What is it like to be a bee? (Part 2)


Author: Heather Broccard-Bell, Ph.D., Honey Bee Health Researcher

It’s safe to say that how honey bees experience the world is quite different from how humans experience the world. While we will always be influenced by our specific sensory systems, we can aim to better understand the honey bee by studying their senses. Knowing more about what the world is like for honey bees can help us to decipher their remarkable behaviour.

In our last blog, we took an in-depth look at how the honey bee uses its unique visual capabilities for navigation, foraging, and more. Like us, bees rely heavily on their vision, but what about the other senses? We’ll now explore beyond sight to further grasp what the world is like from the honey bee’s perspective.

Have You Heard the News?

So, let’s discuss hearing. As humans, we hear the world through our ears by detecting sound waves. Sound waves are a type of compression wave (essentially, bands of molecules that alternate between being tightly compacted and far apart) that travel through the air, as well as through other media, like water, and even solid objects. These compression waves get funneled into our ear canals, transmitted through our ear drums, and into our inner ear. Here, they are picked up by auditory receptor cells that send signals to our brains—and voila! You’ve just heard the latest earworm [a] that will now be stuck in your head for the rest of the day.

In stark contrast, honey bees are incapable of detecting compression waves at all. So, sadly, regardless of what type of music you play in your apiary, your honey bees will never appreciate it. Instead, honey bees have two ways of “hearing” that are more similar to what we would think of as touch. Both hearing methods involve detecting types of mechanical motion, which is different from compression waves.

The Bees’ Knees

The first way honey bees hear is by sensing vibrations through surfaces (e.g., honey comb). These vibrations are detected by an organ located just below the knee called the subgenual organ. The subgenual organ itself is suspended within the leg in a tube filled with hemolymph, the bee equivalent of blood. When the leg is vibrated, the organ moves at a different time than the outer walls of the tube, and this difference is picked up by sensory receptors that send signals to the brain [1,2]. Using the subgenual organ, honey bees can sense vibrations between 100 and 3000 cycles per second (Hertz). It’s no accident then that honey bee vibrational communication signals, such as those produced during waggle dancing and stop signaling signals, fall within this range [3]. Of course, the subgenual organ is also pretty useful for detecting other kinds of vibrations, such as those created by predators attacking the hive or a beekeeper using a string trimmer. It is thought that honey bees’ use of vibration for communication actually came from an earlier ability to use vibrations to detect threats to the colony [4].

The Near Field

The other way bees hear is through “near field sound”. If you’ve ever stood right next to a speaker at a concert, you’ll be familiar with this type of sound: it’s the slight windy sensation you can feel when the bass is pumping! When really close to the source of a sound, humans can detect both compression waves (what you hear) and the near field sound (the actual mechanical movement of the air). However, honey bees can only sense the latter. To do this, bees use special sensory cells located within their antennae called the Johnston’s organ that send signals to the brain when the antenna is bent [5]. Waggle dancers make near-field sound and vibrations, and honey bee dance followers listen to both [6].

The Chemical Senses

What about taste and smell? Unlike senses such as vision and hearing that require some external force to move a receptor in a mechanical sort of way [b], both taste and smell rely on sensory receptors directly detecting molecules. Collectively, we call these senses the “chemical senses” because they depend on assessing chemicals (molecules) in the environment [c]. In evolutionary terms, the chemical senses have been around considerably longer than any other type of sensory system, so there is a lot of similarity between how humans and honey bees taste and smell the world.

Smelling happens when we detect chemicals, called odorant molecules, suspended in the air. Humans sense odorant molecules when they stick to cells called olfactory receptor neurons located deep within our noses. Humans have approximately 400 different types of olfactory receptor neurons, and odorant molecules of different shapes stick to different receptor types. Specific smells are thought to be experienced, not when just one type of receptor is activated, but when specific combinations of them are [7]. Tasting (“gustation”) happens when molecules stick to receptors located in the taste buds of our tongue—and throat, stomach, and even intestines [8]. Although we are used to thinking about taste and smell as being somewhat separate, it is well-known that these two senses are linked, such that if you become unable to smell, taste will often be affected too.

Honey bees detect smells with olfactory receptor neurons located in the antennae, mouthparts, and forelegs. The basic function is the same as it is in humans: odorant molecules stick to the receptor, and a signal is sent to the honey bee’s brain. Also as in humans, the combinations of receptors that are activated seem to produce behavioural responses [9]. Aside from their usefulness in finding floral patches [10], honey bees use odorant molecules to communicate amongst each other. Examples include the well-known alarm pheromone, queen pheromones, and recognition pheromones, among many others [11].

Taste receptors in honey bees are found in the same places on the body as olfactory receptor neurons. In fact, these two chemical senses are even more closely linked in bees than in humans. We know that the structures of the two types of receptors are different, with the primary difference between taste and smell being whether or not the bee has to be in physical contact with the substance in order to sense it [12]. For example, bees are unable to smell newly-prepared lab-grade sugar solution, because sucrose is not an odorant molecule (it does not make it into the air); however, if the solution physically touches the bee’s antennae, she absolutely can taste it. It is worth mentioning, however, that bees can smell solutions of regular table sugar or sugar solution that has been in the environment for a period of time, both of which contain impurities that are sources of odorant molecules.

A Whole New World

In addition to the familiar senses I’ve touched on, honey bees also have a robust set of senses that seem wholly alien to us humans, and I want to tell you about a few of the really neat ones.

Honey bees like to maintain tightly-controlled conditions inside their nests, including relative humidity. The first step toward being able to control something is being able to sense it. Honey bees are able to sense humidity using sensors called hygroreceptors, located in microscopic pits on their antennae [13].

As we learned in part one of this series, honey bees can use polarized light to navigate—but that is not the only way they do so. Two other navigational tricks that I’ll discuss in an upcoming post include recognizing landmarks, and the use of “optical flow”, which is the movement of images across the eye. Yet another method relies on a bizarre sense that has been found in a number of animal species: the ability to detect magnetic fields. We know that honey bees have magnetic structures located inside their abdomens, and experiments during which bees were exposed to artificial magnetic fields have shown that honey bees use these abdominal structures to detect magnetic fields. Related experiments showed that honey bees can use the magnetic field of the Earth to navigate, similar to how humans use compasses [14].

In my opinion, one of the coolest honey bee senses is the ability to detect electrical fields, called electroreception. As they fly, bees accumulate positive charge, whereas pollen grains (indeed, whole flowers) are slightly negatively charged. When a bee lands on a flower, the positive charge of the bee slightly diminishes the negative charge of that flower. With their electroreceptive capabilities, honey bees can detect the charge of flowers, a handy trick to tell which flowers have been recently visited by your competitors. It turns out that bumble bees are especially adept at electroreception, but honey bees can also detect electrical fields—albeit, using a different approach. But how do they do it? Researchers have shown that electrical fields cause the honey bee’s antennae to bend, and that the Johnston’s organ, that group of receptors that responds when the antenna is bent under near field sound, are also activated (in fact, much more strongly) in the presence of an electrical field [15].

Knowledge is Power

Even though we’ll never know precisely what being a bee feels like, I hope that I have given you at least a little bit of the next best thing: an understanding of how they sense the world. I believe that effective honey bee management is improved when we better understand some of the reasons for the behaviours we see. I’ve left out a lot of detail for ease of reading, but I hope you will explore these topics further on your own, and check back soon as we continue to share insights into the wonderful world of honey bees.


FOOTNOTES

[a] https://en.wikipedia.org/wiki/Earworm

[b] Note that that this generalization misses a whole lot of details about how sensory receptors—and indeed, nervous systems in general—work! For a really, really brief introduction to just a tiny fraction of the detail I skipped, see this Scholarpedia entry: http://www.scholarpedia.org/article/Nervous_system

[c] People often use the word “chemical” to mean a harmful substance, but it actually just means something made of atoms and molecules. All matter in the universe is made of chemicals. Yes, even pure water is a chemical. You should be extra-skeptical of products that claim to be “chemical-free”—because that claim alone suggests that the person making the claim either does not understand basic chemistry, or thinks that you don’t.

References

Featured photo credit: H. Broccard-Bell.

[1] Kilpinen, O., & Storm, J. (1997). Biophysics of the subgenual organ of the honeybee, Apis mellifera. Journal of Comparative Physiology A, 181(4), 309-318.

[2] Sandeman, D., Tautz, J., & Lindauer, M. (1996). Transmission of vibration across honeycombs and its detection by bee leg receptors. The Journal of Experimental Biology, 199(12), 2585-2594.

[3] Michelsen, A., Kirchner, W. H., & Lindauer, M. (1986). Sound and vibrational signals in the dance language of the honeybee, Apis mellifera. Behavioral Ecology and Sociobiology, 18(3), 207-212.

[4] Lakes-Harlan, R., & Strauß, J. (2014). Functional morphology and evolutionary diversity of vibration receptors in insects. In Studying vibrational communication (pp. 277-302). Springer, Berlin, Heidelberg.

[5] Dreller, C., & Kirchner, W. H. (1993). Hearing in honeybees: Localization of the auditory sense organ. Journal of Comparative Physiology A, 173(3), 275-279.

[6] Michelsen, A., Towne, W. F., Kirchner, W. H., & Kryger, P. (1987). The acoustic near field of a dancing honeybee. Journal of Comparative Physiology A, 161(5), 633-643.

[7] Zou, Z., & Buck, L. B. (2006). Combinatorial effects of odorant mixes in olfactory cortex. Science, 311(5766), 1477-1481.

[8] Rozengurt, E., & Sternini, C. (2007). Taste receptor signaling in the mammalian gut. Current Opinion in Pharmacology, 7(6), 557-562.

[9] Deisig, N., Giurfa, M., Lachnit, H., & Sandoz, J. C. (2006). Neural representation of olfactory mixtures in the honeybee antennal lobe. European Journal of Neuroscience, 24(4), 1161-1174.

[10] Farina, W. M., Grüter, C., & Díaz, P. C. (2005). Social learning of floral odours inside the honeybee hive. Proceedings of the Royal Society B: Biological Sciences, 272(1575), 1923-1928.

[11] Slessor, K. N., Winston, M. L., & Le Conte, Y. (2005). Pheromone communication in the honeybee (Apis mellifera L.). Journal of Chemical Ecology, 31(11), 2731-2745.

[12] de Brito Sanchez, M. G. (2011). Taste perception in honey bees. Chemical Senses, 36(8), 675-692.

[13] Yokohari, F., Tominaga, Y., & Tateda, H. (1982). Antennal hygroreceptors of the honey bee, Apis mellifera L. Cell and Tissue Research, 226(1), 63-73.

[14] Lambinet, V., Hayden, M. E., Reigl, K., Gomis, S., & Gries, G. (2017). Linking magnetite in the abdomen of honey bees to a magnetoreceptive function. Proceedings of the Royal Society B: Biological Sciences, 284(1851), 20162873.

[15] Clarke, D., Morley, E., & Robert, D. (2017). The bee, the flower, and the electric field: Electric ecology and aerial electroreception. Journal of Comparative Physiology A, 203(9), 737-748.

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