Powered flight using wings has evolved independently on Earth at least four times[1]. But as far as we can tell, insects did it first, and have exploited their innovation like crazy. To the point where they are the most plentiful flying organisms on earth by a massive margin. And the wings are used for everything from a kind of floating aimless flight, to fast-moving and highly-controlled maneuver.

But, one key question is this: what are wings really? What ancestral structure were they derived from, and how did they get to be wings?

The answer to that question is not as obvious as it is for other flying animals, which all developed wings by heavily modifying their front limbs (no mystery there). But the winged insects have the same number of legs and other non-wing appendages as their wingless relatives (silverfish, bristletails, and the like). But, they’ve somehow also sprouted wings as additional appendages.

As it turns out, the origin of insect wings is not a settled question, there are at least four competing hypotheses[3]:

(1) Maybe the wings developed from bulges on the back of the insect thorax – the “Paranotal Hypothesis“.

The diagram shows the ancestral winged insects first developing unjointed flaps on their backs, giving them some aerodynamic benefits. The proto-wings then developed first primitive joints to allow them to be adjusted, which gradually became the highly-functional wing joints that modern insects have. The main objection to this hypothesis is that it would have been difficult to evolve the joints from scratch, and the genetics of wing joints looks suspiciously similar to those of the joints of other insect appendages.

(2) Maybe the wings are heavily modified and relocated abdominal gills – the “Epicoxal Hypothesis”

Here, the ancestral winged insects are taken as being an aquatic form, with gills on the abdomen. These gills already had joints, and so all the insects would have had to do is re-use the gill genes on their backs instead of on their abdomens. I think the main objection to this one is that it involves moving structures from one segment to another.

(3) Maybe the ancestors of insects originally had “biramous” limbs (like most crustaceans do), consisting of a gill branch and a leg branch. The gill branch could have separated from the leg branch and migrated to the back of the thorax, where it could become a wing – the “Exite-Endite Hypothesis”

This was the favored hypothesis for a while, but I understand that it doesn’t agree with some of the evidence from genetic and developmental studies.

(4) Maybe the ancestral insect started by developing mostly immobile winglike projections from the back of the thorax. Then, once these projections were in place, already-existing genes for limb development were expressed on the back as well as in the legs, resulting in the formation of the joints and musculature needed to make functioning wings – the “Paranota plus Leg Gene Recruitment Hypothesis”

If one were inclined to bet on which of these is closest to being correct, it looks like the fourth one is ahead in the running, as it is in fairly good agreement with results of genetic studies and with the fossil record (such as it is). Of course, as more genetic studies are done and more fossils are found, other things will doubtless be discovered that will force yet another hypothesis to be created. And maybe yet another after that. That’s just the way these things work.

The final wing has similar (but not identical) jointing to the insect legs, but has a significant difference from the other appendages – the wing is entirely composed of dead tissue. It is a chitinous flap, stiffened and supported by a network of veins that act as lightweight tubular structural members. Some wings, such as those of earwigs and a number of beetles, have extra hinges in them so that they can be collapsed and folded, but these hinges are worked remotely by muscles in the thorax, not by anything resembling muscles in the wing itself.

As for the benefit that insects get from the wings, the big one is dispersal. Flying is a much easier way to cover big distances than crawling, so winged insects could spread themselves much more effectively than their non-winged relatives. They are useful for this even before they are fully-developed powered wings. An insect with winglike structures can use them to scoot across the surface of the water like a sailboat (some stoneflies still do this), or can climb up a high place and then glide for a significant distance to locations that they could not have easily walked to. So once there was a proto-wing in place on the distant insect ancestor, there was a lot of selective pressure making the wing even better so that it could scoot faster, glide further, and ultimately actually take flight.

And once insects were properly in the air, they could start using flight not only to spread to new locations, but to actively hunt down food. Either as scavengers looking for dead things, or as aerial predators (like dragonflies) pouncing on their less-aerial relatives. Flight became almost a defining characteristic of insects.

Although, there was still a big problem. Wings are fragile, and can’t heal, so a winged insect has to avoid damaging them. This problem is greatly reduced by the nymph/larval forms of insects not having wings at all, so they can spend most of their lives not worrying about them. But, in addition to this, a few lineages came up with a way of having wings, but also protecting them. Beetles, “true bugs”, and earwigs all hit on a similar solution: while insects started with four wings, only two wings are strictly necessary to fly with. So they repurposed their front pair of wings to make protective shells for their flight wings. So now, they can get into the rough-and-tumble of rummaging around the leaf litter, or chasing down prey, or burrowing into carrion, and not worry much about damaging their wings. They aren’t quite as strong of fliers as the insects with unprotected wings, but they are good enough to get all the major benefits of wings without most of the drawbacks. Which has lead to beetle species being more numerous than species of any other insect group by a large margin[4].

Other insects (Diptera, the “true flies”) took sort of the opposite approach. Like the beetles, they only needed two wings to fly, but they kept the front wings. The hindwings got reduced to “halteres”, basically weights on the ends of short sticks that they could vibrate quickly. These seem to act as gyroscopes and balancers, giving the flies significantly better maneuverability than most of the four-winged insects, even though they tend to have smaller wings.

Of course, what can be evolved can also be lost. A number of insects have gotten into lifestyles where wings are of limited use, and so they lost them again. Many of these have the wings on some individuals, but not on others. Ants and termites, for example, have wingless workers (who spend most of their time dragging stuff around on the ground, and so wouldn’t benefit much from wings), but the fertile males and females still have the wings so that they can fly off to mate and establish new nests. And there are a number of moths where the females have lost their wings, giving them more body volume to use for producing eggs, while the males can still fly around and spread their genetics over a wide area.

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[1] Or five times, if you consider the ability to construct aircraft to be an evolutionary development of the human species (which I kind of do). Other than that, true powered flight was evolved by insects, pterosaurs, birds, and bats, in approximately that order. Of course, if we want to stretch a point there are a bunch of other aerodynamic organisms that can get into the air for extended times, but can’t quite manage sustained, powered flight[2]. Like flying fish, ballooning spiders, flying squid, flying squirrels, sugar gliders, flying snakes, draco lizards, flying frogs, several kinds of geckos, and others that either haven’t been discovered yet, or whose aerodynamic qualities are disputed. Some would go so far as to say that even house cats have very limited flight ability. And then there are all of those plants that have wings or parachutes on their seeds . . .

[2] One of the arguments that regularly gets trotted out by people opposed to the theory of evolution, is “What good is half a wing?” The idea being that they think a flying organism can’t evolve in small steps, because there supposedly isn’t any advantage to having anything short of a full-blown wing. Given the number of “half wing” organisms I just listed above, though, that argument is clearly nonsense – any number of organisms have structures ranging everywhere from the simple ability to flatten out their bodies, to just short of a full wing, but all of which are still a big improvement over jumping off of a high place and plummeting straight down.

[3] I’d like to point out here that this isn’t a case of “nobody has any idea how wings evolved!”, but of “we have too many ideas for how wings evolved, and we don’t have enough data yet to choose between them!” Sooner or later, we will discover information that rules out most of the alternatives, a theory will be developed that is consistent with all the actual data, and it will get sorted out. Eventually.

[4] And lead J. B. S. Haldane to note “God has an inordinate fondness for beetles”.