Honeybees (Apis mellifera) are renowned for constructing perfect hexagonal honeycombs, hailed as the pinnacle of biological architecture for its ability to maximize storage area while minimizing building material. However, in natural nests, workers must regularly transition between different cell sizes, merge inconsistent combs, and optimize construction in constrained geometries. Using automated image analysis of naturally built honeycombs, scientists have found that some building configurations are more difficult for the bees than others, and that workers overcome these challenges using a combination of building techniques, such as: intermediate-sized cells, regular motifs of irregular shapes, and gradual modifications of cell tilt.
Honeybees begin constructing honeycombs in multiple locations, which they will eventually bridge together. Image credit: Michael Smith / Cornell University.
Honeycomb is hailed as the pinnacle of biological architecture for its ability to maximize storage area while minimizing building material.
In the 4th century, Pappus of Alexandria marveled at the ‘geometrical foresight’ of the bees, while Charles Darwin dubbed it the ‘most wonderful of all known instincts.’
To build a single cell, multiple bees must deposit, mold, and shape the precious wax; given that an unsupervised collective builds the structure, it is unknown whether each bee possess architectural abilities, or if they act as simple automatons.
“In this fundamental study, we looked at a naturally evolved system which solves similar challenges in a near-optimal manner,” said Dr. Kirstin Petersen, a researcher in the Department of Electrical and Computer Engineering at Cornell University.
“Understanding how evolution can lead to these organisms that have architectural tricks gives us insights into how you can build structures that are multipurpose, strong, and resilient to different environmental perturbations,” added Dr. Michael Smith, a researcher in the Department of Biological Sciences at Auburn University, the Department of Collective Behaviour at the Max Planck Institute of Animal Behavior, and the Centre for the Advanced Study of Collective Behaviour at the University of Konstanz.
Bees build two types of hexagonal honeycomb cells: small ones for rearing worker bees and larger ones for rearing drones, the male reproductive bees.
A challenge arises when the bees must link lattices made of smaller cells with the larger ones, because the geometries don’t allow for a seamless fit. One issue is that bees don’t remodel their cells.
“Whatever action they take in one place effectively decides what’s going to happen later,” Dr. Petersen said.
“Also, for honey bees, wax is the most expensive material energetically. When they build something out of wax, they’re being as frugal as possible.”
As a result, the bees employ other shapes — pentagons or heptagons — in order to link together panels of perfectly hexagonal drone and worker cells.
Along with building cells of different shapes, they bees also build irregular-sized cells, and sometimes even combine multiple types of irregular cells.
The researchers refer to these pairs and triplets of irregular cells as ‘motifs’ and show that particular combinations occur more often than expected by chance.
“Sometimes the bees will switch from building one type of cell to the other, but they make that change gradually, over multiple cells, which suggests they are thinking ahead,” Dr. Petersen said.
Cells marked with different colors to show their orientations reveal how different patches in the comb are built with a consistent tilt when the bees merge two patches; note that irregular five- and seven-sided cells are also used along the merge lines. Image credit: Kirstin Petersen / Nils Napp / Cornell University.
In the study, the authors set up 12 colonies in the field with frames that lacked the usual wax and wire inside, so the bees could build natural honeycombs without guides.
At the end of the season, they took specially lit images and then wrote a custom software to automatically identify, sort and measure the vertices, angles, sizes and geometries of thousands of cells.
The researchers developed a theoretical computer model that allowed them to analyze configurations, and test optimal ways cells might fit together in a continuous manner under the space constraints.
“We used the model to ask, how much better could the bees do? And it turns out, not that much better,” Dr. Petersen said.
More than 200 years ago, Swiss entomologist Francoise Huber suggested that bees might use intermediate cells to merge a honeycomb together, but he lacked the modern tools to measure thousands of cells and validate his idea.
“It really required these tools to rigorously show that. So it’s not surprising that no one has done this before,” Dr. Smith said.
A paper on the findings was published in the Proceedings of the National Academy of Sciences.
Michael L. Smith et al. 2021. Imperfect comb construction reveals the architectural abilities of honeybees. PNAS 118 (31): e2103605118; doi: 10.1073/pnas.2103605118