Introduction.
A couple of days ago I woke to the unmistakable trumpeting sound of whooper swans (Cygnus cygnus) overhead. I scramble into my dressing gown and slippers and out the front door in time to see around 15-20 swans disappear over the treeline. Four stragglers fly directly over my head calling frantically as if appealing to their companions to wait up. They travel thousands of miles down from Iceland every autumn to overwinter mostly further south in England. We are below their flight-path here in the Scottish Highlands and they are pretty regular in their migration habits. Two years ago, I heard them on exactly the same day, while last year it was two days earlier. You can listen to them here: https://www.youtube.com/watch?v=6rHY8FF5IJY
Hexagons.
I was refilling my dish washing up detergent bottle the other day and noticed the bubbles change from the initial round shape to form hexagons on the surface of the funnel. I thought, hm! hexagons? I remembered something about honey bee comb building in Professor Brian Cox’s Forces of Nature published in 2016. He discusses the “latest” research from 2013 when three engineers published a paper on how the hexagons appear because the bees heat the wax to melting point and, as the liquid thins out (like soap bubbles) it settles in the hexagonal shapes we know so well. Cox is clear to point out though that the debate on the origins of hexagons will not stop with the Karihalloo et al. paper and their research on thermoplasty. He is not wrong. Bauer and Bienefeld1 will disprove the Karihalloo theory and show, using thermal imaging, that honey bees cannot heat wax to the melting point of around 40°C needed for thermoplasty to change cell shape.
Vince Gallo Investigates.
So, how do honey bees construct hexagonal cells with a downward tilt to prevent nectar and honey from leaking out? Engineer and curious beekeeper Vince Gallo from England has been working hard for several years learning how honey bees make this happen. Gallo has even taken on PhD studies to find out, and is in the final stages of his study working under the supervision of Professor Lars Chittka the author of The Mind of a Bee, https://www.beelistener.co.uk/book-review/the-mind-of-a-bee-reviewed/
Adaptability.
Gallo reminds us just how amazingly adaptable honey bees are. We often read about how perfect honey comb is, but the reality is different. If you look closely at naturally built honey comb, you will notice that the first row of cells is slightly different from the rest. Sometimes the bees make mistakes which they might rectify by moving wax about, or else use those cells for honey storage instead of brood. The have the ability to measure the flexibility of wax which is crucial to cell building because the cells need to be thin enough to interact with each surrounding cell. For this they use their antennae to gauge pliability, and if the cell wall is not flexible enough, they simply remove some of the wax.
Stigmergy.
Honey bees have no leader managing the building project, nor do they have slide-rules and set-squares tucked into their leg baskets. Gallo says that the geometry of the cells arises by active construction. I learn a new word, stigmergy, as I research this fascinating but complicated cell construction topic. Apparently, it all hinges on stigmergy which originated in biology but it now being used increasingly in the developing world of robotics.
Hӧlldobler and Wilson (2009)2 explain that stigmergy was coined in 1959 by French zoologist Pierre-Paul Grassé to explain the concept of self-organisation in nest architecture when studying nest building among termites in Africa. It also applies to other social insects such as bees. The origin of stigmergy is Greek and means “inciting to work”.
Grassé demonstrated that no direct interactions between the ants is needed when building a nest, but that what they actually did affected the other workers. So, if an ant placed a faecal pellet, or other material, on the ground in a particular spot then the change in the environment stimulated the other ants to respond by placing another pellet on top. An anthill is built by stigmergy. A rapidly growing anthill acts as a strong stimulus for worker to add more material and this positive feedback mechanism sees the anthill reach a certain size when negative feedback will come into play and there is a decline in construction as ants stop adding to the mound.
What Bees Do.
Back to bees. As Gallo explains, honey bees construct individual cells with a hemispherical base like a football cut in half. These bases are surrounded by circular tubes like those ovoid egg-shaped cells produced by bumble bees and solitary bees. But because many other bees are working nearby on individual cells, they become densely packed and move closer to one another. The shapes arise from the work of other cell builders constructing cells on all sides. They would have been curved if not packed together so the geometry arises from cell compaction. You can hear Gallo give an excellent presentation on his work to a National Honey Show audience here: https://www.youtube.com/watch?v=V93ramxh4yk . The referenced discussion papers are well worth reading to get a clear picture of what is going on when honey bees construct cells. The articles contain useful photographs to help one understand exactly what happens. Hats off to Vince Gallo and all the other scientists working on comb construction to help us better understand our bees!
References
1Daniel Bauer & Kaspar Bienefeld (2013) Hexagonal comb cells of honeybees are not produced via a liquid equilibrium process DOI 10.1007/s00114-012-0992-3 ORIGINAL PAPER
2 Hӧlldobler B & Wilson E.O. (2009) The Super Organism: The Beauty, Elegance, and Strangeness of Insect Societies, W.W. Norton & Company Inc. New York.
Vince Gallo and Lars ChittkaStigmergy versus behavioral flexibility and planning in honeybee comb construction 73/pnas.2111310118 https://doi.org/10.10
Gallo V and Chittka L (2018) Cognitive Aspects of Comb-Building in the Honeybee? Front. Psychol. 9:900. doi: 10.3389/fpsyg.2018.00900