Journeys Environment

Butterfly migration through a metropolis

It is migration time for the Common emigrant. Photo: M. Periasamy  

This particular afternoon in mid-October was like any other in Bengaluru. The arterial NH44 was choked with bumper-to-bumper traffic inching towards the city centre and back. But that day, there was another kind of restless traveller on the highway. A kaleidoscope of thousands of tiny, powdery yellow wings filled the air: the common emigrant butterflies were migrating. And no one quite knows where they came from or where they were headed to, because research on this species has just begun.

A week later, I was standing on top of a building at the National Centre for Biological Sciences (NCBS) with Sanjay P. Sane, an associate professor at the Centre who specialises in the physics, neurobiology and ecophysiology of insect flight. The sky was blue, a gentle breeze blew as we waited to spot one of these colourful clouds now so ubiquitous across the city skies.

Occasionally, when the sun shone through the clouds, we could spot a few emigrant butterflies flitting above a cork tree in full bloom. Their behaviour was distinctly different from the solitary white common albatross that hovered about, settling every now and then on a white flower. The emigrants moved quickly, in the same direction, without resting. Sane explained: “Migrating butterflies come out when it’s sunny and make use of the breeze when it’s strong. Like other migrating animals, birds and insects, they move instinctively in a straight direction without stopping to eat or mate.”

A feat

While we do not know very much about the migration and behaviour of common emigrants, we know a little more about the blue tiger butterfly, which is also migrating now. Krushnamegh Kunte, an associate professor at NCBS, studies various facets of butterfly biology, the evolution of wing patterns and their migration, including that of the blue tiger.

In a paper published in the Journal of the Bombay Natural History Society he observed the migration of four other species of milkweed butterflies, the blue tiger being one. He explained that these butterflies travel some 500 km to the eastern plains of the peninsular from the Western Ghats, just before the arrival of the south-west monsoon, to escape the torrential rains of the highlands. At the end of the rainy season, their offspring make the journey back to the Western Ghats. Milkweed butterflies are named after the plants of the milkweed family, such as Nerium oleander, on which they lay their eggs and on whose leaves caterpillars feed.

Large animals and birds are known to migrate seasonally over long distances, sometimes multiple times during their lifespan, storing body fat to fuel up. But how does a butterfly undertake this feat given its short lifetime that ranges from a few weeks to a few months?

We know about the sweeping migration of the monarch butterfly in the north American continent. Thousands of these insects migrate from southern Canada and eastern and north-eastern U.S. down to Mexico to avoid the harsh winter, possibly taking cues from falling temperatures, the angle of the sun, the length of day, or decreasing availability of host plants. In spring, they travel back. These short-lived butterflies make this great trans-continental journey of thousands of kilometres over at least four generations.

The Blue tiger. Photo: E. Arun Kumar

The Blue tiger. Photo: E. Arun Kumar  

Vaishali Bhaumik, a Ph.D. student at NCBS studying butterfly migration and body morphology, has compared the thorax and abdomen in migrating and non-migrating butterflies. “The flight muscles in a butterfly are located in the thorax and the reproductive organs are located in the abdomen. Our studies on milkweed butterflies revealed that migrating females had smaller abdomens compared to non-migrating females that were reproductively active.” This suggests that by temporarily halting reproduction during migration, females can spare more energy to move.

City lights

But busy cities such as Bengaluru pose many hurdles in their migration path. Sane says that large buildings can block the path of these delicate creatures, lights can trap them, and they collide with moving vehicles. And loss of green cover in cities means they have fewer trees and bushes to roost on.

As for the city, the butterflies enrich it with their arrival. And this season, it was the common emigrant that connected Bengaluru to the natural world that lies beyond it.

The writer works with Nature Conservation Foundation and lives in Bengaluru.

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Printable version | May 6, 2021 1:23:36 PM |

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