Insects are losing the ability to fly, samples from sub-Antarctic Islands back Darwin’s theory

On the small islands that lie halfway between Antarctica and Australia, almost all the insects have lost their ability to fly.

The flies there walk and the moths crawl.

"Of course, Charles Darwin knew about this wing loss habit of island insects," says PhD candidate Rachel Leihy, from the Monash University School of Biological Sciences.

"He and the famous botanist Joseph Hooker had a substantial argument about why this happens. Darwin's position was deceptively simple. If you fly, you get blown out to sea. Those left on land to produce the next generation are those most reluctant to fly, and eventually evolution does the rest. Voilà."

But 160 years ago, other scientists agreed with Hooker when he expressed his doubt and disagreed with Darwin.

However, almost all of these discussions have ignored the place that is the epitome of flight loss – those 'sub-Antarctic' islands. Lying in the 'roaring forties' and 'furious fifties', they're some of the windiest places on Earth.

"If Darwin really got it wrong, then wind would not in any way explain why so many insects have lost their ability to fly on these islands," said Rachel.
Using a broad, new dataset on insects from the sub-Antarctic and Arctic Islands, researchers at Monash University looked at every idea suggested to account for insect flight losses, including Darwin's idea of the wind.

Reported on December 9, 2020, in Proceedings of the Royal Society B, they show that Darwin was right for this 'most windy of places'. None of the standard theories (such as those put forward by Hooker) clarify the degree of flight loss in sub-Antarctic insects, but Darwin's idea does. But in a slightly different form, in line with new theories about how flight loss actually occurs.

Windy conditions make insect flying more difficult and energetically expensive. As a result, insects avoid investing in flight and its costly underlying machinery (wings, wing muscles) and redirect resources to reproduction.

"It's remarkable that after 160 years, Darwin's ideas continue to bring insight to ecology," said Rachel, the lead author of the paper.

Professor Steven Chown, also from the School of Biological Sciences, added that the Antarctic region is an extraordinary laboratory in which to resolve some of the world's most enduring mysteries and test some of its most important ideas.