Marine biologist Charles Anderson saw swarms of globe skimmer dragonflies landing at the Maldives during autumn before they took off towards East Africa. Back in 2009, these observations led Anderson to an interesting proposal: the migratory globe skimmer dragonfly, which is only five centimetres long, flies seasonally from India to East Africa by crossing the Indian Ocean and then journeys back.
It is believed that the dragonflies ride on high-altitude seasonal winds to cross the ocean. Taking into account the dragonfly’s tiny body size, this voyage would be the world’s longest regular non-stop migration – surpassing the distance travelled by the famed Monarch butterflies in North America.
But this epic journey is only a small part of a giant annual migratory circuit. It is believed that over several generations, globe skimmers fly between India, East Africa and Central Asia covering more than 14,000 km. Scientists are only beginning to unravel the details of this circuit and how this tiny dragonfly makes it across the Indian Ocean.
“It is not ‘a migration’, it is a whole web of movements,” notes Anderson, who has studied the arrival of globe skimmers at the Maldives.
While the migration of Monarch butterflies is well studied, relatively little is known about the annual migratory cycle of the globe skimmer dragonfly (Pantala flavescens). Because of their migratory behaviour, globe skimmers or wandering gliders, as they are also known, are distributed worldwide except for Europe and the polar regions. A 2016 genetic study found high rates of gene flow among populations around the world suggesting all populations can interbreed owing to their high dispersal abilities.
“There is quite likely only one population of globe skimmers – a global population,” Anderson said. “That is not confirmed, there is some uncertainty. It may be that there are two populations and if that is the case, it is likely to be the Americas and the rest of the world.”
“They are all mixing up,” Anderson said.
Johanna Hedlund, a behavioural ecologist at Lund University said, “The globe skimmer is much lighter than the Monarch butterfly, weighing around 300 mg whereas the Monarch weighs around 600 mg.”
“The Monarch migrates mainly over land, and what makes the globe skimmer migration from India so incredible is the hypothesis that it regularly flies over open ocean without landing,” Hedlund said.
In the nineteenth century, seafarers encountered the migrants hundreds of kilometres offshore. Since then, individuals were noted on remote islands in the Indian and Pacific Oceans leaving scientists intrigued about their migratory behaviour. Mountaineers in the Himalayas have spotted globe skimmers flying over 6,000 metres. “Extraordinary, they can fly very high,” said Anderson.
Unlike some other migratory insects that are pests, globe skimmers have a positive effect on human health and agriculture, notes Anderson. “They do not harm people,” Anderson said. “They do not eat crops. They do not spread diseases.”
“They eat other insects such as mosquitoes,” Anderson said. “As larvae, they eat mosquito larvae and as adults, they eat adult mosquitoes and other insect pests.”
Belonging to an ancient group called Odonates (which includes damselflies), around 5,000 species of dragonflies are estimated to exist. The earliest fossils of an ancestral group of huge dragonflies go back to 325 million years.
Globe skimmer migration
Flying across the open ocean is no ordinary task for a tiny insect the size of a globe skimmer. It requires extreme endurance and many perish during the trip. So, why do swarms of dragonflies undertake the perilous journey?
Compared with other dragonfly species, globe skimmers have an unusually short larvae period, reaching from egg to adult in 5 weeks to 7 weeks. Most other dragonfly species spend a large part of their life, a minimum of 10 months, as larvae – years, in some cases.
“For a dragonfly larva to emerge after just five, six, seven weeks in the water is actually quite remarkable,” Anderson notes. “They can do that because they specialise in [laying eggs] in warm, temporary water pools.”
“By laying its eggs in a temporary pool, which by its nature is likely to be shallow, and in the tropics, it will be warm … so [globe skimmer larvae] can grow faster,” explained Anderson. “And because there are no long-lived predators, [larvae] do not spend their life hiding under rocks and branches and holes, they are actively out hunting for food, they can consume more, they can eat more, and because it is warm, they develop more quickly.”
By the time larvae emerge as adults, it is likely that the monsoon is over or retreating. But the flies need pools of fresh water for breeding. In search of ephemeral rain pools, the dragonflies “go where the winds take them”, Anderson explains. “The potential range is so huge that if you get up into the air and drift, carried by the winds, then on average you will end up somewhere suitable.”
Globe skimmers peak in abundance in India post-monsoon, between September to December, after which they disappear. When the monsoon in India has retreated, the dragonflies are proposed to take off to equatorial East Africa where it rains have moved to. Another generation is thought to arrive in India from East Africa in spring.
While flying across the open ocean, globe skimmers encounter many challenges. How do these tiny insects accomplish this feat? They have acquired morphological, behavioural and physiological adaptations for long-distance flights.
“The globe skimmer is perhaps the ultimate dragonfly flying machine and therefore the assumption is that its wings must be the very pinnacle of perfection in terms of evolutionary design,” said Anderson. “So, it is assumed that the lumps and bumps, the corrugations on the globe skimmer wing are fine-tuned to promoting lift and reducing drag and these corrugations must play some part.”
“We also know that dragonflies that migrate long distances, and in particular glide, which is a mechanism for reducing fuel consumption and therefore increasing potential range, have a particularly enlarged hind wing at the base near the body so the wings are very triangular with a pointed tip,” Anderson explains.
Sanjay Sane, professor at the National Centre of Biological Sciences, Bengaluru, who studies flight in insects, said, “One would expect that dragonflies have mechanisms that lock their wings and other body parts in place so that there is no energetic expenditure in actively maintaining their wing or body postures over long periods of flight.”
“The wings will also need to be structurally robust,” Sane said.
Riding the wind
The Indian Ocean crossing is thought to be completed by one generation in autumn and in spring covering between 900 km to 2,400 km depending on the starting point. However, the arrival of Indian globe skimmers in Africa in autumn and African globe skimmers in India in spring has not been confirmed yet.
Globe skimmers are believed to follow a path where high-altitude trade winds converge, known as the Inter-Tropical Convergence Zone, which shifts north and south seasonally. It appears as a band of clouds consisting of rain showers in the tropics.
“The Inter-Tropical Convergence Zone is extraordinarily important for the story of the globe skimmer,” stresses Anderson. A newly-emerged adult dragonfly needs to find where the rain is because that is the habitat needed for her offspring. So, it would fly up and, under the influence of the Inter-Tropical Convergence Zone, would be carried by the winds. On average, the winds will take it to where it is raining, Anderson explained.
Sane proposes that “the powered flight could then just be used by dragonflies to rise up in altitude until they get caught in these winds, and then to descend whenever they are in sight of a major landmass”.
Because of their small size, tagging the flies with transmitters as is done in birds to track their movement is not possible. Instead, scientists have to rely on tools such as radar, modeling and naturally-occurring stable isotopes in their wings to map their migration.
So, can globe skimmers fly across the Indian Ocean?
To answer this question, Anderson and Hedlund’s team estimated how long a globe skimmer can remain airborne using only its fat reserves. The researchers also used wind trajectory analysis to determine if favourable tailwinds are available to transport these tiny migrants across the Indian Ocean both in spring and autumn.
Two flight modes were considered: active flapping and gliding flight. The former relies entirely on burning fat reserves, which expends a lot of energy, while the latter consumes much lower energy. Migration events from different sites from western India in autumn and from Africa in spring were simulated.
The findings reveal that globe skimmers cannot cross the Indian Ocean relying solely on their own power (active flapping) as they will quickly deplete their energy reserves before making it across. Even by using a mixed strategy consisting of gliding and active flapping, the globe skimmer dragonfly can only complete the Indian Ocean crossing at the shortest distance with an exceptionally high gliding speed (9.4 km/hr) and little headwinds.
Since this scenario is unlikely, favourable tailwinds are crucial to transport the flies across the ocean. Such winds are more available in spring from Africa to India than in autumn from India to Africa. In autumn, only 15% of the simulated migrations from India to Africa were successful, taking on average 127 hours, whereas in spring 41% of simulations from Africa to India took only 55 hours.
“Migration in September [India to Africa] is very difficult, whereas migration in November-December [India to Africa] has many more favourable winds available. Autumn migration from the Maldives appears difficult whereas autumn migration from Gujarat seems easier. Spring migration from Africa has the most opportunities in June-July, explained Hedlund.
“A globe skimmer aiming to succeed at migrating over the Indian Ocean thus needs to have the adaptation to be able to select favourable winds,” especially during autumn, Hedlund concludes.
Keith Hobson, an ecologist at the University of Western Ontario in Canada, is not surprised by these findings. “It has been clear for a long time that for this crossing and multigenerational seasonal migration to exist in the first place, use of seasonal winds aloft is critical.” He added that “this is the first time an energetics flight simulation was conducted and “provided a more rigorous argument based on physiology”.
Sane of the National Centre of Biological Sciences found the study very interesting “because it suggests some new, testable hypotheses such as which locations and seasons to find the flyways, as well as perhaps testing the insects in the lab for the percentage of active flying to gliding”.
Landing at Maldives
Anderson observed masses of globe skimmer dragonflies landing at the Maldives, 430 km from India, from October to December and a small number in spring. But there are no freshwater pools in the archipelago for breeding. In 2012, his team traced the origin of globe skimmers in the Maldives to northern India using evidence from stable hydrogen isotopes in the wings of the flies. This suggests that the flies are travelling a distance longer than the crossing of the Indian Ocean.
So, why do the dragonflies land in the Maldives?
Scientists do not know yet but offer some likely possibilities.
The global population of globe skimmers are all being swirled around by the winds, said Anderson. “Some from India will follow the wind and it happens to take them past the Maldives, so they will come down and have a look and find that it is pretty useless and so they go up again and carry on and it so happens that the winds will take them all the way to East Africa.”
Hobson said, “If the dragonflies are coming from northern India, it begs the question whether or not these dragonflies are using a refuelling strategy at stopover sites in much the same way we see in birds.”
Hedlund also said, “we do not know whether arriving in the Maldives is an adaptation somewhat similar to the stop-over behaviour of birds, and that the landing on the Maldives improves survival or whether individuals that have taken the route over this archipelago are already ‘off course’ and will suffer high mortality.”
Climate change threat
Since globe skimmers need seasonally favourable winds to migrate over the Indian Ocean, any disruption to wind patterns could affect their migration. “If the movement of the Inter-Tropical Convergence Zone is sped up or delayed or advanced by climate change, then that could have an impact,” said Anderson. On the other hand, if climate change affects rainfall, Anderson believes it could seriously affect the dragonflies.
“Many insects that are considered pests (rice planthoppers, the fall armyworm) are migratory, and it is believed that their reactive mobility enables them to disperse widely and utilise new resources where and when these resources peak,” said Hedlund. “Dragonfly adults and nymphs hunt and eat many of these pests, and so it is of great importance that we do not disrupt the movement of these beneficial, predator insects.”
Many bird species could also be affected. Amur falcons, for example, also migrate from India to East Africa and its route is presumed to coincide with that of the globe skimmers. It is thought that they co-migrate along with the globe skimmer and may even prey on them en route. “If any of these birds are relying on in-flight re-fuelling – relying on the presence of globe skimmers to feed en route – and if climate change disrupts the migration for the dragonflies, then yes that would have a profound impact on the success of the crossing,” said Anderson.
He will be looking at trends in the migration dates of globe skimmers. In the future, Anderson hopes to untangle the wider picture of the migratory circuit of globe skimmers using genetics and stable isotope analysis. “There are plenty of things to be done. It is a never-ending story.”
This article first appeared on Mongabay.
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