson pieced them together in an elegant and convincing manner. “This just illustrates how little we still know about the natural world,” he says.

And, of course, it is exciting to think that the islands of Seychelles could be a vital link in the migratory chain of the globe skimmer dragonflies. I suppose biologists and conservationists here will be all fired now to collect data on when these and other migratory dragonflies appear in numbers. Poets are no longer alone to watch as

La frissonnante libellule
Mire les globes de ses yeux
Dans l'étang splendide où pullule
Tout un monde mystérieux !
(Victor Hugo)

by Pat Matyot

The Island Conservation Society promotes the conservation and restoration of island ecosystems.