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In flying vertebrates, almost all have only two wings (with the exception of Microraptor and its close relatives). This makes some sense because you need some way to walk also, and a wing isn't very good as a leg, hence why bats are so bad at walking. Of course, bats do demonstrate that being able to walk well is not necessary to be a successful clade.

As far as insects are concerned this might seem like an odd question, since most insects technically have four wings. But among the four largest insect orders (which together make up a sizeable majority of insects), Coleoptera, Lepidoptera, Diptera, and Hymenoptera, all of them functionally have two wings. In Diptera and Coleoptera, only one pair of wings is used to fly. In Hymenoptera and Lepidoptera the two pairs are attached and don't move indepedently.

So my question is, why is two-winged flight so much more common than four-winged? If two-wingedness is advantageous, why do other insects retain four-winged flight (most notably Odonata)?

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Note you are talking about two very distinct conditions

In vertebrates, 4 "wings" only evolves once in vertebrates (in early birds) and only one set function as wings the others "wings" are control surfaces for low speed stability, they generate insignificant amounts of lift. for large animals like vertebrates you can't fit two sets of functional wings on the same animal. Large wings need a lot of space to flap and you would need to make the torso longer to give enough room for a second set of flapping wings, which would add a lot of weight and defeat the benefits.

In Insects four wings is the default, the groups with two wings got them by transforming one set of wings into something else useful. Wing covers in beetles ,which protect wings from damage, and Halteres in flies which act as a sort of sensory organ to determine position. Two wings are more common numerically becasue those two groups are extremely successful. Basically if you have two of something but one will also work and evolution can turn the other set into something useful then you have a chance to lose one set of that thing. Two wings are more common numerically becasue those two groups are extremely successful. By turning four wings into two those groups ended up with more abilities than their four winged relatives, becasue they did not just lose the wings they turned them into something else.

In insects wings actually evolved from gill covers, insects could have had six wings, why the first set never evolved would be an interesting question. It may be due how HOX genes are expressed in insect segments.

Source

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    $\begingroup$ Thanks for your answer! It still doesn't explain why hymenoptera and lepidoptera have 'decided' to get rid of independent wing motion; they did not transform a second pair of wings into something else useful. I'm wondering if there's some unifying reason why two wings is better than four? The same thing seems to hold for airplanes, though I suspect that may be a different reason. $\endgroup$ – Dark Malthorp Apr 14 at 14:35
  • $\begingroup$ Perhaps this is not the right place to ask this question. $\endgroup$ – Dark Malthorp Apr 14 at 14:36
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    $\begingroup$ It is the perfect place ot ask it you just need to clarify your question. hymenoptera and lepidoptera still have two wings, they beat together becasue the mechanism insects use to flap the wings can generate more force that way, insect wings are not like bird's the muscles do not pull on the wings dirrectly, the mechanism driving their motion is different. There is not one unifying answer, each different mechanism has a different functional answer. $\endgroup$ – John Apr 14 at 14:43
  • $\begingroup$ Thanks. It just seems like an odd coincidence that across (most) insects, dinosaurs, and airplanes, there's been an 'experiment with four wings' phase, but the two-winged forms are more successful in the long run. $\endgroup$ – Dark Malthorp Apr 14 at 14:48
  • $\begingroup$ There is no four wing phase in dinosaurs, there is a two wing and two rudder phase, If you see a picture of microraptor with the lower "wings" horizontal the artist has broken the creatures legs to get them in that position. there is a whole question about this biology.stackexchange.com/questions/62502/… $\endgroup$ – John Apr 14 at 14:51
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Four winged insect's ancestors were lucky because they had the sky all to themselves for perhaps 50 million years, giving them valuable time to develop more complex wings which can compete with simpler forms of flight. The first terrestrial insects came 380 million years ago, and the first dragonflies are from 300 million years ago. The high oxygen at the time made all forms of flight easier to develop.

They had to go through various stages of inefficiency which are developmentally expensive. Earlier animals could afford to fly clumsily compared to today's ones because there were less areal hunters to catch them on the wing.

Robot flight tests suggest that four-winged flight is generally less efficient than two-winged, so if another four-winged insect developed independent control of the up and down muscles on both sets of wings, it would require 20 or 50 percent more food to travel the same distance for thousands of generations to compete with the two-winged variants, added to the fact of being easier prey.

Dragonflies can also not fold their wings for protection and burrowing and rely on high energy food sources, so they have a limited habitat today compared to most two-winged insects.

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This is far from being the complete explanation (which I don't have). But:

From a molecular standpoint, the symmetry between left and right is achieved often by presence / absence of gene expression of certain developmental genes.

In some cases, these genes could be expressed in a gradient, forming gradual transitions of patterns.

A four winged animal would require a dev gene to be expressed on one axis (generating left and right) but also on another, being expressed in two pulses (two sets of wings). This is much more prone to deleterious mutations, since there are more factors weighing in. Hence, it could be evolutionary more disadvantageous.

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  • $\begingroup$ This makes logical sense to me but if I understand correctly, it would only explain an actual reduction in the number of wings (like diptera), not a re-purposing of one pair (as in coleoptera), or a linking of two pairs to make a functional single set as seen in hymenoptera and lepidoptera. In vertebrates, the four-winged forms didn't have a different number of limbs from normal tetrapods either. $\endgroup$ – Dark Malthorp Apr 12 at 12:59
  • $\begingroup$ This is not how wing development works, by this logic any organism with more than a single set of limbs is at a major disadvantage. It is also disproven by insects in which four wings is the default both originating from the same developmental pathway. $\endgroup$ – John Apr 12 at 14:16

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