How does a moth hover?

Our first Humming-bird Hawk-moth sighting in our Shropshire garden

Now, I've seen humming birds in the cloud forests of the Andes and they are amazing creatures – the only birds able to hover. Sustained hovering by the bird requires the highest metabolic rate of any vertebrate. There appear to be multiple genetic mutations to the genome of the Apodiformes (swifts, treeswifts and hummingbirds) that allow this: the ability to metabolise sugars in nectar at a very high rate, the cardiac output to deliver this energy to the tissues of the body at about ten times the usual rate and, of course, the wing speed and control to generate enough lift to hover and hold position while feeding. It's a truly amazing product of evolution.

So, I wondered, how can a moth perform the same trick? A moth is clearly a simpler organism than a bird, without its folding wings, powerful muscles and cardiovascular system. How can it meet the same challenges of power-to-weight, energy consumption and controlled hover? It's a fascinating example of convergent evolution.

I'm not the only person to wonder this. Recent studies in the field of micro air vehicle (MAV) research are taking inspiration from flying insects and birds to achieve new flight capabilities. The power consumption and control systems necessary for such small robots are beyond current technologies but the quest to learn from nature is bringing together an unlikely combination of engineers, biologists and roboticists. An MAV carrying a small camera was used to peek inside the Fukushima Nuclear power plant after the 2011 earthquake. More generally, there are plenty of areas where the study of fluid dynamics suggests that mimicking natural structures leads to greater efficiency. You may see fish-scale patterns on the wheel rims of riders' bikes in this year's Tour de France!

It should be remembered that hovering takes considerably more power than level flight, because there's no air movement across the wing and no glide between power strokes.

Hovering flight

The Humming-bird Hawk-moth (Macroglossum stellatarum) typically weighs less than 0.3g. A 2021 study in the Journal of Experimental Biology [1] calculated that the moth needed a constant power output of 0.0017 Watts (1.70mW) to hover while feeding. Scaled up to the size of an elite athlete (weighing say 75kg), that's equivalent to about 425W – roughly what the winner of this year's Tour de France will need to deliver in the Alps and Pyrenees.

The study also set up multiple high-speed cameras, some operating at a thousand frames per second, to study the flight dynamics of the moth's wings. A laser illuminated smoke particles in the air to show the downdraft created during the hover. This revealed that the moth's wings create downforce on both downward and upward strokes. Also, while moths don’t have the flexibility to twist their wings on the upstroke as birds do, they do adjust their wing angles and shed vortex structures from the wingtips in a complex way that supports the need to manoeuvre properly, as well as producing a nice humming sound. Even with that ability, compensating for gusts of wind must require rapid and accurate control of the wings, beating about 69 times a second!

[1] Journal of Experimental Biology, Volume 224, Issue 10. 'Hovering flight in hummingbird hawkmoths: kinematics, wake dynamics and aerodynamic power'. Warfvinge et al.

Sight and sound

For the musically trained, 69Hz is D♭2 (D-flat 2), or the first stopped half-tone on the cello's lowest string (listen to 69.3Hz here). My photograph was exposed for 1/400th of a second, which is about 1/5th of a beat, so it's no surprise that the wings are blurred.

To top this all off, the moth has highly effective trichromatic (like us) eyesight to select its high nectar food sources, enable it to direct its long (25-28mm) proboscis to the target and triangulate a fix for its position during the hover. This discrimination is not simple to achieve with the small sensor sites of the compound insect eye. The resolution is limited by a phenomenon known as diffraction because light is a wave. Moths have a superposition optical system [2], unlike butterflies but like bees and glow-worms. A bee's eye resolves around one degree. To get an idea of how the world would look through such eyes, imagine the scene divided into pixels about the size of your fingernail at arm's length.

What a marvel the Humming-bird Hawk-moth is! I hope this little piece provides some ammunition to use the next time someone tells you that moths aren't interesting…

[2] Eyes to see. The astonishing variety of vision in nature. Michael Land. Oxford University Press.

Thinking about those 1.7mW of power generated, you can see some of the associated heat in a thermal image of a hovering Humming-bird Hawk-moth on the West Midlands Moth Atlas