Matt Walker, BBC News 14 Jul 09;
Every year, millions of dragonflies fly thousands of kilometres across the sea from southern India to Africa.
So says a biologist in the Maldives, who claims to have discovered the longest migration of any insect.
If confirmed, the mass exodus would be the first known insect migration across open ocean water.
It would also dwarf the famous trip taken each year by Monarch butterflies, which fly just half the distance across the Americas.
Biologist Charles Anderson has published details of the mass migration in the Journal of Tropical Ecology.
Each year, millions of dragonflies arrive on the Maldive Islands, an event which is well known to people living there.
"But no-one I have spoken to knew where they came from," says Anderson, an independent biologist who usually works with organisations such as the Maldivian Marine Research Centre to survey marine life around the islands.
Their appearance is especially peculiar because the 1200 islands that make up the Maldives lie 500 to 1000km from the mainland of southern India, and all are coral cays with almost no surface freshwater, which dragonflies need to complete their lifecycle.
Anderson noticed the dragonflies after he first arrived in the Maldives in 1983. He started keeping detailed records each year from 1996 and now collates data collected by local observers at other localities in the Maldives, in India and on vessels at sea.
When Anderson compared these observations with those made of dragonflies appearing in southern India, he found a clear progression of arrival dates from north to south, with dragonflies arriving first in southern India, then in the Republic of Maldives' capital Male, and then on more southern atolls.
Each year, dragonflies first appear in Male between 4 and 23 October, with a mean arrival date of 21 October. Dragonfly numbers peak in November and December, before the insects then disappear once more. The insects arrive in waves, with each staying for no more than a few days.
Over 98% of the dragonflies recorded on the islands are Globe skimmers ( Pantala flavescens ), but Pale-spotted emperors ( Anax guttatus ), Vagrant emperors ( A. ephippiger ), Twisters ( Tholymis tillarga ) and Blue perchers ( Diplacodes trivialis ) also appear in some numbers.
The dragonflies then reappear between April and June.
Longest journey
The dragonflies are clearly migrating from India across the open sea to the Maldives, says Anderson.
"That by itself is fairly amazing, as it involves a journey of 600 to 800km across the ocean," he says.
Quite how they do it was a bit of a mystery, as in October at least they appear to be flying against the prevailing winds.
However, in October, and continuing into November and December, a weather system called the Inter-tropical Convergence Zone moves southwards over the Maldives.
Ahead of the ITCZ the wind blows towards India, but above and behind it the winds blow from India. So it seems that the dragonflies are able to reach Maldives by flying on these winds at altitude above 1000m.
But that is not the end of the animals' epic adventure.
"As there is no freshwater in Maldives for dragonflies, what are they doing here?" asks Anderson.
"I have also deduced that they are flying all the way across the western Indian Ocean to East Africa."
Anderson has gathered a wealth of circumstantial evidence to back his claim.
Large numbers of dragonflies also start appearing in the northern Seychelles, some 2700km from India, in November, and then in Aldabra in the Seychelles, 3800km from India, in December.
That matches the slow southerly movement of the Inter-tropical Convergence Zone weather system, behind which winds blow steadily from India to East Africa.
It is also known that Globe skimmers appear in large numbers through eastern and southern Africa.
In Uganda, they appear twice each year in March or April and again in September, while further south in Tanzania and Mozambique they appear in December and January.
Record breakers
That strongly suggest that the dragonflies take advantage of the moving weather systems and monsoon rains to complete an epic migration from southern India to east and southern Africa, and then likely back again, a round trip of 14,000 to 18,000km.
"The species involved breeds in temporary rainwater pools. So it is following the rains, taking sequential advantage of the monsoon rains of India, the short rains of East Africa, the summer rains of southern Africa, the long rains of East Africa, and then back to India for the next monsoon," says Anderson.
"It may seem remarkable that such a massive migration has gone unnoticed until now. But this just illustrates how little we still know about the natural world."
The monarch butterfly is often cited as having the longest migration of any insect, covering around 7000km in an annual round trip from Mexico to southern Canada.
On average, it takes four generations of butterflies to complete the journey.
Anderson believes that the dragonflies survive the ocean flights by gliding on the winds, feeding on other small insects.
They too, take four generations to make the full round trip each year.
He says the migratory paths of a number of insect-eating bird species, including cuckoos, nightjars, falcons and bee-eaters, follow that of the dragonfly migration, from southern India to their wintering grounds in Africa. That suggests the birds feed on the dragonflies as they travel.
"They [fly] at the same time and altitudes as the dragonflies. And what has not been realised before is that all are medium-sized birds that eat insects, insects the size of dragonflies," he says.
Extraordinary ability
"There are earlier records of swarms of Globe skimmers flying out to sea, and at sea," Anderson continues.
"But it was always assumed that those dragonflies were doomed. Which says rather more about our earth-bound lack of imagination than it does about the globe skimmers' extraordinary flying abilities."
From India to Mozambique, the insect world's greatest migrant
A species of dragonfly has been revealed to travel an amazing 12,000 miles every winter – using a tropical weather system to flit between India and Africa
Andrew Buncombe, The Independent 21 Jul 09;
It is known as the globe skimmer or wandering glider, but no one ever knew just how far this remarkable dragonfly could actually travel. Now a British naturalist living in the Maldives has claimed that Pantala flavescens may hold the record for the longest migration of any insect. If it is confirmed, his theory would mean that this dragonfly, which measures no more than 5cm, migrates from southern India to Africa and then back.
"It's an amazing story," said the naturalist, Charles Anderson, speaking by telephone from his home in Male, capital of the Maldives. "But what is beautiful is that the pieces of the puzzle fit together."
Mr Anderson first started thinking about the dragonflies after he arrived in the Maldives in 1983. Every year in October, millions of the creatures arrive in swarms, a phenomenon that is well known to local people and which they say heralds the beginning of the north-east monsoon.
What the naturalist found particularly strange was that the Maldives – a string of more than 1,200 coral atolls located off the south-west coast of India – possessed only a tiny amount of fresh water on its surface. Fresh water, rather than salt, is essential for the breeding and life-cycle of dragonflies. Intrigued by the appearance of these creatures, he began collecting data and maintaining records about the dragonflies' arrival and departure.
He discovered that the dragonflies in the Maldives arrived somewhat after similar swarms of the insects appeared in southern India. On the more southerly atolls of the Maldives, they appeared later still. The numbers peaked in November and December.
Mr Anderson believes the dragonflies are heading to southern and east Africa, slowly making their way eastwards on the tradewinds. In the northern Seychelles, around 1,700 miles from India, the dragonflies appear in November. In Uganda they appear twice a year – in March and April and again in September, while in Mozambique and Tanzania they arrive in December.
Mr Anderson, who has published his findings in the Journal of Tropical Ecology, believes the creatures are making the most of the weather system of the so-called Intertropical Convergence Zone, that moves southwards by way of the Maldives every year. It follows those winds at a height of more than 3,000 feet. "Circumstantial evidence suggests that the dragonflies fly with north-easterly tail winds, within and behind the ITCZ," he writes.
The naturalist said he had also collated circumstantial evidence to suggest the dragonflies returned to the Maldives in the spring, en route back to Africa. In all, the journey would total around 12,000 miles and would involve the dragonflies passing through four generations.
Mr Anderson said not only did the dragonflies' journey make use of rain-providing weather systems that would create temporary pools of fresh water for the larvae to grow, but that the life-cycle of the insect had been modified to allow it to make this journey. Whereas the life-cycle of most dragonflies involves a larval stage in which it lives underwater for up to a year, the globe skimmer is able to climb out of the water and metamophosise within just six weeks. "These guys take a different approach to the typical dragonfly," he said.
As they make their way eastwards to Africa, the dragonflies attract company. Mr Anderson says that many medium-sized migratory birds including falcons, cuckoos and nightjars, make a similar journey as they head for their wintering grounds. He said that these birds probably flew at about the same altitude as the dragonflies, made use of the same winds and ate the insects as they went. The significance of the birds' journey had not previously been noticed, he said.
While remarkable, the journey postulated by Mr Anderson is not without precedent in the insect world. For years, there was a mystery about monarch butterflies, which are found in large numbers in the north-east of the US and southern Canada during the summer. It was not until 1975 that scientists were able to confirm that – unable to over-winter in a northern climate – the butterflies migrated south to Mexico. Again, this remarkable journey of up to 4,500 miles takes place over four generations of the insect.
Professor Karen Oberhauser, a monarch butterfly specialist at the University of Minnesota, said that when the monarch's journey was confirmed, people were astounded. "It was beyond comprehension that they could do it," she said.
While she said she had not yet studied Mr Anderson's theory in detail, she was ready to keep an open mind. "Animals and plants do amazing things," she added. "This is another example of wonderful things they can do to evolve to make their way in the world."
Great migrations: Birds and butterflies
*The odyssey of the globe skimmer will take an honoured place in the annals of epic migratory journeys. It is already clear that dragonflies are capable of travelling enormous distances, for in September 1988, the green darner, an American dragonfly of the hawker family, succeeded in crossing the Atlantic.
In fact, several green darners were found in the Isles of Scilly and Cornwall, in the aftermath of hurricane conditions on the east coast of the US.
Britain is also the scene of major butterfly migrations for, each year, many thousands of painted ladies and clouded yellows migrate northwards to us from their breeding grounds in southern Europe and northern Africa. May this year witnessed what may have been the greatest painted lady migration ever recorded, with millions of the insects – perhaps between 10 and 20 million – pouring into Britain.
The longest aerial migrations, however, are still those of birds. The Arctic tern travels every winter from Britain to the Southern Ocean and sometimes even reaches Australia – a round trip of more than 22,000 miles.
The longest single non-stop journey is believed to be that of the bar-tailed godwit, pictured, a wading bird, which has been shown to fly 8,000 miles across the Pacific from Alaska to New Zealand in a continuous uninterrupted flight lasting eight days. When the godwits arrive and land on the coast near Christchurch, the Christchurch cathedral bells are rung.
Michael McCarthy
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