Darwin knew why insects would give up their ability to fly

A new study from Monash University has revealed that nearly half of all insects that are native to the Southern Ocean Islands have become flightless over time. On some of these remote islands, almost all of the insects have lost the ability to fly.

When the researchers investigated the potential factors driving this flightlessness, they confirmed a 160-year-old theory by Charles Darwin that flightlessness evolved in response to wind. 

“Of course, Charles Darwin knew about this wing loss habit of island insects,” said study lead author and PhD candidate Rachel Leihy.

“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.”

Many scientists, including Hooker, questioned Darwin’s theory on flightlessness and simply believed that he got it wrong. However, most of these debates have overlooked the sub-Antarctic islands where flightlessness is evolving the most rapidly, and these are 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 Leihy. The researchers set out to analyze every idea that has been proposed to explain flight loss among insects.

“Terrestrial species on islands often show reduced dispersal abilities. For insects, the generality of explanations for island flight loss remains contentious. Although habitat stability is considered the most plausible explanation, others are frequently highlighted,” wrote the study authors.  “Adopting a strong inference approach, we examined the hypotheses proposed to account for the prevalence of flightlessness in island insect assemblages, for a region long suspected to be globally unusual in this regard – the Southern Ocean Islands (SOIs).” 

“Combining comprehensive faunal inventories, species’ morphological information, and environmental variables from 28 SOIs, we provide the first quantitative evidence that flightlessness is exceptionally prevalent among indigenous SOI insect species.”

The study showed that no other ideas besides Darwin’s wind theory could explain the extent of flight loss in sub-Antarctic insects. 

“The outcomes redirect attention to Darwin’s wind hypothesis. They suggest, however, that wind selects for flightlessness through an energy trade-off between flight and reproduction, instead of by displacement from suitable habitats,” explained the researchers.

Their findings suggest that windy conditions make insect flight more difficult and energetically costly. As a result, insects stop investing in flight and invest more in resources for survival and reproduction.

“It’s remarkable that after 160 years, Darwin’s ideas continue to bring insight to ecology,” said Leihy. 

Study co-author Professor Steven Chown concluded 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.

The study is published in the journal Proceedings of the Royal Society B.

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By Chrissy Sexton, Earth.com Staff Writer