Why are some insects flightless?

Wingless beetles of Madeira

Why do some insects not have wings when having wings is an advantage? Wings on insects are described as “being the key evolutionary innovation leading to the success of the group” (Daly 1978). I think the key (to repeat the word) aspect of that quoted text is that wings normally lead to the success of an animal in terms of survival. It makes sense when you think about it.

Wingless beetles of Madeira

Wingless beetles of Madeira. Image: This website based on an image deemed to be in the public domain.

However, there are certain situations under which having wings harm the prospects of survival. This was recognised by Charles Darwin who, today, is back in the news in The Times newspaper. The reason why some insects lost the ability to fly has caused scientists to scratch their heads for a long time. Charles Darwin writing in On the Origin of Species in 1859, wrote about the beetles of Madeira.

Two hundred of these beetles, out of a total inventory of 550, were flightless. Charles Darwin wrote that, “Each individual beetle which flew least will have had the best chance of surviving from not being blown out to sea”.

Some of his contemporaries disagreed with him because flightless insects were and are not limited to islands. However, researchers from Monash University in Melbourne, Australia have concluded in a recent study that if a species of insect lives on a windy island it is more likely to be flightless which distinctly supports Charles Darwin’s theory.

On some islands almost all the insects have given up on flying. Scientists found that the best guide to whether they had become flightless or not was how windy the island was. Other factors for the evolution of flightless insects such as the absence of predators or lower temperatures are much less influential than how windy the island was where the insect is living.

The researchers from Monash University looked at insects on the Antarctic and subantarctic islands of the Southern Ocean. These are some of the most windy islands on the planet. Clearly they felt they were onto something having read Darwin’s theories published 160 years ago.

About 5% of insect species have evolved to fly to then lose that ability. A much higher proportion of insects lose their wings when they are living on an island, particularly a blustery island where flies are more likely to crawl along the ground.

There are other theories about the evolution of flightless insects. One website from a university in Arizona, USA, writes that the most accepted hypothesis for the loss of wings is because the individuals became fitter; to quote, “The most broadly accepted hypotheses for loss of flight in insects have focused on loss of wings resulting from increased fitness of individuals”. They may decide to refresh their page. Althiugh there are probably other reasons as well for insects losing the ability to fly. On occasions there may be a combination effect although other factors are less important.