# Air Fish
**URL:** https://boards.straightdope.com/t/air-fish/823424
**Category:** Factual Questions
**Created:** [October 24, 2018, 8:00am UTC](https://boards.straightdope.com/t/air-fish/823424 "2018-10-24T08:00:05Z")
**Posts on this page:** 12
**Page:** 1
### Author:  [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)
#### Post date: [October 24, 2018, 8:00am UTC](https://boards.straightdope.com/t/air-fish/823424/1 "2018-10-24T08:00:05Z")
I was considering the fact that [viscosity increases as your size shrinks](https://www.youtube.com/watch?v=clxn9rNYzNo).
This would imply that at some size, air is possibly as viscous to some tiny critter as water is to a fish.
Fish are fish-shaped because that is the most efficient shape for moving through a medium of that viscosity.
This implies that there may be some tiny critter that is fish shaped, swimming through the air we breath.
Is there?
---
### Author:  [@Riemann](https://boards.straightdope.com/u/Riemann)
#### Post date: [October 24, 2018, 8:44am UTC](https://boards.straightdope.com/t/air-fish/823424/2 "2018-10-24T08:44:54Z")
Some small insects generate drag-based thrust similar to the paddling mechanism of fish. But bear in mind that fish don’t exclusively use this mechanism, they also use hydrofoil lift.
> **[2011\_PRL\_Ristroph.pdf](https://dragonfly.tam.cornell.edu/publications/2011_PRL_Ristroph.pdf)**
>
> 881.97 KB
[http://dragonfly.tam.cornell.edu/insect\_flight.html](http://dragonfly.tam.cornell.edu/insect_flight.html)
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.
[http://www.pnas.org/content/110/7/2575](http://www.pnas.org/content/110/7/2575)
---
### Author:  [@Mangetout](https://boards.straightdope.com/u/Mangetout)
#### Post date: [October 24, 2018, 9:45am UTC](https://boards.straightdope.com/t/air-fish/823424/3 "2018-10-24T09:45:29Z")
> [@Riemann](#):
>
> Some small insects generate drag-based thrust similar to the paddling mechanism of fish.
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.
---
### Author:  [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)
#### Post date: [October 24, 2018, 2:35pm UTC](https://boards.straightdope.com/t/air-fish/823424/4 "2018-10-24T14:35:34Z")
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?”
---
### Author:  [@Riemann](https://boards.straightdope.com/u/Riemann)
#### Post date: [October 24, 2018, 5:00pm UTC](https://boards.straightdope.com/t/air-fish/823424/5 "2018-10-24T17:00:50Z")
> [@Mangetout](#):
>
> 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.
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):
> [@](#):
>
> By analyzing high-speed video of the fruit fly, we discover a swimminglike mode of forward flight characterized by paddling wing motions…Reduced-order models and simulations reveal that the law for flight speed is determined by these wing motions but is **insensitive to material properties of the fluids**. Thus, paddling is as effective in air as in water and represents a common strategy for propulsion through aquatic and aerial environments.
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.
---
### Author:  [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)
#### Post date: [October 24, 2018, 5:29pm UTC](https://boards.straightdope.com/t/air-fish/823424/6 "2018-10-24T17:29:51Z")
> [@Riemann](#):
>
> 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.
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.
Stranger
---
### Author:  [@Riemann](https://boards.straightdope.com/u/Riemann)
#### Post date: [October 24, 2018, 5:43pm UTC](https://boards.straightdope.com/t/air-fish/823424/7 "2018-10-24T17:43:32Z")
> [@Stranger\_On\_A\_Train](#):
>
> 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…
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.
---
### Author:  [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)
#### Post date: [October 24, 2018, 6:32pm UTC](https://boards.straightdope.com/t/air-fish/823424/8 "2018-10-24T18:32:04Z")
> [@Riemann](#):
>
> Right, since no living creature can maintain neutral buoyancy in air […]
My expectation was that this fact would negate the hypothesis, but I figured it didn’t hurt to ask (there was some chance that I’d learn something new).
That flying insects wag their bodies a bit like fish seems like a reasonably close response that also answers how the buoyancy issue was resolved.
---
### Author:  [@Riemann](https://boards.straightdope.com/u/Riemann)
#### Post date: [October 24, 2018, 9:15pm UTC](https://boards.straightdope.com/t/air-fish/823424/9 "2018-10-24T21:15:16Z")
> [@Sage\_Rat](#):
>
> That flying insects wag their bodies a bit like fish seems like a reasonably close response that also answers how the buoyancy issue was resolved.
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.
---
### Author:  [@md2000](https://boards.straightdope.com/u/md2000)
#### Post date: [October 25, 2018, 3:52pm UTC](https://boards.straightdope.com/t/air-fish/823424/10 "2018-10-25T15:52:41Z")
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.
---
### Author:  [@Sage\_Rat](https://boards.straightdope.com/u/Sage_Rat)
#### Post date: [October 25, 2018, 10:09pm UTC](https://boards.straightdope.com/t/air-fish/823424/11 "2018-10-25T22:09:40Z")
> [@Riemann](#):
>
> 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.
Well I mean, we’re comparing vertebrates to exoskeletal invertebrates. Getting that close is sufficient for me to feel like I’ve learned something thanks to asking the question.
---
### Author:  [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)
#### Post date: [October 25, 2018, 10:39pm UTC](https://boards.straightdope.com/t/air-fish/823424/12 "2018-10-25T22:39:04Z")
Here is a video of a Bumble Bee showing how they fly mostly with back an forth movements similar to how a fish uses fins.
[ ](https://www.youtube.com/watch?v=hGhjFGKdTEM)