How do insects fly? The question sounds simple enough (they flap their wings), but there's actually a lot of interesting physics behind it.
Last year, scientists discovered that flying critters, ranging in size from insects to birds, only need passive forces enabled by symmetrical wingbeats to slow down during a slow-motion turn. Active control is required for slowing down after more complex flying maneuvers, but passive forces are sufficient for many occasions.
Another question regarding insect flight that may raise interesting physics questions is how termites fly in rain. This is an important issue; termites establish new nests in rain.
This is a problem for any insects with a large wing surface area relative to their body mass. To avoid getting stuck on water (such as ponds) or on wet surfaces (such as leaves), such insect wings are often water-repelling.
However, termites are weak fliers. Since they clearly fly well in rain, there may be more to their wings than is currently known.
Gregory Watson (James Cook University, Australia) and coworkers have shown how the micro- and nanoarchitecture of termite wings aids flight in rain. They have also shown how this small-scale architecture minimizes the weight and materials required in the wings, further aiding flight.
Discovering small-scale architecture in termite wings.
The scientists first found that termite wings appear to be exceptionally water-repelling. A drop of water makes a 180° contact angle with the wing.
However, when viewed at higher magnification (tens of micrometers), the scientists found that the hairs on the wing (there may be thousands of them per wing) prevent the water drop from coming into contact with the wing at all. The hairs keep the water drop suspended above the wing.
On even further magnification (one micrometer or less), the scientists observed nanometer-scale open troughs running the length of the hairs. Such troughs are seen on the legs of other insects that walk on water.
These troughs are thought to prevent leg penetration into water. The physical basis for this hypothesis is that surface roughness is known to impart increased water repulsion from the surface as long as the size of the water drops is sufficiently larger than the surface roughness (sufficiently larger than the size of the channels).
Testing the small-scale architecture hypothesis.
The scientists tested the hypothesis that open troughs on the termite hairs are required for water repulsion by filling in the channels with a water-repelling polymer. They then attempted to force the hairs through a drop of water.
Coated hairs penetrated a water drop at a force of 1.6 micronewtons. Uncoated or thinly coated hairs did not penetrate a water drop at a similar force.
The thickly coated and uncoated hairs were both similarly stretchy (the spring constants were very similar), and the thickly coated hairs were far more adhesive to water (meaning that the water spreads across the hairs). These results strongly suggest that the open troughs on the hairs impart exceptional water repulsion to termite wings.
The scientists further found that small star-shaped hairs on the wings (there may be over one million of them per wing) are also highly water-repellant, yet add only a few percent of body mass to the insects. Other scientists have speculated that these hairs weigh termites down, facilitating predation by birds and other critters.
However, this is clearly not the case. Small-scale architecture, not extraneous material, imparts remarkable water-repelling properties to termite wings, and aids their flight in wet environments.
Future investigations.
It's possible that this small-scale architecture may enable termites to escape from spider webs and other solid surfaces, and reduce charge-based interactions with surfaces, in addition to the clear water-repelling properties discussed here.
On a practical note, the small-scale architecture in termite wings uncovered by these scientists may inspire other scientists looking to impart new and improved water-repelling properties to small-scale devices.
for more information:
Watson, G. S., Cribb, B. W., & Watson, J. A. (2010). How Micro/Nanoarchitecture Facilitates Anti-Wetting: An Elegant Hierarchical Design on the Termite Wing ACS Nano, 4 (1), 129-136 DOI: 10.1021/nn900869b