By Athayde Tonhasca
Insects made their first appearance on this planet between 450 and 500 million years ago. But they really took off evolutionarily – and literally – some 80 million years later when they acquired the ability to fly. From then on, insects could explore a three-dimensional world to occupy every nook and cranny of a habitat, escape predators, disperse widely and search for food more efficiently. Insects soon became the dominant creatures on Earth.
The ability to fly gave insects so many advantages and opportunities that it may seem inconceivable to give it up. And yet, many species have done just that. Brachyptery (wing reduction) or aptery (loss of wings) is widespread among insects. It is easy to understand the uselessness or even disadvantage of wings for bedbugs, fleas, lice and other sedentary creatures. But winglessness seems odd for insects we commonly see flying about such as wasps, beetles, and butterflies.
For these insects, wing reduction or wing loss almost always happens to females: males usually retain fully functional wings. The large velvet ant (Mutilla europaea), is a case in point; the male is winged and a capable flier, while the female is apterous, a trait that makes her look like an ant – hence the species’ common name. But in fact this creature is a wasp that parasitizes several species of bumble bees.
The reasons for the loss of flight in insects have baffled scientists for a long time, and Charles Darwin was one of the first to come up with a theory to explain it. Intrigued by the unusual number of apterous beetles on the island of Madeira, Darwin suggested that flightlessness was a survival strategy. To avoid being blown into the ocean by the strong winds that buffet the island year round, the local insect fauna adapted by losing their wings and keeping their feet firmly on the ground.
Darwin’s theory was tested recently with data gathered from 28 Southern Ocean Islands, a collection of isolated, wind-swept specks of land in the southern regions of the Atlantic, Pacific and Indian oceans. About half of the islands’ indigenous species are unable to fly, which is nearly ten times the global incidence of flightlessness among insects.
By analyzing variables such as wind speed, temperature, air pressure, habitat fragmentation, and presence of predators or competitors, researchers validated Darwin’s hypothesis: wind speed was the main environmental contributor to flightlessness in insects. But Darwin didn’t get it quite right: the risk of being blown away is not the main evolutionary driver – after all, even a tiny island is a huge mass of land for an insect. Instead, the enormous energetic cost of flying seems to be the cause.
Indeed, brachypterous or apterous insects are more common in areas where a great amount of energy is required for flight such as arctic regions, mountains and deserts; or in stable habitats where dispersal is not vital for survival, such as caves, termite and ant nests, and on vertebrate hosts. Flight muscles comprise 10-20% of an insect’s body weight, and sustained flights consume a great deal of the insect’s resources. If flying does not give it significant advantages, energy could be spent on some other function – such as laying more eggs, for example.
Egg production explains why it’s mostly females that are wingless. Free of the costs of flying, a female can produce lots of eggs, which are considerably more expensive energetically than sperm. In fact, for many flightless species the female’s abdomen is greatly enlarged to hold as many eggs as possible, which increases the species’ chances of survival. Flight is retained in males probably because it increases their chances of finding females.
In Britain, the belted beauty (Lycia zonaria), the winter moth (Operphthera brumata) and the vapourer moth (Orgyia antiqua) are three of the better known species with wingless females. The belted beauty is a scarce species confined to coastal areas, but the other two are abundant and widespread; the winter moth is an invasive in North America.
Wings were the morphological feature that assured insects’ success on Earth, but many species made a U-turn in the evolutionary road. For them, flightlessness was the best life strategy. This apparent throwback is another demonstration that evolution is not teleological, that is, it has no objectives or ‘improvement goals’. It just provides the best means for a species to adapt and survive.