Yesterday we began looking at arthropods that engineer things. Another arthropod family displays enviable skill at architecture. If you’ve ever watched ants busily tunneling in an ant farm, you may have noticed that the intricate finished product rarely collapses. Why is that? Engineers would like to know, since cave-ins pose a serious threat to miners’ safety. It turns out that ants, with no foreman or architect, instinctively build on the principle of natural arches as they remove sand grains one by one. This was found by scientists at Caltech, who wrote about “The Science of Underground Kingdoms.” A short video in the article notes that ant colonies can extend down 25 feet, host millions of inhabitants, and last for decades.
Jose Andrade, a Caltech mechanical engineer, was impressed with the intricate casts of ant tunnels that have been made by pouring molten metal into them and retrieving the architecture (see photo in the article). He asked, “What are ants thinking (if anything)?” Do they dig blindly, or just “know” what to do? He teamed up with biologist Joe Parker to investigate. What they figured is that the know-how is not in the individual ant, but in the colony. They call it a “behavioral algorithm.”
“That algorithm does not exist within a single ant,” he says. “It’s this emergent colony behavior of all these workers acting like a superorganism. How that behavioral program is spread across the tiny brains of all these ants is a wonder of the natural world we have no explanation for.”
To survive, ants have to build according to the laws of physics. It might be that ants have sensors that help them avoid removing particles that provide load bearing, much as Jenga players pull out sticks that keep the pile from collapsing. The remaining sticks create a “force chain” that stabilizes the pile.
As ants remove grains of soil they are subtly causing a rearrangement in the force chains around the tunnel. Those chains, somewhat randomized before the ants begin digging, rearrange themselves around the outside of the tunnel, sort of like a cocoon or liner. As they do so, two things happen: 1.) the force chains strengthen the existing walls of the tunnel and 2.) the force chains relieve pressure from the grains at end of the tunnel where the ants are working, making it easier for the ants to safely remove them.
The research was published in the PNAS by de Macedo et al., “Unearthing real-time 3D ant tunneling mechanics.” The Abstract says that natural arches form as the ants instinctively know which grains to remove.
We discover that intergranular forces decrease significantly around ant tunnels due to arches forming within the soil. Due to this force relaxation, any grain the ants pick from the tunnel surface will likely be under low stress. Thus, ants avoid removing grains compressed under high forces without needing to be aware of the force network in the surrounding material. Even more, such arches shield tunnels from high forces, providing tunnel robustness.
One more example of arthropod architecture is the honeycomb. Everyone is familiar with the hexagonal cells that honeybees build, but bees (and engineers using fabricated honeycomb material) face a problem building the hexagons around corners and curves. A photo in the Cornell Chronicle shows the problem: the growing honeycombs start in different locations and will eventually merge. One cannot use perfect hexagons at the junctions, but prefab materials, used in “everything from airplane wings, boats, and cars, to skis, snowboards, packaging and acoustic dampening materials,” tend to be manufactured in straight lines, not curves. Because bees are good at solving this interface problem, “Engineers may learn from bees for optimal honeycomb designs.”
Challenges arise when space constraints or repairs require engineers to keep a structure mechanically strong when linking together industrial honeycomb panels that each have cells of different sizes. High performance computers used with 3-D printers may solve this problem in the future, but could bees provide a more efficient and adaptable strategy?
A new study finds they can. It turns out that honey bees are skilled architects who plan ahead and create irregular-shaped cells and a variety of angles to bridge together uniform lattices when limited space constrains them.
By careful observation, Cornell engineer Kirsten Petersen noticed that bees are as frugal as possible with their “expensive” material, beeswax.
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, the bees also build irregular-sized cells, and sometimes even combine multiple types of irregular cells. The authors refer to these pairs and triplets of irregular cells as “motifs” and show that particular combinations occur more often than expected by chance.
The bees even seem to be “thinking ahead” as they construct “intermediate cells” to link dissimilar combs together. All the while, they keep the structure strong and robust around curves and corners. As part of the study,
Coauthor Nils Napp, assistant professor of electrical and computer engineering in the College of Engineering, 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. They used the model to ask, how much better could the bees do? “And it turns out, not that much better,” Petersen said.
Petersen attributes this engineering know-how to evolution, but those in the design community know to expect that kind of narrative gloss added like whitewash on the engineered structure. Is any gloss needed? Not really. One can observe these ingenious arthropod engineers, understand what they do, and apply it. The whitewash can be sandblasted off without damaging the structure.
Nevertheless, the origin of these capabilities in arthropods must be addressed at some level. If the arthropod body plan mystically “emerged” by evolution, did the engineering expertise of a spider, ant, or honeybee also emerge by unguided natural processes? The same answer applies that Stephen Meyer gave in Darwin’s Doubt: complex specified information is a hallmark of design. Only an intelligent cause provides the best explanation for it, wherever it is encountered. Chance never does. By default, then, intelligent causes should be preferred for the origin of engineering expertise in arthropods.