Viscosity isn’t the only consideration. Since biological reactions take place in aqueous solution, there is no difficulty for a sea-dwelling creature to maintain approximately neutral buoyancy. Neutral buoyancy is not practical for an airborne organism of any size. But if you’re small enough, air currents completely dominate the tendency for a denser object to eventually drift downward with gravity. So in addition to everything smallish that gets blown around at lower levels there are plenty of microorganisms way up high.
I think if we’re looking for comparisons with aquatic organisms, the flight mode of very small insects is sort of similar to the swimming mode of things like diving beetles (albeit using wings in the former case and legs in the latter)
Many fish do ‘swim’ using their pectoral fins, but mostly, they move by flexing their tail or whole body - I can’t think of any insects that do that.
Also keep in mind that “fish shaped” isn’t the only choice for swimming in the water–there are (or were) many ocean and fresh-water swimmers shaped nothing like fish. Octopusodii, for example, and eurypterids. And plenty of fish shaped nothing like fish–think catfish vs manta vs sunfish vs lanternfish–which one is “fish shaped?”
Well, I think the reason that no airborne flyers are designed quite like fish is probably the buoyancy issue - even if they were small, they would simply fall down. Airborne flyers cannot maintain neutral buoyancy, so they need to generate larger forces to stay airborne - larger wings, faster flapping.
So I was considering the size-viscosity question. However, I think I failed. Having read that paper more carefully, the authors say that the drag-based thrust mode is almost independent of fluid density. From the summary of that paper on their webpage (linked above):
So it’s not the greater viscosity experienced by a small flyer that allows it. I can’t see any discussion of why only small flyers use it, but it could be just the biomechanical strength required, the square-cube law.
There are so-called aeroplankton, which are small organisms from microbes and protists up to fungal spores and tiny arthropods (spiders, aphids), and of course pollen and seeds which are buoyant enough to be carried in light winds, and a few of these have some degree of self-locomotion in air, but because of the relative scales involved they cannot exert enough force upon the surrounding medium to move as fish do through water. This is in part because there just isn’t sufficient surface area to make way against moving wind, but also because water is essentially incompressible while air is definitely not, so when you push against water you can get a net directed momentum exchange whereas when you push against air it just puffs out of the way in all directions. This is why you can redirect a stream of water with your hand but smoke will just billow around it, and why astronauts on the ISS can’t swim through the air like you can swim in a pool on Earth.
That said, some spiders are able to deploy a web filament and then use their limbs to control direction, somewhat analogous to how a sailboat can tack against the wind. This is only transitory but it is sufficient that they can direct themselves toward a landing spot rather than just going wherever the wind takes them.
It would be possible for normal air to be sufficiently dense that you could swim in it, but then you’d be in an environment like Venus, and the oxygen molarity would be so great that virtually any interaction would result in a violent explosion. So…don’t even think about it.
Right, since no living creature can maintain neutral buoyancy in air, possible strategies are to be large enough (down to insect size) that you can generate significant force for both propulsion and lift, in which case you won’t look like a fish, because greater forces are required - larger wings, faster flapping; or to be so tiny that air currents dominate gravity, in which case air currents blowing you around will also dominate any efforts at active self-propulsion, you’ll be largely passively blown around and you still won’t be designed like a fish.
I’m not sure where you are drawing this conclusion from. I think the opposite is true. Insects cannot generate sufficient thrust to overcome negative buoyancy using fish-like biomechanics. The paper I linked to said that the aerodynamics of the drag-based thrust paddling mechanism in some insects are similar to the aerodynamics of the pectoral fins of fish, but it did not say that any of the biomechanics involved are similar to fish.
There’s the additional issue hat the distinction between gas and liquid is that fluids are essentially incompressible. Therefore the effort to “push” against the fluid is less when it’s a liquid, the fluid mass you displace backwards to go forwards matters (which is why kicking works for swimming but not for basketball jump shots). (And to be fair, density and mass of displaced fluid is different in air vs water which is also a factor) So a gas may have a similar viscosity at VERY small scale, but still nowhere near as convenient for motion, even ignoring the buoyancy issue.
Although I did attend a presentation once, “Life at a Low Reynolds Number” but I forget the details other than they were talking about bacteria scale.
I do recall an article about how insects hover, which suggests they have a circular wing motion not unlike the hand motions for treading water.