Rusty Burlew’s Bee-Vision Guest Blog.

I’m excited to announce this week’s guest blog on bee vision by the world’s number one bee blogger, Rusty Burlew. I’ve been a fan of Rusty’s for years, and avidly read her frequent articles in the American, and worldwide beekeeping journals. I admire the way she describes even the most complicated subjects in simple terms, and brings in a good measure of humour. So, I’m really grateful to her for her contribution to this week’s Beelistener. Thank you, Rusty.

Rusty has studied agriculture, honey bees and the environment for over 30 years. She is passionate about the conservation of native bees and founded the Native Bee Conservancy in Washington state. You can read more of her work on the popular and world famous blog: Honey Bee Suite.

Harnessing the Power of Honey Bee Vision

“The better to see you with, my dear!” said the big bad wolf to Little Red Riding Hood. The words sound ominous. Were they spoken by a bee, even a wolf might quiver in fear, especially when he saw those outsized peepers and the three little backup cameras. Time to run!

Why so many?

Honey bees are not alone in having multiple eyes. Nearly all the Hymenoptera have five eyes, including the bees, wasps, and ants. Many flies also have five eyes and most spiders have eight. And don’t forget that some mollusks and crustaceans have eyes on stalks. Talk about creepy.

Bees’ eyes come in two varieties. In female honey bees, the two large compound eyes dominate the sides of the head and the three ocelli form a triangle on the vertex—a forehead of sorts. Although the compound eyes of workers are quite large compared to the body, the eyes of drones are crazy big, extending to meet at the top of the head. In fact, the compound eyes of drones actually displace the ocelli, pushing them towards the base of the antennae.

A worker’s compound eyes

Shiny and black, the compound eyes of a worker take up most of her face. Although these eyes appear smooth and glassy, each comprises roughly 6900 hexagonal plates. Each of the exposed surfaces is flat like a piece of glass, and all the plates fit together in a pattern similar to a piece of honeycomb—each of the six sides butting against another to form a continuous surface.

We call each of the flat plates a facet, just like the cut surfaces of a diamond. Directly beneath each facet is a lens that collects light, and below that is a long cone that extends into the bee’s head. The cone contains retinal cells that detect light. After the light travels through the lens, the retinal cells pass the information to the central nervous system. Like a complex computer, the central nervous system combines all the visual messages into a mosaic picture of the bee’s surroundings.

Do you remember disco balls from the 1970s? They were spherical and covered with individual mirrored facets that reflected light. As the ball rotated, light hit the mirrors and bounced off them, traveling in thousands of directions. I like to imagine the compound eyes working like a disco ball in reverse. Instead of reflecting light, the eye facets collect and combine the images into one big picture stitched together by the bee’s brain.

Eyes for every purpose

The large compound eyes that meet in the middle of the drone’s forehead displace the ocelli, pushing them towards the antennae. Photo copyright: Ron Matsumoto.

Drones and queens have the same eye arrangement as the workers, but the number of facets is different. The compound eyes of a drone are much larger, each having 8600 facets.1 The larger size and the top-of-the-head placement enable the drones to find queens in the three dimensions of open space.

As they all cruise around a drone congregation area, the drones must be able to find, track, and approach a virgin. Even if the virgin is flying above the drone, he can pinpoint her location with his extra-large sensitive eyes.

Queens, the largest of the bees, have the fewest facets in their compound eyes, numbering about 4000 in each. Since queens don’t spend much time foraging and navigating, their eyes are less important. Even when the queen does fly—during mating and swarming—workers stay by her side, guiding her along. Everything about the queen’s body centers on survival and reproduction, and all else is secondary.

A flicker of danger

The curved arrangement of flat plates is especially good for detecting subtle movement. If you wiggle a diamond ring in a ray of light, the slightest movement is amplified into a noticeable flash of light. In a similar way, the facets capture a slight movement in a bee’s environment and send a warning message to the brain.

“What was that?” says the brain to the bee. It could be something like a light breeze or a cloud passing over the sun, but it could also be a bird, a wasp, or a hungry mantis looking for a meal. It seems like the smallest creatures have the most eyes, all of them constantly checking for creatures that are bigger and hungrier.

Color vision

Honey bees have trichromatic vision, much like humans. Trichromatic means there are three separate types of receptors for receiving color information. Humans, and many other mammals, are sensitive to blue, green, and red. Honey bees are sensitive to ultraviolet, blue, and green—a shift to shorter wavelengths.2

In fact, honey bees are especially sensitive in the range that includes ultraviolet, blue-violet, green, yellow, and a shade called “bee purple,” which is a synthesis of ultraviolet and yellow.

The color red, which has a very long wavelength, is invisible to bees. Red objects appear like “black holes” simply because the color receptors fail to detect them.

Flowers that have petals on the red end of the color spectrum—for example red, orange, and brownish-purple—often have ultraviolet nectar guides that point to the center of the flower. By following these “street signs,” the bees can find the center of the flower even if they can’t see the main petals.

Ultraviolet navigation

Ultraviolet light also aids in bee navigation. As Karl von Frisch discovered, dancing bees direct their sisters to a flower patch by describing the location of the patch relative to the position of the sun. The really crazy part—the closest thing to a miracle I can imagine—is the bees recalculate the angle as the day wears on. So when the bee goes home or returns to the patch later in the day, she can recalculate the direction based on the new position of the sun.3

On cloudy days when the sun is not visible, the bee knows where the sun is because, unlike white light, ultraviolet light penetrates clouds. The bee can “see” the sun, even on days when we cannot.

Alternative sight navigation

When bees are close to home or in familiar territory, they often use other types of navigational markers. They may follow hedgerows, rivers, roadways, fields, buildings, tree lines, or anything else that looks familiar. Experiments have shown that older bees are more likely than younger ones to use landmarks, which makes sense because the landscape becomes more familiar with every trip.

In The Buzz about Bees, professor Jurgen Tautz describes it this way, “Bees use earthbound and celestial cues as aids to orient themselves outside the nest, and will make their way from one landmark to the next along each part of the journey to their goal. For this, they use trees, bushes, and other conspicuous features in the landscape. . .­ During [orientation flights] bees leave the hive each time in different directions, and so map the location of the nest relative to its surroundings.”4

Experiments show that honey bees are more likely to use the angle of the sun for distant, high-speed travel into unfamiliar territory. But as bees approach their home territory on the return trip, they slow their flight speed and use familiar visual landmarks.

The ocelli

A bee’s ocelli are much smaller than the compound eyes and do not form images in the brain. Instead, each of the three ocelli has a single lens topping a series of retinal cells that detect changes in light levels. The retinal cells of the ocelli are especially sensitive to blue and ultraviolet light.5

Because the ocelli are near the top of the head, they can collect information about the sky and the horizon. They aid navigation and orientation, alerting the bee to its position in relation to the sun. In addition, they are helpful for detecting danger approaching from above. If you’ve ever tried to stalk a bee with a net or a camera, you know how well the system works!

Hairy eyeballs

Hairy eyes: the hairs on a honey bee’s compound eyes grow between the facets and help the honey bee detect wind speed and direction. Photo copyright: Rusty Burlew.

Honey bees are one of the few bee species that have hairy eyeballs. The hairs—technically called setae—grow out of the spaces between the facets, just like weeds jutting through cracks in a sidewalk. The hairs detect air movement across the eyes, something that helps them navigate in windy conditions. Just like the pilot of a plane or boat needs to compensate for wind and tides, so must a honey bee compensate for varying wind speed and direction.

Most wild bee species forage within a few hundred yards of their nest, so assorted navigational tools, such as hairy eyeballs, are not necessary. But since honey bees travel so far afield—several miles or more—they need dependable ways to find their way back home.

Of course, honey bees seem to have backup systems for just about everything. Besides splendid vision and hairy eyes, honey bees also use olfactory cues and magnetic sensors to find their way around.

The ground below

When they are flying full tilt, honey bees can travel up to 20 miles per hour. Unless the eyes are quick to respond to images, the foragers might miss things like flowers. In fact, the ground would appear blurry and vague.

Laboratory experiments have shown that honey bees can discern black and white stripes at 300 stripes per second. By contrast, humans can only detect 15–20 stripes per second before the images blend together.

If you are old enough, you may remember 16 mm home movie cameras. They recorded many images on a strip of film. After processing, projectors replayed them at 18-24 frames per second. This was fast enough that, instead of looking like a series of still images, our brains “thought” they saw moving images.

If the playback was too slow, the images looked jerky. If played too fast, they looked comical or cartoonlike. But even at proper speeds, a honey bee watching a movie at 24 frames per second would yawn. Instead of perceiving moving images, the bee would see a series of still shots, much like a sleep-inducing PowerPoint presentation. Ho-hum, just another B(ee)-rated movie.

Applying what we know

Beekeepers are ingenious at harnessing bee vision. For example, knowing how bees navigate allows us to reduce drift in the bee yard. Drift occurs when bees go home to a different hive than they left. It probably arises from confusion, especially when multiple hives are close together and their scents intermingle.

By tracking bees, we know they are likely to go to the outermost hives, rather than those in the middle. If they are carrying payloads of pollen or nectar, those outermost colonies are happy to let them in. By the end of the season, the outer hives may contain significantly more bees than the others. Along with the extra bees, they may accumulate extra varroa mites as well.

When hives were marked with bold markings such as broad horizontal or vertical stripes or bright colors, the bees were more likely to find the right home. But Jurgen Tautz warns us, “Shape and color are not learned as quickly as odors, taking three to five training sessions to achieve proficiency.”

Some beekeepers use this information to train bees before moving a hive a short distance. One beekeeper painted a long, wide board with bright stripes like a zebra crossing and placed it in front of her hive like a runway. She let the bees get used to it for a week (the training period) and then moved the hive and the board to the new location. The bees adjusted to the new location easily because they could recognize their private runway from the air.

It also works when moving multiple hives long distances because the bees can easily sort themselves into the proper hive. The visual clues are easier for the bees to identify than a tangled mix of pheromones.

It’s not all pheromones

Bee vision is one of those things we easily overlook in our zeal to attribute most bee behavior to pheromones and instinct. So the next time you gaze deeply into the enormous eyes of a honey bee, remember this: they have powers of vision we can only imagine. Knowing how that vision works will give you expanded opportunities for skilled honey bee management.

References

  1. Winston ML. 1987. The Biology of the Honey Bee (pp. 163-168) Cambridge, Mass. Harvard University Press.
  2. Mattingly, RL. 2012. Honey-Maker: How the Honey Bee Worker Does What She Does (p. 63). Portland, Oregon. Beargrass Press.
  3. Sammataro D and Avitabile A. 1978. The Beekeeper’s Handbook (pp. 22-23).. New York. Charles Scribner & Sons.
  4. Tautz J. 2008. The Buzz about Bees: Biology of a Superorganism (p. 89). Berlin, Germany. Springer-Verlag.
  5. Snodgrass RE, Erickson EH, and Fahrbach SE. 2015. The Anatomy of the Honey Bee. In JM Graham (Ed.) The Hive and the Honey Bee (p. 153). Hamilton, Illinois: Dadant & Sons, Inc.

6 thoughts on “Rusty Burlew’s Bee-Vision Guest Blog.”

  1. I like Rusty’s style of writing – fun, witty, yet still scientifically precise. Very good.

    1. That’s good, Philip: I’m you got such a lot from Rusty’s article, and thank you for commenting.

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