Every October, the globe skimmer dragonfly, Pantala flavescens, migrates from India to Africa via the Maldives in the Indian Ocean. The nonstop journey from the Maldives across the Indian Ocean to Somalia is 2500 kilometers.
The dragonfly is just a few centimeters long so for its body size, this is the longest nonstop journey of any creature.
This extraordinary feat has long puzzled biologists who wonder how such a small creature can endure over such long distances. Various researchers have debated the role of wind, rain and life cycle on this phenomenon. But in the absence of sensors small enough to track insect flight over these distances, nobody has settled the question of how P. flavescens achieves this feat.
Enter Kumar Sanat Ranjan and colleagues at the Indian Institute of Technology in Kharagpur in West Bengal. This group has brought together all these factors along with a detailed study of dragonfly endurance to work out how long they can fly, what factor the wind must play and the annual timing of the migration.
In this way, they show how P. flavescens perform their extraordinary migration, why it is only possible at a certain time of the year and how the dragonflies become so widely dispersed across Asia.
At the heart of the work by Ranjan and colleagues is their model for studying the energetics involved in dragonfly flight. Their model includes factors such as lift, drag and energy involved in flying with wings beating at a certain frequency. It then compares it with the chemical power the insect has available from onboard fuel storage.
“Energetics calculations reveal a P. flavescens can endure 90 hours of steady flight at 4.5m/s,” say Ranjana and co.
That’s an important finding that suggests the migration is not possible without help. “The prevailing winds play a pivotal role; a direct crossing of the Indian Ocean from Africa to India is feasible with the Somali Jet, whereas the return requires stopovers in Maldives and Seychelles,” say the team.
The team analyzed wind patterns throughout the year and found that they only became favorable for a crossing from India to Somalia after September with the greatest probability of success in December.
Indeed, that’s exactly what entomologists observe. P. flavescens arrive in Somalia in November and December having travelled from India via the Maldives and the Seychelles. The researchers say that the prevailing wind means that each leg of the journey can be done in under 90 hours.
The return journey is assisted by the Somali Jet, which sends moisture towards Asia for the Summer monsoon. Again this becomes possible within the 90-hour endurance limit.
Ranjana and co say that more than one route satisfies the conditions for flight. That probably explains why the routes dragonflies take are not always the same and why they end up elsewhere in Asia too — the same pattern of flight, wind, moisture and other factors can send them to entirely different places.
That’s interesting work that shows how important the timing of the journey must be. The dragonflies can only make the journey when the winds at their chosen altitudes are strong enough and in the right direction.
Even then, this can send them to far-flung destinations. And yet a huge number make the journey from India to Africa, presumably when the triggers to fly are exactly right. That raises interesting questions about the way climate change will influence migratory patterns, probably not always for the best.
The research leaves numerous other questions unanswered, such as how the dragonflies know when the winds are optimal for the journey, how they navigate to tiny islands in the Indian Ocean along the way and most puzzling of all, how this knowledge is passed from one generation to the next, since the same flight patterns occur every year.
Researchers have recently made Important progress in understanding how birds navigate so accurately. But dragonflies must have similarly capable navigation systems to achieve their feats. For now, these mysteries remain unsolved.
Ref: Transoceanic migration of dragonflies and branched optimal route networks : arxiv.org/abs/2206.11226