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The Globetrotting Dragonfly That Follows the Monsoon

The Globetrotting Dragonfly That Follows the Monsoon

A male globe skimmer in Kadavoor, Kerala, May 2017. Photo: Jee & Rani Nature Photography, CC BY-SA 4.0

  • The globe skimmer is a dragonfly found around the world and is known to travel great distances. And it weighs less than a gram.
  • Controlling for body weight, its migrations around the planet are the longest non-stop migrations by any living thing.
  • A new study suggests the only way for globe skimmers to make these journeys is if they actively seek out the intertropical convergence zone.
  • Remarkable though it is for this tiny insect to be able to seek out favourable winds, the findings – if true – will have many ecological implications.

The west coast of India has for many successive years been ravaged by unseasonal rains, and many cities have suffered lasting damage. The Arabian sea and the Indian Ocean, historically calm facilitators of trade, exchange of culture, cuisine and language, have been reporting stronger cyclones and storms every year.

Among the many humans that occupy the coasts of these water bodies is also a truly impressive insect that weighs less than a single gram. The globe skimmer (Pantala flavescens) is an aptly named dragonfly. It is found around the world and is known to travel great distances. It crisscrosses the same waters that spice merchants have for centuries. And controlling for body weight, its journeys are the longest non-stop migrations by any living thing.

A new study, published in August 2019, suggests the only way for globe skimmers to make these journeys is if they actively seek out the intertropical convergence zone – the buoyant area over the world’s oceans where the northeast and southeast trade winds converge. As remarkable as it is for a dragonfly so small to be able to seek out favourable wind patterns, if the findings are true, they will have many ecological implications.

Insects regulate pests, pollinate crops, decompose organic matter and are an important food source for vertebrates. And as apex predators of the insect world, dragonflies are crucial for controlling mosquito populations. The global decline of insects is being driven by many forces – from changing temperatures to environmental toxins. Around the world, hoverflies, locusts, butterflies and moths take to the skies and migrate long distances. These migrators are particularly vulnerable to changing temperatures, wind patterns and the rains, but – with notable exceptions – they remain less than well-studied.

In 2009, Charles Anderson, a marine biologist and an “old fashioned naturalist” in the Maldives, documented the arrival of globe skimmers to Male in October and their departure over November and December. Male, Anderson hypothesised based on independent observations, could be a stopover for globe skimmers destined for Africa’s east coast.

To find out where they were coming from, Anderson collaborated with Keith Hobson at the Western University, Canada, to examine globe skimmers collected in the Maldives between October and November. These insects go through their early life stages underwater, which means their wings remain unchanged during the adult phase of their lives. The wings carry chemical signals of their place of origin in the form of stable hydrogen isotopes. Studying these isotopes allowed Anderson and Hobson to generate a probability map of where the Maldivian globe skimmers may have originated. The most likely location was the northern Indian subcontinent.

This was a remarkable discovery: even though the globe skimmers’ onward migration to Africa was yet to be confirmed, the trip from northern India to Maldives is already 400 km over the ocean. How do these dragonflies do it?

That is what Johanna Hedlund at the Exeter University in the UK set out to investigate. In comparison to the migratory birds she has studied in the past, “globe skimmers are quite small, only weighing about 300 mg,” Hedlund said. “And like many other insects, this makes them impossible to track with tags that have heavy batteries, for example tags that can produce GPS positions. Thus, their migration has to be studied in other ways, for example by the use of stable isotope analysis, radar, observations or modelling.”

Resting globe skimmers, probably during migration, in Talakaveri, Coorg. Photo: Shyamal/Wikimedia Commons, CC BY-SA 3.0

A closer look at the anatomy of globe skimmers, their wings and flight muscles shows that they are well adapted to gliding for long periods of time. Unlike active flying, gliding only requires them to spend a little more energy than they might at rest. Hedlund and her group used their knowledge of dragonfly physiology and calculated that a mixed gliding-active flying strategy would allow globetrotters to stay in the air for 230.45 hours before they ran out of energy reserves.

They then picked three equally spaced take-off points: Dwarka (Gujarat), Vasco da Gama (Goa) and Male (Maldives). Their closest arrival sites across the ocean were Ras Al Jinz (Oman), Ras Madrakah (Oman) and Kap Hafun (Somalia), respectively. At a reasonable speed of 1-2 m/s, the globe skimmers would only make the shortest of the westward crossings, from Dwarka to Ras Al Jinz.

So Hedlund concluded that “the globe skimmer needs wind assistance in order to succeed in crossing the Indian Ocean. To achieve such a feat, an individual would have to be able to select favourable winds, which appear at certain time windows in the Indian Ocean region.”

In 2009, when Charles Anderson observed the October influx of globe skimmers over the Maldives, he noted that the surface wind was actually in the wrong direction. Favourable winds could only be found much higher in the atmosphere, and that, Anderson surmised, is where the insects must be flying. These winds are associated with the intertropical convergence zone.

Around the equator, as hot air rises, it flows towards the poles, leaving behind an area of low pressure. Winds just north and south of the equator rush into this area, creating the intertropical convergence zone. Winds blowing towards the convergence zone from the northeast in October-November bring monsoons to East Africa and those blowing from the south west bring India’s summer rains.

The dragonfly swarms that Anderson observed in the Maldives arrive at exactly the time these winds are blowing towards East Africa, at 1,000-2,000 meters above sea level. Many species of birds, including Amur falcons, the enigmatic Indian rollers and the insect-eating bee eaters, also use these winds to cross the Indian Ocean.

Now, could these monsoon winds suffice to carry the globe skimmers across the Indian Ocean? Hedlund turned to her collaborators Gao Hu and Hua Lv. “They used available, actual meteorological data and analysed it using an algorithm that Gao Hu had developed,” she said. Her team then incorporated this analysis into their calculations.

(The new study was coauthored by Hedlund, Lv, Philipp Lehmann, Hu, Anderson and Jason Chapman.)

Starting from any of three eastern locations, globe skimmers could indeed make all their journeys if they travelled these winds 500-1,500 metres above ground. The autumn wind trajectories touched land in Africa between October and December – when dragonflies have been observed to arrive.

For eastward migrations from Kenya, the winds could take the insects across in 55 hours, well below their physiological limit, and most eastward wind trajectories hit land in June and July coinciding with the observed arrival of dragonflies and the Indian monsoon.

The intertropical convergence zone is visible as a band of clouds encircling Earth near the equator. Image: GOESS-11/NOAA, NASA

If globe skimmers are actively seeking favourable winds to make transoceanic migrations, the question arises: why? Evolution seems to have landed on a strategy that maximises their potential to reproduce. Unlike other dragonflies, the underwater stage of globe skimmers can be as short as six weeks, which means they can breed in temporary pools of water produced by wet seasons.

For a dragonfly larva (or nymph), that means not getting eaten by bigger predators like fish, and themselves feasting on mosquito larvae growing in the same pools. And before these pools dry out, the dragonflies could emerge and follow the winds that are always converging where the rain is.

The complete circuit, according to Anderson, could mean three breeding seasons in the monsoons of western India, East, and South East Africa. Over four generations, the globe skimmer could complete a circuit 16,000 km long.

Scientists need more work to confirm the new findings and to fully understand the globe skimmer’s life cycle. According to Hedlund, “using stable isotope analysis could reveal more about the hypothesised spring migration from Africa.”

Many observations that undergird insect studies of this kind are driven by are driven by nature enthusiasts and citizen scientists, and for Hedlund, they can be helpful for further study of the timing “of globe skimmer movements in the Indian Ocean region.” She receives samples of dead globe skimmers from citizen scientists across the world and urges people to get in touch with her if they think they can help.

If the study’s findings hold up, they could mean there are seasonal highways high up in the sky, carrying birds and insects across continents, connecting the biological world in ways we have only begun to understand.

Some research already suggests that the intertropical convergence zone itself could be changing as the planet warms. How dragonflies and other migrators will be affected by this also needs further research. As citizen scientists, nature enthusiasts and professional scientists pool their knowledge together, we gain – as biologist E.O. Wilson said – “a more complete and productive understanding of how to care for the life that we’ve inherited.”

Siddhant Pusdekar is a candidate at the University of Minnesota’s Ecology, Evolution and Behaviour PhD programme.

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