On the Wings of Insects (part 1)

In his address to the Royal Society in 2016, evolutionary biologist Gerd Müller described some of the most serious challenges facing the theory of evolution today. They involve the inability of the theory to explain, a) the non-gradual modes of transition (abrupt appearances in the fossil record), b) the origin of anatomical novelty, and c) the origin of phenotypic complexity[1]. It is because of these failures that biologists like Müller and others are looking for different ways to explain these aspects of life.

All three of these challenges to evolutionary theory are represented within the attempts to explain the wings of insects – a feature which arguably makes insects the most successful class of organisms on the planet (nearly a million described species and counting). In this three-part series, I will outline these challenges in terms of insect flight. The inability of Neo-Darwinism to account for insect flight will be examined along with the ability of some alternatives to explain this characteristic of insects.

The first difficulty to be considered is the situation we find throughout the fossil record in which new taxonomic groups appear very suddenly (i.e. they have no transitional forms which precede them in the fossil record). The most commonly known example is referred to as the Cambrian explosion which marks the beginning point for almost every phylum existing today (and some which have become extinct). There are no transitional fossils which link any of these phyla to a common ancestor. Other abrupt appearances include the appearance of jawed fish (425-415 mya), four-legged animals (251-240 mya), flowering plants (130-115 mya), modern birds (65-55 mya) and placental mammals (62-49 mya).

Dragonfly fossil from the Green River Formation; National Park Service photo

Another of these abrupt appearances involves insects. Wingless forms of insects start showing up in the fossil record about 400 mya. The earliest fossils of winged-insects discovered date to about 300 mya. Among all the fossils between those two time periods, however, there are no fossils which possess any structure which could be considered a transitional wing. This is perplexing for a couple reasons.

Functional wings are exceptionally complex structures, and would require significant modifications to the body plan of a non-flying insect to accomplish flight (more details on this in the next post). From an evolutionary perspective, a considerable number of small steps or variations which accumulate over time is required to move from the wingless insect to a winged insect. So, it would be reasonable to expect at least some of these changes to be recorded in the fossil record.

Even more perplexing is that insects produce a lot of offspring. Fossilization does not capture every living thing, but with the abundant populations of insects, the odds of being caught in this process are greatly increased. Consequently, it is not uncommon for insects to be found as fossils. So, it is quite reasonable to expect that a good transitional record should be available to us in the fossils. But there is nothing.

The origin of insect wings has been a standing debate among evolutionary biologists for over a century. The two competing hypotheses suggest wings developed either from tracheal gills or from lobes that developed on the thorax.[2]  Both hypotheses have strong weaknesses, but neither of these hypotheses has gained traction over the other primarily for lack of evidence.

Mayfly nymph – notice the gill structures emerging from the abdomen

Tracheal gills are most commonly seen as abdominal structures among aquatic insects, but are present on the thorax of some species. However, the placement of thoracic gills is along the ventral surface – opposite of where wings are located. The “veins” within these gills are extensions of the tracheal system which is designed for the diffusion of gases. Wing veins on the other hand accommodate the movement of fluid – the insect’s blood. Some wing veins even contain trachea themselves. It would be a stretch to consider wings and gills to be homologous structures.

A recent study reexamined fossils of insect nymphs from the early-late Carboniferous boundary (325 mya) which were of interest because of the presence of node-like structures associated with the nymph’s wing pads. The researchers suggest the presence of these nodes lends support to the hypothesis that wings developed from thoracic nodes.[3] One of the difficulties in drawing any conclusions from this study is the inability to connect the nymph fossils with an adult insect. This evidence is suggestive at best because wing pads in nymphs are structures found on insects which already have wings – they are not transitional.

The researchers from the study mentioned above speculated how nodes on the thorax could have been useful for thermoregulation which later developed into flaps which could have been used for gliding. The gliding flaps could later have developed articulation with the thorax, and later still developed muscle which could move and maneuver the wings.

These speculations are less than compelling as there are no fossils which corroborate this narrative. Moreover, no current insect species has been demonstrated to utilize thoracic lobe structures for thermoregulation, so the suggestion of this adaptation has little basis in fact.

Green snaketail dragonfly (Ophiogomphus cecilia)

It is not uncommon for evolutionary biologists to construct possible scenarios for evolutionary development which suggest what the possible incremental steps might have been, and how they would have benefited the organism. However, these imaginative scenarios have somehow become misconstrued as evidence in support of evolution when in fact they are nothing more than untestable hypotheses.

In terms of the transformation from nodes to gliding structures, one could just as easily argue that these gliding structures would have impaired mobility (e.g. evading predators), and would have not been a selective advantage. There is no way to test whether these supposed structures would have been helpful or not.

A healthy imagination is helpful in the creative process of developing explanations for how things came into existence. Imagination allows us to develop hypotheses for testing, but it would not be safe to say that as long as we can imagine it, it happened.  This unfortunately is all evolutionary biology has to offer in explaining the existence of insect flight.

So, from the fossil record we are left with no verified naturalistic account for how insects developed wings. That a change occurred is undeniable. What is questioned here is whether the processes described by evolutionary theory have the power to explain the change which occurred. The evidence which should exist according to evolution is absent. The fossil record leaves this an open question, and may even suggest there was more involved than evolution alone. To determine which is most likely the case, we need to turn to other lines of evidence which I will examine in upcoming posts.

[1] Gerd Müller, “Why an extended evolutionary synthesis is necessary,” Interface Focus 7: 20170015. http://dx.doi.org/10.1098/rsfs.2017.0015 (accessed 5/2/2020).

[2] Andrew Ross, “Insect Evolution: The Origin of Wings” Current Biology vol 27, issue 3 (06/02/2017), R113-R115. https://doi.org/10.1016/j.cub.2016.12.014 (accessed 05/02/20).

[3] Jakub Prokop, etal., “Paleozoic Nymphal Wing Pads Support Dual Model of Insect Wing Origins,” Current Biology 27, 263-269 (01/23/2017) http://dx.doi.org/10.1016/j.cub.2016.11.021 (accessed 05/02/2020).

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