On diaphanous wings they soar

Imagine being born with wings. As you chew your way out of your cell, you slowly become aware of something weird. In addition to the requisite six legs and two antennae, you also have these…um…well, what are they exactly? According to the book, they are not appendages but simply outgrowths of the cuticle. As you continue to excavate chunks of wax with your mandibles, you wonder where you learned the words “appendage” and “cuticle.” And what is simple about it? You finally pull yourself free. Wow. Life is going to be some kind of trip!

Indeed, our little bee read the right books. Wings are not appendages, but arise from a structure called a wing bud.¹ Pretend for a minute that you are inside the thorax of a bee chewing some bubble gum. You see a slit in the integument (the skin) and decide to blow a bubble right through the slit to the outside of the bee. As the bubble gets larger and larger, the walls of the bubble get thinner and thinner. Then finally it pops. The two thin layers collapse and lie one atop the other on the outside of the bee.

Layers with channels

What you have is something similar to the basic structure of a bee wing. Of course, they are not made from bubble gum, but from cuticle. And through the millennia, the two thin layers evolved to have veins that give the wing shape and strength, and transport hemolymph and oxygen throughout. The veins are not separate structures like our own veins, but more like reinforced channels that separate the two thin layers of wing.

Like most hymenopterans, bees have two pairs of wings. In bees, the forewings are larger and extend from the first thoracic segment, while the smaller hindwings extend from the second thoracic segment. The wings begin to form from the buds just before the larva becomes a pupa. When the young bee emerges from her cell, her integument is light-colored and soft, and her wings have not yet hardened. This youngster, called a teneral, must wait a few hours to mature before she becomes a fully-functioning adult. She can’t yet fly and she doesn’t yet produce venom.

The pattern of wings

The wing veins are in no way random but are clearly defined by genetics. In fact, the length and shape of the veins, and the shape of the cells formed by the veins, are commonly used for bee identification. Each vein and each cell has a standard name, and the basic pattern of veins is consistent throughout all the families of bees.²

An experienced melittologist can often identify a bee to the genus level after a quick glance at the wings. Refining the classification down to the species level, however, may require a dissection of the mouth or reproductive parts. At least for now. With advances in facial recognition and similar types of software, we may soon be able to simply scan a wing vein to learn the species, subspecies, or even the race of a bee.

Bees in the genus Apis have unmistakable wings. The marginal cell is particularly long and curved, and the three sub-marginal cells have unusual shapes compared to other bees. If you live in an area with only one Apis species—like we do in North America—you can distinguish a honey bee from all other bees just by looking at that one cell. This can come in handy if you are trying to identify a bee that is diseased, moldy, wet, torn up, or partially eaten. You only need one good forewing for honey bee identification.

Honey bee’s marginal cell: The marginal cell of a honey bee wing is long and narrow, running almost to the wing tips. © Rusty Burlew.

Other wing structures

In addition to carrying hemolymph and oxygen, some of the veins also contain nerves. Because of these nerves, some beekeepers speculate that queen wing clipping—once widely practiced as a deterrent to swarming—may actually be painful. In addition, drops of liquid may ooze from the clipped veins, indicating a loss of hemolymph. Since clipping is an unreliable method of swarm control, wing clipping is best left to the history books, much like bloodletting.

Wings have other features that are harder to see. Hairs are often found on the outer surfaces, both above and below. The hairs vary in position, length, and density depending on the species. The forewings of bees have a stiffened area running along the front edge. Made of two parts called the pre-stigma and stigma, these reinforce the leading edge of the wing—the part that cuts through the air. Both the size and shape of the stigma vary by genus, so they, too, are used for identification.

Bee wings are fragile because they have no structural support beyond the veins. As a consequence, the wing tips quickly fray and tatter. It is not unusual to see an old forager whose wing membranes have disappeared all the way back to the veins—or even further. This makes flight difficult, so damaged wings signal the end of life.

Stigma: The size of the stigma varies with the bee species. The stigma and the thickened portion of the wing edge leading up to it add strength to the wing. This Lasioglossum bee has a large, clearly visible stigma. © Rusty Burlew.

Hooks and loops

To assure that all the wings work together and don’t get tangled, the bee hooks her two sets of wings together before taking off. The rear edge of the forewing has a fold running along its length, and the leading edge of the hind wing has a corresponding row of upturned hooks called hamuli. Before flight, the bee drags her fore wings over the hind wings, which causes the hamuli to latch onto the fold. The coupled wings act as one, keeping them in sync.³

All bees have this system of making two small wings into one large wing. Other insects with wings, including the dragonflies, have wings that act independently during flight.

Connected wings: When the wings are hooked together with the hamuli, each wing pair looks and acts like a single large wing. © Rusty Burlew.

Wings and flight

Within the bee thorax are two complete systems for moving wings. One system is known as the direct system, and the other as the indirect system.¹ Although they work together, they control different movements.

The direct muscles attach to the wings themselves and allow the bee to move each wing independently. A bee can move her wings out—perpendicular to her abdomen—or back in, she can twist them forward and aft, she can move one over the other, or she can rest them over her back. These muscles are similar to those that control your arms and legs: you can move an extremity while holding the rest of your body completely still.

The second set—the indirect muscles—are not connected to the wings. Instead, they are attached to the insides of the thorax, which is actually quite flexible. Since the wings are outgrowths of the thorax, muscles that move the thorax also move the wings.

The indirect muscles come in two types, vertical and horizontal. Vertical muscles run from the top of the thorax to the bottom. When these muscles contract, the thorax compresses from top to bottom. When the thorax is compressed, it gets wider, forcing the wings up. These are sometimes called elevator muscles, since they elevate the wings.

The second set of muscles, the horizontal ones, run from the front of the thorax to the back. When these contract, the thorax becomes shorter from front to back but taller from top to bottom, which forces the wings down. Not surprisingly, these muscles are known as depressors.

When these two sets of indirect muscles work together, alternately contracting and relaxing, the bee’s wings are raised and lowered at an incredibly fast rate. In fact, the wings of a honey bee beat at about 250 cycles per second, faster than the nerve impulses can travel from the brain to the muscle. So how does that work?

Bee wings as springs

It turns out that the bee can store potential energy in her muscles. This allows the wing to beat several times after receiving one nerve impulse. The mechanism has been compared to a spring. The spring stores energy quickly when the nerve impulse arrives, but it parcels the energy out over several wing movements until the next impulse arrives.⁴ If a bee were allowed only one movement per nerve impulse—the way we are built—she would never be able to fly.

In spite of all that wing speed, a bee still needs some aerodynamic refinement to get where she is going. If you were able to watch a bee’s wing from the side, you would see that instead of lifting straight up and then pushing straight down, the wing tips trace a figure eight pattern in the air.¹ The wings tilt forward and backward much like the oars of a racing shell. This type of movement gives the bee maximum power during the down strokes but minimum drag during the recovery strokes.

Although all bees work basically the same way, other insects do it differently. Those dragonflies I mentioned earlier also have four wings, but their wings do not hook together and they beat out of phase with each other. Depending on what the dragonfly is doing, the wings may beat totally out of phase (hovering) or only slightly out of phase (taking off). But a dragonfly’s wings beat at only about 40 cycles per second, which is drastically slower than a honey bee’s rate of 250 cycles per second. As you can see, these two insects take completely different approaches to the problem of flight.

Although bees can vary the rate of their wing beats, the normal rate we hear as they forage from flower to flower is fairly constant. When I’m staring through a camera lens trying to photograph bees, it is very easy to hear the different species. If I’m focusing on a bumble bee, I might hear a honey bee to my right and a wool carder to my left. Not only are the frequencies different, but the rhythm of stops and starts is characteristic of each species. Hang around them long enough, and you can tell them apart by sound alone.

Wings in motion: Bee wings move incredibly fast in a figure eight pattern. The speed is made possible by the indirect flight muscles. © Rusty Burlew

Hovering

A common misconception about bees is that they can’t hover. In fact, I’ve often been told that’s one way to tell a bee from a fly. By that is simply not true. Bees are some of the best hover craft around, but different bees do it in different ways. Some bees, especially male Anthidium and Habropoda can be extremely intimidating. They select a spot about six inches from my face, stop dead in the air like a flying saucer, and dare me to come any closer. They can’t sting but their message is clear: “Don’t mess with my women!”

Honey bees don’t hover mid-air in the fashion of some other bees. Instead, you are likely to see a honey bee hover at a flower. She assumes a more vertical position, with her legs dangling and her proboscis extended. If she finds a good stash of nectar, she may actually land on the petals. If not, she may just go on to the next one without ever actually landing.

Hovering at flower: When a honey bee hovers at a flower, she is nearly vertical. Some bees can hover in the horizontal position, much like a hover fly. © Rusty Burlew.

More than flight

In the life of a honey bee, wings and the associated muscles have many uses other than flight. The workers use their wings to work for all sorts of projects such as cooling the hive, drying honey, distributing queen pheromone throughout the hive, and signaling each other with the Nasonov gland.⁵ For these jobs, the honey bee must make adjustments to her wing system. If she didn’t, she wouldn’t be able to remain stationary and beat her wings at the same time.

Since the wings are controlled by two sets of muscles, the bee can control her movements by using different combinations of direct and indirect flight muscles. For example, the tilt of the wings can prevent her from taking off when she is trying to stand in one place and fan. Or, if she keeps her wings folded over her back, she can make her indirect flight muscles contract rapidly to produce the heat required to keep her—or the rest of the colony—warm during cold weather.

Problems with wings

As cool as wings are, they are often the weak point in a bee’s anatomy. Because wings wear out with use, they are much like a set of tires—they come with an expected life span. How many miles can you get from one set of wings? Apparently, it varies with the species. The few estimates I’ve seen for honey bees predict a 500-mile maximum, but there are many variables. As the wings wear, the bee has to work harder to fly. She also loses maneuverability, which makes her more susceptible to predation and other misadventures such as wind and rain.

Another downfall of wings is their susceptibility to a number of debilitating bee viruses. In honey bees, these include deformed wing virus, K-wing virus, and cloudy wing virus. Some of the viruses, such as deformed wing virus, affect the development of the wing before the bee reaches adulthood. As a result, the affected bees emerge with their wings already ruined. Some other viruses can affect the wing or the wing muscles after the bee has reached maturity.

The extremely thin membranes of the wings also makes bees vulnerable to environmental toxins such as pesticides. Once toxins are absorbed through the very thin layers of wing, they can be delivered throughout the bee’s body via the hemolymph that runs through the wing veins. In one experiment, researchers were able to kill honey bees by exposing only the wings to pesticide.⁶

In addition, we mustn’t forget the normal, everyday problems with wings. The large surface area makes moisture—whether it’s from rain, mist, fog, or a sprinkler system—extremely hazardous to a bee because of the added weight. And don’t forget the surface tension of water. We’ve all seen bees with wet wings stuck upside down on a moist landing board or a dewy leaf while she pedals the air, frantically trying to free herself. The very properties that make wings work in good weather—such as being light and thin—make them hazardous in bad weather.

Worn wings: When a bee’s wings are this tattered, flight becomes difficult. This bee hasn’t much longer to live. © Rusty Burlew.

Hope soars

Regardless of the problems, the wings of the bee power the plant world, effectively pollinating many species, both wild and cultivated. Shakespeare said, “True hope is swift, and flies with swallow’s wings.”⁷ Instead, I would say, “Hope soars on the wings of the bee.” How something so fragile can be so powerful, and so profoundly affect our planet, is truly a wonder.

Rusty
Honey Bee Suite

References

1. Mattingly RL. 2012. Honey-Maker: How the Honey Bee Worker Does What She Does. Portland, Oregon. Beargrass Press.
2. Michener CD. 2007. The Bees of the World. The Johns Hopkins University Press.
3. Snodgrass RE, Erickson EH, Fahrbach SE. 2015. The Anatomy of the Honey Bee in JM Graham (Ed) The Hive and the Honey Bee (pp 111-165) Hamilton IL. Dadant & Sons, Inc.
4. Clark CJ, Mountcastle AM, Mistick E, Elias DO. 2017. Resonance frequencies of honeybee (Apis mellifera) wings. Journal of Experimental Biology 2017 220: 2697-2700; doi: 10.1242/jeb.154609.
5. Winston ML. 1987. The Biology of the Honey Bee. Cambridge, Massachusetts. Harvard University Press.
6. Poquet Y, Kairo G, Tchamitchian S, Brunet JL, Belzunces LP. 2015. Wings as a new route of exposure to pesticides in the honey bee. Environ Toxicol Chem 34: 1983-1988.
7. 1592-1593. King Richard III. Act 5, Scene 2, Line 23.