Insect Flight: Origins & Aerodynamics

There are two competing hypotheses for the origin of flight in insects. One is that the first terrestrial hexapods had extensions coming out of their thorax that helped them glide between the tall vegetation of the time; an evolutionary path from simple paranota (unmoveable stubs) to an insect wing is relatively easy to imagine. Paranota are found in silverfish and are presumed to be homologous to the pterygote wing. This is the classical view, and was even mentioned in a less structured form by Darwin (not that that means anything). The best (though by no means conclusive) proof of this would be find an insect from those ancient times with a similar structure.

We know from rainforest ants that they are capable of controlled gliding – but this is irrelevant, since ants are very derived insects. If we are to draw any conclusions from recent taxa, then basal hexapods are needed. It’s now also known that bristletails – members of the apterygotes, the non-winged basal sister group to the winged insects, can also jump from trees and glide, using their caudal appendages (‘tails’) to control the manoeuvre. This may be a derived behaviour. Or it may be representative of the insect ancestors! But again, in the absence of fossils, we cannot say anything for sure. Let that be a lesson to all those biologists who regard the fossil record as useless (and they are plentiful, believe you me).

The second hypothesis is more modern and is partly a result of developmental biology. It states that insect wings are derived from the gills or the gill plates of their marine crustacean ancestors; in more precise terms, they are the modified exites of the crustacean gill. Crustacean trunk appendages (‘legs’) are biramous, with an epipod to the inside and an exopod to the outside (which acts as a gill). This ramus is then imagined to have fused with the body wall, becoming the tergal lobe present in all insects and other hexapods. Hence, even here the paranota of the silverfish is homologous to the pterygote wing, however the silverfish paranota must have secondarily lost its ability to move.

In this scenario, wings first served as respiratory surfaces. The first insects had aquatic larvae at least, and they had these moveable gill plates sticking out of their sides. It’s also not hard to imagine a pathway to flight, with these gill plates getting co-opted to become aerodynamic airfoils and true wings. The best (and again not conclusive) proof of this would be to find a fossil marine hexapod (one, Devonohexopdus bocksbergensis, was reported as such, but the claim was swiftly shot down and proven to be false).

The key to the debate is therefore to decide whether the ancestral insects were terrestrial (hypothesis 1) or aquatic (hypothesis 2). Given that the most basal hexapods are exclusively terrestrial, we can’t reasonably presume an aquatic ancestry without having even a single fossil to back us up. All in all, it’s hypothesis two that has more to prove, and hypothesis one is simply more intuitive and imagineable – however, definitely backing one over the other is a matter of personal preference, as no conclusive evidence exists either way; the diagram below is a plausible summary for the origin of flight.

Path to flight. Hasenfuss (2008).

The basic situation we’re in today is that the terrestrial origin of insects is the current generally accepted paradigm – albeit with few extremists unwilling to budge if proven wrong. But the fact is that we don’t have the key basal winged insect – the Archaeopteryx of palaeoentomology, if you will. The early insect fossil record is gappy, and that affects not only the study of the origin of wings, but also of holometabolism (the larva – pupa – imago lifecycle; metamorphosis). It also screws around with phylogenetics, since it makes efforts to root the tree of the insects that much harder – how can we know what’s more basal when the insect orders all appear at once? (Okay, it’s not that dire in most cases; I’m being hyperbolic as a rhetorical strategy.)

Let’s summarise what we expect of a terrestrial origin and an aquatic origin of flying insects, if we look at the ‘extreme’ views of each scenario.

  • In the terrestrial model, the ancestral insects were closely related to the myriapods (centipedes, millipedes). In the aquatic scenario, insects are crustaceans.
  • In the terrestrial model, we expect cockroach-like insects to have been the earliest truly flying insects; the earliest insects were silverfish-like. In the aquatic model, we expect an amphibian pterygote, something mayfly-like: aquatic larvae and terrestrial adults. This would also elegantly explain the origin of metamorphosis: in modern odonates and mayflies, the gradual moulting and maturing coincides with a gradual move from the water to land. In this case, when conducting our phylogenetic analyses, we need to take special consideration of larval characters.
  • In the terrestrial model, metamorphosis would have evolved as a way of more efficiently splitting resources between adults and larvae.
  • In terms of anatomy, the terrestrial model says that the trachaeae are ancestral features (hence the naming of the Myriapoda + Insecta grouping as Tracheata). As for the wings, they are derived from paranota used for gliding. In the aquatic scenario, the trachaeal system evolved independently in all arthropods; it first came about in the insects during terrestrialisation. The wings derived from the epicoxa of the crustacean leg and are homologous to the gills of mayfly larvae.

Granted, taking such fundamental hypotheses to the extremes, while necessary, may be a bit disingenuous. It is silly to think that in the evolution of the insects, they only had one habitat transition from water to land, or land to water. It must have happened many times, and every time, there’s bound to be major mix ups in anatomy, since there’s no such thing as ‘de-evolution’; they can’t just revert back to a primitive state, they need to redevelop their adaptations when going into a new environment (Moral-in-a-nutshell: Evolution has no memory and certainly no consciousness. Do not personify it.)

With the sparsity of and difficulties inherent in interpreting the early fossil record, let’s look at what we know from recent arthropods, and how that knowledge affects how we view the early evolution of the insects and the origin of flight. First of all, the position of the insects relative to the other arthropods. The classical Tracheata grouping (Myriapoda + Insecta) has come under fire recently by the new hypothesis that insects are crustaceans. The evidence comes not only from genetic data, but also from neuroanatomy (crustacean and insect brains are remarkably similar), as well as developmental biology (same master genes expressed in the developing epicoxae of crustacean and of insect wings) – this all lends support to the aquatic scenario. This is the major source of contention when discussing pterygote origins at the moment.

From a functional morphology point of view, the most important parts of the pterygotes’s flight system are the wing’s skeleton (the veins), which provides stability, and the thoracic muscles, which power the flapping. On top of that, there’s also the places where the thoracic muscles attach to the wing, i.e. the sclerites, which can determine the type of flight, the frequency of the flapping and even how the wing folds – the key innovation of the neopterans, their ability to fold back their wings, was first caused by a change in the number/positions of the sclerites on the wing base.

Keep in mind that not the entire thoracic muscular system is involved in flight! While the flight muscles make up the most mass in the thorax (they are few, but much larger, bulkier and powerful), there are also the muscles that close the spiracles or that move the legs. There are also many small muscles that directly affect the way the wing moves – the ones that attach to the sclerites.

To move away from the origin of the wing, let’s look at what function it has today: aerodynamics. This automatically plays into the first hypothesis for the origin of the wing, but it simply can’t be ignored that the way insects fly can be baffling to the biology-ignorant physicist, and the structure and morphology of the wing plays a large role in that. The fact is that under regular aerodynamic theory, insect flight should be impossible; but just a short walk through a park will show that insects can fly and even perform various stunts such as flying backwards, hovering and taking off very quickly. Research in this area is booming not only because of advances in computing (CFD in particular), but also because insects are seen as the best models to base microaerial vehicles (MAVs) on.

The main enabler of insect flight is the extraordinarily high wingbeat frequency, often exceeding 100 Hz (beats per second). This is demanded by the insects’ small sizes and is permitted by asynchronous muscle, a muscle type present in the thorax of over 75% of all the insects (including three of the largest orders: beetles, dipterans and hymenopterans). It is not ancestral, but evolved independently 7 to 10 times, indicating just how important it is. Again, the consequence of having asynchronous muscle is much more efficient aerodynamics. Not only does it allow the high wingbeat frequency, it also allows having much smaller wings relative to the body size (the most notorious example is the bumblebee). Asynchronous muscle cannot be given the status of a key innovation since it’s a convergent feature, but it certainly was a crucial step in asserting the dominance of those insect orders that have it.

A small sidenote: the regular muscles in insects are similar in structure to vertebrate skeletal muscles.

All muscle works by contracting and relaxing, accompanied by uptake of calcium ions. This is energetically costly even at a regular pace, so in order to achieve the wingbeat frequency required for flight at an insect’s scale, asynchronous muscle does not contract and relax at every nerve impulse, but remains active all the time, operating faster than the nerve impulses.

Just to make sure we’re all on the same page, here’s a simple introduction to striated muscle. Myofibrils are arranged hexagonally, and stack together to form sarcomeres, which are the building blocks of muscles. The sarcomeres making up the asynchronous flight muscle can be regarded as a single protein crystal because the myofibrils in them are very specifically arranged in a lattice; this is unique, and part of the reason behind asynchronous flight muscle’s properties.

One of these properties is the stretch-activation of flight muscle: when the myofibrils are stretched beyond a certain threshold, they will contract – even in the absence of nerve impulses. For human physiologists, this is similar to what human hearts do. Another interesting set-up in flight muscle myofibrils is the fact that between individual filaments, many mitochondria are found. This allows ATP to diffuse along all the active area more or less evenly; this is then useful during active flight to keep the muscles powered.

To go back to aerodynamics, just consider the odonates. They live near water and have extremely long wings that they can’t reach to wipe water off when wet. Water is destructive to a wing’s aerodynamics because it weighs it down – so odonates evolved nano-scale, superhydrophobic (water-repellant) structures on their wings (basically tiny, ~200nm spikes), just to preserve aerodynamic efficiency. This is also seen in many other insects, but research into it is still in its infancy – but will be picking up when combined with nanomaterial and other engineering-related research.

This is a biology blog, so I will not go into details or even an introduction into pure aerodynamics. Suffice it to say that for a long time, physicists used a very inadequate model of insect wing and wing motion. Since the 1980s though, they realised that comparing insect wings to aeroplane wings is idiotic; aeroplane wings are modelled as moving through straight air; insect wings move through turbulent air (the turbulence being created by the wing motion itself, much like in a helicopter). Insect wings produce all sorts of turbulence patterns and have evolved to take advantage of them in order to maximise the amount of lift they produce. A real discussion of these would involve plenty of formulae, diagrams and animations, but I will not go into any real detail beyond that unless asked.

Let’s look at the odonate gliders. They have no asynchronous flight muscles, as one can tell from their long wings that flap at frequencies between 30 and 50 Hz. However, some dragonflies can also glide for up to 30 seconds without losing speed or altitude. The advantage of gliding is clear: no overheating. Compare an odonate wing to the wings of an albatross and of a gliding plane and you’ll see that they both have a high aspect ratio, i.e. that they are much longer than they are wide; for a relative look, a dragonfly wing has an aspect ratio of ~11, while a fruit fly’s is ~2.

Dragonfly wings are also morphologically adapted to be aerodynamic, with the wing surface area gradually getting larger from the hinge to the end – so most of the airflow is controlled by the ends of the wings, just as in an aeroplane. On top of that, as I said before, insect wings are not two-dimensional sheets. They have deep channels criss-crossing them, providing stability and stiffness – both characters varying across the wing, depending on how deep and large the ridges (i.e. veins) are. This is the most baffling part for physicists and engineers, as any common aerodynamics model would say that the more structured a surface is, the greater the turbulence and the lesser the aerodynamic efficiency.

However, they forget the millions of years of natural selection acting on the wings and perfecting them. In fact, when modelled properly, dragonfly wings are found to increase lift, purely as a result of their venation patterns (of course in coordination with the high aspect ratio and other features). Replicating this effect artificially has so far had limited success, because it does not work as clearly on the relatively enormous human scale. The best replicants are MAVs – and, as usual, are pioneered first by the military. What this all combines to is an ability to glide with only needing to flap once to get in the air – the aerodynamics takes care of the rest.

But only some dragonflies regularly glide – capability does not automatically mean use. Let’s look at their actual flight. As I mentioned in my odonate post, they have muscles that attach right on their wing base, allowing them to control the wing’s flapping directly. In fact, the odonate wings flap independently of each other. Their control is great enough to allow them to change flying style on the go, from hovering to high-speed flight, from coordinated mating flight to low-speed tactical manoeuvring. The way this all works out aerodynamically can be investigated in wind tunnels with smoke.

What we find is that the level of coordination is very great. The amount and pattern of air left by the front wing for the hind wing (the wake) has to be within certain parameters for flight to work, and this is more or less perfected behaviourally/intuitively by the dragonfly. Even if the front wing produces a turbulent pattern (a swirl or any kind of vortex), the hind wing can correct it – some energy goes to waste, but at least it stays in the air. The picture below shows several air patterns produced by odonate wings. The key point to remember is that these aren’t random, but the wings and associated musculature and nervous control have all evolved to produce these to fit the odonates’ particular ecological requirements.

Odonate wake patterns. Bomphrey (2006).

Insect wings can be articulated in three ways: in addition to regular up and down flapping, they can also move forward and backwards; in between those two extremes, the wing can flip (supinate) or fling (pronate). In neopterans, where the wings are folded, the wings first have to spread out (abduct) and unfold before flight; correspondingly, they have to fold back (adduct) when landing.

To look at the unfolding of the wing, we’ll use an interesting example: the beetles. Their front wing is modified to a hardened elytra that is not useful for flight, but serves to protect the hindwings. Therefore, its unfolding is critical for the beetle to fly – it has to give enough space for the hind wings.

The beetle elytra functions by having many locks along the the elytra-abdomen border, between the two elytral halves and the elytra and the hindwing – it basically seals everything up. The locks can simply be clamps or they can be fields of hair functioning like Velcro. The only direct movement that the elytra undergoes is during opening and closing – in flight, it only moves because of the metathorax’s contraction (remember, no direct flight muscles in beetles, so wing flapping is achieved by the thorax’s flexing); all related muscles in the mesothorax are reduced/gone.

Elytral abduction is outwardly simple: they unlock (most importantly, the beetle moves its head and abdomen down to unlock the strongest ones locking the elytra with the body, then pull the elytral halves apart with muscle power), the elytra moves out to the side, goes forward/rotates and up, letting the hind wings unfold. This has been known since the late 1820s, but the way it happens is not so straightforward; it’s a series of steps which have been successively uncovered since those times, with advances in technology and research methods (compare purely anatomical works, where they looked painstakingly at how the muscles were arranged and indirectly tried to reconstruct the process, to today, when you can take a high-speed camera and film the process). And given the diversity of beetles, it would be absurd to expect them all to work the same; the degrees of rotation, the way the elytra is pitched during flight, etc all vary depending on the species.

To fold back, gravity pulls it down and a muscle contracts and locks it again. It’s not really hard to imagine any of this; anyone with an inkling of knowledge of how any engineering part works can picture it. In a way, Velcro and electrical plugs are nothing more than us mimicking elytral locks.

I seem to have wandered off any sort of logical path from the origin of wings and flight by now, so I’ll just stop here. As always, elaborations and more info can be requested (privately or by comment).

References:

Bomphrey, R. J. 2006. Insects in flight: direct visualizations and flight measurements. Bioinspiration & Biomimetics 1, S1-9.

Hasenfuss, I. 2008. The evolutionary pathway to flight – a tentative reconstruction. Arthropod Systematics & Phylogeny 66, 19-35. [Warning: PDF!]

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