Lift off…

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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.

Here’s a Spanish robberfly roosting for the night on a flower head. The fore-wings are folded along the back, and the vestige of the right hind-wing, one of the halteres, is clearly visible as a yellow, stalked knob at the back of thorax. The halteres act like tiny gyroscopes and they are one reason why flies are among the most accomplished flying insects (Ross Piper)
Beetles still have two pairs of wings, but their first pair have evolved into protective cases, the elytra, while the hind-wings are membanous and can still be used for flight. The elytra protect the hind-wings and the abdomen. The majority of beetles can fly, although some of the larger species, such as this metallic green chafer from Trinidad are not the most graceful animal in the air (Ross Piper).
The Strepsiptera have to be among the weirdest of all the insects. This is a male Strepsiptera from Trinidad (not sure which genus, since the antennae are broken). In these insects it is the male’s fore-wings that have evolved into halteres, while the hind-wings are used to propel the animal through the air (Ross Piper)

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.

The mayflies are amongst the most ancient of all the insects. As juveniles, like the one in this photo, they have a pair of tracheal gills on each of their abdominal segments. Similar gills on the thorax of an ancient insect may represent the origin of the modern insect wing. In mayflies the winglets that will become the fully developed wings of the short-lived adult can clearly be seen extending from the thorax and covering the front part of the abdomen. This mayfly nymph is beautifully adapted for a life in fast flowing streams. Its hydrodynamic shape and strong legs keep a tenacious grip on slippery stones in the powerful current (Ross Piper).

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.

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