On the Wings of Insects (pt 2)

In the first post of this series, I outlined three main challenges faced by evolutionary theory – all of which are represented in the theory’s inability to explain the presence of wings in insects. The first of these was the abrupt appearance of organisms in the fossil record. Previously, I examined the inability to provide any meaningful account through biological history for the development of insect wings. Scientists speculate how this occurred, but without any evidence (kind of important).

The abrupt appearance of winged insects in the fossil record is closely associated with another challenge to evolutionary theory which is the origin of anatomical novelty – the appearance of structures which are completely new compared to other similar organisms – they cannot be accounted for simply as modifications of preexisting structures. To appreciate what a substantial transition this was, it will be helpful to outline the modifications which needed to occur in order to accomplish flight.

The most obvious modification would be some planar outgrowth of the exoskeleton. Wings did not derive from legs as is supposed in vertebrates because the body sections of insects which possess wings also possess legs. Insect wings have several features which make them distinctly different from any other part of the exoskeleton. They are transparently thin and do not have the multilayered structure found elsewhere in the exoskeleton. Wings also have an embedded network of veins which is also atypical of the rest of the exoskeleton. Wing veins are essential for both wing deployment and wing stability. Most importantly, wings useful for flight must have a shape which is adequate to function as an aerodynamic foil (not just any old flap will do).

Internally, insects do not possess arteries, capillaries and veins for the circulation of blood. Invertebrates have open circulatory systems, so the vein structure in wings is itself a novel structure. Insects do have a tracheal system used for gas exchange which branches much like blood vessels, but they are designed for changing air pressure, not fluid pressure which is the case in wing veins. Along with these veins, there needs to be a mechanism which allows the fluid from the body to pressurize and fill the wing veins as the adult emerges from the nymph or pupal stage so that the newly developed wings can unfold and spread out to form the flat wing foil. For many insects, after the wings are unfolded and hardened, the veins then serve as structural support. However, in insects such as beetles which repeatedly fold and unfold their wings, they need a hydraulic system which can operate in both directions.

As a structure designed for flight, the wings that developed could not be solidly fixed to the body. Much like the legs, the wings need to have an articulation with the body. In modern winged insects, the wing structure pivots with an edge of a plate at the top of the thorax, and are held in place by membranous tissue. The articulation must allow for not just a simple up and down movement, but must permit very complex movement which includes forward and backward motion as well as an angling of the wing that occur during each wing beat.

Closely tied to the modification of wing articulation is the modification of muscles which direct wing beat motion. For most insects, the up and down motion is accomplished by muscles which are not connected to the wings themselves, but instead cause the thorax of the insect to compress in shape which in turn move the wing by means of a lever system. Muscles that deform the thorax would find little or no utility beyond wing movement. This unique mechanism can generate wing beats at a rate as high as 1,000 beats per second! Other muscles are connected to the wing or around the region of wing articulation which cause a change to the angle of the wing in order to control things like the roll, yaw and pitch during flight.

Like all muscle movement, these flight motions must be initiated and coordinated by a nervous system. Sensory input from the eyes as well as the hairs on the body and wings of the insect can signal the initiation of flight muscle action. They will also provide ongoing feedback to the insect brain indicating changes in visual targets and air movements which can result in adjustments to the wing beat motion. What is of great interest in regard to the nervous system and flight is the presence of innate behaviors (i.e., those that are not learned or practiced). Following the emergence of an adult insect, once the wings have hardened, the insect is ready to fly – they “know” how to fly without any practice or training.

If an anatomical novelty is a challenge for evolution to explain, then insect wings are doubly challenging because each of the modifications outlined above are anatomical novelties in and of themselves. Furthermore, none of these modifications would seem to have a benefit apart from the role they have to play in insect flight – all of them would need to be present at the same time to make flight possible.

The weakness in the ability of the modern synthesis to explain novel structures (e.g.,the origins of wings) is its reliance upon mutations to explain any changes in phenotype (what an organism looks like). Mutations are limited to the modification of existing information to do its work, and is unable to account for the advent of new information which would be necessary to account for the many novel structures associated with insect flight. Small incremental changes could not account for this highly complex structure. It would require the simultaneous appearance of multiple pieces of new information.

In his latest book, Michael Behe surveys the progress made by evolutionary biologists, who are trying to come up with new ways to fill in the gaps left behind by the modern synthesis. While some of these latest ideas may be true to what happens in real life changes to phenotypes, they primarily suggest more possible ways to obtain simple variation. No advances have been made which could explain the origin of insect flight. Behe says, “neither neutral theory nor complexity theory, neither the ideas of the extended evolutionary synthesis nor the latest Darwinian innovations – none of them even try to account for the sophisticated machinery of life. None even try to account for the purposeful arrangement of parts.”[1] – what Behe refers to as functional complexity.

To account for the presence of these anatomical novelties, some explanation would be expected to be found in the genetic material of the organism. This however is where we find deeper challenges still, and will be addressed in the next post in this series.

It is a happy world after all. The air, the earth, the water teem with delighted existence. In a spring noon, or a summer evening, on whichever side I turn my eyes, myriads of happy beings crowd upon my view. “The insect youth are on the wing.” Swarms of new-born flies are trying their pinions in the air. Their sportive motions, their wanton mazes, their gratuitous activity testify their joy and the exultation they feel in their lately discovered faculties … The whole winged insect tribe, it is probable, are equally intent upon their proper employments, and under every variety of constitution, gratified, and perhaps equally gratified, by the offices which the author of their nature has assigned to them.[2]

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[1] Michael Behe, Darwin Devolves: The New Science About DNA That Challenges Evolution (New York: Harper Collins, 2019), 137.

[2] William Paley, Natural Theology: or, Evidences of the Existence and Attributes of The Deity, Collected from the Appearances of Nature (1802), 490-1

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The wings in the header of this blog are those from a flesh fly (family Sarcophagidae). While the natural history of these flies is somewhat gruesome, the wings may still be a thing of wonder and beauty.