Insects are an incredibly diverse and diverse group that make up more than half of all known life and describe more than a million species. Many are social while others are lonely.
Most can fly while some had this ability but lost it at some point in the past, especially on islands. When Charles Darwin noticed this trend, he speculated that it was for a very simple reason: so that the insects would not be blown out into the sea. Those who fly a lot are more likely to be blown, so evolution favors those who don’t.
Many biologists disagreed with Darwin’s simple assumption. Now, a new study suggests that maybe he was right after all – at least in part.
Image credit: Cedric VT.
Flies run, moths crawl
Between Antarctica and Australia, some islands known as the Southern Ocean Islands are almost exclusively home to flightless insects. It’s an extremely peculiar thing with so many insects flying, and it’s a trend that’s present on many other islands as well.
“Of course, Charles Darwin knew about this wing loss habit of island insects,” says Ph.D. Candidate Rachel Leihy from Monash University School of Biological Sciences.
“He and the famous botanist Joseph Hooker had a major argument about why this was happening. Darwin’s position was deceptively simple. If you fly, you will be blown into the sea. Those who stay on land to produce the next generation are the ones who fly the least, and eventually evolution does the rest. Voilà. “
But Hooker, who was an accomplished explorer himself, had other ideas – and Hooker’s own trips to Antarctica only cemented his ideas. Over time, biologists appeared to be more on Hooker’s side than Darwin’s. Surely there must be some other mechanism that most believed. But there didn’t seem to be a clear pattern to explain this. Island size is a poor predictor of inability to fly, and climate is also difficult to correlate with.
But only a few thought of testing the idea on the Southern Ocean Islands (SOIs). Leihy and colleagues believe the sub-Antarctic SOIs are an excellent test bed for these hypotheses. They’re pretty cold, food is scarce, and most importantly, they’re some of the windiest places on earth.
“If Darwin really got it wrong, the wind would in no way explain why so many insects have lost their ability to fly on these islands,” Leihy said.
They found that 47% of native SOI insects are flightless, compared with 8% for arctic species and the global average of 5%. In other words, the windier the island, the more likely the insects will get rid of the fly – which Darwin is doing essentially right.
However, the researchers gave Darwin’s idea a new twist. Wind is indeed a deterrent to flying, but perhaps not because it blows the insects out into the sea, but because it uses more energy.
Flying is very exhausting, it takes a lot of energy to do it. The reason so many insects can fly is because they are generally lightweight, which works very well when flying. However, when you struggle with a lot of wind you need to use more energy than normal, which leaves less energy for other things like reproduction. Less reproduction means you are less likely to spread your genes, and voilà.
Instead, insects on windy islands can redirect the energy for their wings and flying muscles to other activities, which seems a viable strategy for many species.
It is noteworthy that the ideas of Charles Darwin, the father of evolution, can turn out to be so valuable to this day.
“It is noteworthy that after 160 years, Darwin’s ideas continue to provide insight into ecology,” concludes Leihy, the paper’s lead author.
The study was published in the Proceedings of the Royal Society B.