This micromoth (wingspan around 12mm) is found throughout temperate North America and is positively adorable. Like other plume moths, this tiny lepidopteran has a wing composed of small, hairy, feather-like appendages (6 on each wing, 24 total, to be precise) that interlock, flapping as a single unit. Of course, when we think of “normal” butterfly and moth wings, we think of their wings acting like a single, very solid, paddle-like structure. Plume moths, on the other hand, have a wing that more closely resembles a tennis racket than a ping-pong paddle. But if we replaced bird’s wings with wire tennis rackets, they certainly wouldn’t be able to fly anymore, right? So how do these tiny moths do it? What is it about their tennis rackets that make them act more like ping-pong paddles?
Well, plume moths are tiny. And when you’re tiny, you interact with fluids in a much different way than you would if you were gigantic. For example, imagine you are a giant, 30-meter whale. You wave your tail once and it sends you coasting for hundreds of meters through the ocean. Now imagine you are a bacterium and we apply the exact same force to your tiny body. How far do you think you would coast before coming to a stop? 100 meters? 10 meters? Less than a meter?
As a matter of fact, you would coast to a stop before you had even traveled the width of a single water molecule. That’s less than one ten-billionth of a meter! Crazy, right?
If we think about the size of a bacterium relative to the size of water molecules, this analogy begins to make some more sense. As a whale, even the smallest movements can push a huge volume of molecules. Whereas a bacterium swimming through those same molecules feels more like a child trying to swim through a ball-pit at McDonald’s.
This idea that large things interact with fluids in different ways than small things was summarized into a single number by the British fluid mechanist, Osborne Reynolds in the 19th century. This number, known as the Reynolds number, is a ratio that describes the size of the swimmer, its speed, and the density of the fluid in relation to the viscosity of that fluid. So, a whale moving through a swimming pool full of water has a higher Reynolds number than if it was swimming through molasses, and humans have a higher Reynolds number when we’re jogging than when we’re swimming.
We can also think of the Reynolds number as an object’s susceptibility to the inertial forces of a fluid. A gust of wind does not have the same effect on a stone as it does on a leaf, while a “gust” of water, moving at the same speed, is likely to move the stone much farther than the gust of wind ever could.
So how does this affect the wings of plume moths? Well, Alucita's wings are so small, and the gaps between the barbs on their plumes are so minuscule that they affect air molecules in a way very similar to the solid wings of butterflies. So, in a bird with tennis-racket-wings, although its rackets are much larger and could potentially push more air than the plume-moth-wings, the gaps between the wires on the bird’s rackets are much larger in relation to air molecules than the gaps in the plumes, allowing air to pass right through with little resistance.
So how tiny would those gaps have to be to give the bird’s racket-wings the ability to move air molecules sufficiently for flight? Well, they’d have to be about as tiny as the space between barbules in a feather. As a matter of fact, thinking about a bird wing as a racket rather than a paddle really isn’t that farfetched. Bird’s wings, like plume-moth wings, are made up of feathers (duh; analogous to the moth’s plumes), which are made up of thousands of tiny barbs (analogous the hairs on the moth’s plumes) that branch into millions of interconnecting barbules (see SEM image and diagrams above). The gaps between these barbules are around 20 millionths of a meter (μm). It’s no surprise then, that the gaps between the hairs on Alucita's plumes are also 20μm apart. So, although a bird’s wing most certainly acts like a solid paddle, its structure is much more similar to that of the moth’s tiny plumes than a butterfly’s sheet-like wings.
[Now, this analogy superficially appears quite solid, but if we take a closer look, we’d quickly realize that it’s actually full of holes. Differences in the body-mass-to-wing-size ratio of birds and plume moths, as well as minute structural characteristics of barbules, make the ways in which each organism interacts with air very, very different in spite of the similarities in the gap-sizes of their wings.]
If you’re still curious, check out my list of references below. If you’d like to learn more about Reynolds numbers and the differences between sperm and sperm whales, check out this excellent TEDed Video.
And if your desire for fluid dynamics has not dissipated, you can always follow me as I continue learning more about the mechanics of flight at the University of Montana’s Flight Lab, or check out the blog F*ck Yeah Fluid Dynamics.
Links and References:
by Robert Niese, PhD student in the College of Humanities and Sciences Division of Biological Sciences Flight Lab at the University of Montana. Reprinted with permission from the blog So Much Science. Image of plume moth wings by Robert Niese.