Insect wings are right up there with metamorphosis in the adaptations that have allowed these animals to become as successful as they are. Insects are pioneers of the sky and they took to the wing at least 350 million years ago in the steamy forests of the Carboniferous period, some 120 million years before craniates took their first faltering forays into the air. Today, you only have to watch a hoverfly for a short while to see why the insects are arguably the most accomplished of all the flying animals.
The primitive state in all insects is two pairs of wings, each of which are essentially offshoots of the thoracic segments. Many of the more ancient insect taxa retain these two, almost identical pairs of wings (just imagine a dragonfly or a damselfly), while the more derived insects, such as flies, have tinkered with this original set-up so that the hind-wings have evolved into tiny, yet important vestiges known as halteres. In the beetles the fore-wings have evolved into very tough wing cases (eltyra), which protect the animal’s abdomen and its membranous hind-wings. In some of the beetles and the Strepsiptera it is the fore-wings that have evolved into haltere-like structures in an excellent example of evolutionary convergence. Some insects, e,g. certain stick insects have adapted to niches where there’s no need for wings and they have almost completely lost them.
The huge variety of wing forms we see in the insects today had a starting point, but what was it? What structures did they evolve from hundreds of millions of years ago? Did they start off as as moveable bits of cuticle that helped terrestrial insects regulate their body temperature or did they evolve from the gills of juvenile aquatic insects? No one will ever know for sure, but some living insects give us a tantalizing glimpse of the possible origins of these structures. The nymph of a humble mayfly is one such creature.
On each of its abdominal segments the mayfly nymph has a pair of gills, each of which is equipped with a network of tiny channels much like the veins of insect wings. They’re also vibrated continuously by their own muscles to increase water flow around them thus improving gas exchange.
It’s not hard to imagine the nymph of an ancient aquatic insect breaching the surface of a forest pool on the cusp of becoming an adult. As it shed its skin to begin its fleeting life as a terrestrial animal a mutation meant that the gills on each side of its thorax were retained. These ‘winglets’ let it scoot around a little better than others of its kind; even making short glides possible. With the passage of time and countless generations, evolution honed these gills into perfect wings that made true flight possible for the first time.