The Physics of Bees

In this third installment of The Physics of Native Gardening series, I look at the physics of bees. Each year we get more and more bees coming to our garden. I especially like the big, fat bumblebees. Some days there are so many bees that they swarm their favorite flowers. You can tell they are hard at work gathering pollen; busy as a bee.

Bumblebee Flight

Sometimes you’ll hear nonsense claiming that according to the laws of physics bumblebees can’t fly. A focus article about a 2016 Physical Review Letters paper states

Relatively small insects, like bees and house flies, have a different flying strategy than larger insects and birds. They have fairly rigid wings, which they flap as much as a hundred times per second. The flapping motion is not strictly up and down like for birds but more forward and backward… Starting with the wings “clapped” together and vertical behind the insect’s back, the wings then tilt steeply away from one another as the forward stroke begins. As they move, the wings produce low pressure in the region just behind the leading edge… The air in front of the wings curls up and over them and forms a horizontal, tornado-like vortex along the back side of the leading edge. Previous experiments and computer simulations have shown that leading edge vortices produce a suction effect that gives the necessary lift for keeping an insect aloft.

Ultraviolet Vision

An Immense World, by Ed Yong.

Bee vision is different than human vision. The human retina has three color receptors, each sensitive to different frequencies in the visible spectrum (blue, green, and red). The bee also has three receptors, but only two are in the visible spectrum (blue and green); the other is in the ultraviolet (light with a wave length shorter than what the human eye can see). In his book An Immense World: How Animal Senses Reveal the Hidden Realms Around Us, Ed Yong wrote

Flowers use dramatic UV [ultraviolet] patterns to advertise their wares to pollinators. Sunflowers, marigolds, and black-eyed Susans all look uniformly colored to human eyes, but bees can see the UV patches at the bases of their petals, which form vivid bullseyes. Usually, these shapes are guides that indicate the position of nectar.

Oxygen Diffusion

Bees don’t have lungs. Yet, all that flying means they need a lot of oxygen to power their high metabolism. How do they supply oxygen to their muscles?

Instead of having vessels to transport blood, insects use small, air-filled pipes (trachea) that deliver oxygen. The pipes typically have a dead end, so you can’t just flow air through them. The oxygen is supplied by diffusion.

Diffusion is the movement of a molecule from a region of high concentration to low concentration. It works quickly over short distances, but takes a long time over long distances. Russ Hobbie and I discuss diffusion in Chapter 4 of Intermediate Physics for Medicine and Biology. Oxygen can diffuse through air ten thousand times faster than it can diffuse through water. Therefore, filling these insect pipes with blood would make oxygen diffusion way too slow. But having them filled with air means plenty of oxygen can diffuse in, powering flight.

One potential problem for bees would be if water was sucked into their trachea by capillary action. This would drastically lower the diffusion of oxygen. To avoid this, the trachea may be coated by a waxy substance that repels water. The pipes are so small that it is difficult to know for sure, but capillary action is so powerful there must be some way to keep water out.

Thermoregulation of Flight Muscle

Air and Water, by Mark Denny.

In his book Air and Water, Mark Denny discusses how tiny bees stay warm. As we saw last week when describing hummingbirds, one way to produce a lot of heat is having a high metabolic rate. But it’s complicated. Denny writes

It is a tribute to the metabolic and respiratory machinery of bees and other flying insects that this magnitude of metabolism is possible. Note, however, that this high metabolic rate is closely tied to activity on the part of the bee. It must raise its metabolic rate well above that at rest (by shivering, for instance) to heat itself up. As soon as it becomes inactive, its body temperature returns to that of the ambient air.

There is a flip side to this puzzle. If a flying bee can heat its muscles to 30° C on a cold day, it seems likely that it could overheat on a hot day. Indeed this is the case, and bumblebees have evolved mechanisms that allow them to dump heat effectively from the thorax to the abdomen, from where it is shed to the air.

If you want to learn more about bees, there’s an entire website about physics for beekeepers. It’s mainly about honey bees, which are a little like cattle: non-native domesticated livestock that are commercially important. I’m not so interested in honey bees, but focus more on the many solitary, ground-nesting or cavity-nesting species of bees native to Michigan that live in our yard, such as sweat bees, carpenter bees, mason bees, and miner bees.

Many of the physical constraints faced by bees are shared by their fellow insects, the butterflies. Next week we’ll examine in more detail the physics of butterflies.

Native bees of Michigan with Dr. Rebecca Tonietto

https://www.youtube.com/watch?v=EICFH-DIYGU