BeetleJuice, BeetleJuice, BeetleJuice!!!

Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics

There are plenty of obvious differences between animals like insects with exoskeletons, and animals like us with endoskeletons, but we don’t often give much thought to how joints in these skeletons might differ.  Our joints are fully sealed and bathed in lubricating synovial fluids. Run a human knee dry, and its coefficient of friction (the ratio of the normal force holding the knee surfaces together to the force needed to slide one over the other) might be as high as 0.27 but bathed in synovial fluid it falls to a fraction of that figure: somewhere in the range 0.005-0.023. So, the force required to slide the knee bearing surfaces over each other while supporting bodyweight on one leg would be somewhere between 2% and 0.5% of the supported weight. Of course, the knee isn’t designed as a sliding joint – the two sides roll over each other as the knee bends. So, very little sliding actually happens but it’s clear that being bathed in synovial fluid makes our movement much easier (and less painful) than it would be without it. Clearly, if you are going to move your limb joints, staying hydrated is a good idea.

For insects, and other animals with an external skeleton, keeping joints hydrated and lubricated is much trickier.  With their joints on the outside of their bodies, lubricating them with fluids would lead to evaporation and dehydration. Unless they can find a constant source of hydration, they would dry out.  Clearly insects aren’t all immediately dying of dehydration, so something else must be going on.

Now, Stanislav Gorb and his team have shown that the knees of a beetle work using a dry-lube more slippery than PTFE (the key component in Teflon). The legs either side of a beetle’s knee are basically tubes, and the knee joint is essentially a pin-jointed hinge. The pin-joints are external (it’s an exoskeleton), so they are not bathed in body fluids like human knees. Instead Gorb’s team showed that there are pores in the surface of the knee where a protein-based dry-lubricant is secreted. To see how effective this lubricant is, they tested the coefficient of friction for dry glass-on-glass joints, glass-PTFE-glass, and glass-Beetle lubricant-glass joints, being 0.35, 0.14 and 0.13, respectively.  The beetle lubricant reduced the friction of a dry glass-on-glass joint by nearly two thirds, showing it’s an effective solution to their external joint friction challenge. 

Five-horned rhinoceros beetle (Eupatorus gracilicornis) also known as Hercules beetles, Unicorn beetles, or Horn beetles
Five-horned rhinoceros beetle (Eupatorus gracilicornis) also known as Hercules beetles, Unicorn beetles, or Horn beetles

Based on that result, I’m now considering replacing the Squirt PTFE-based lubricant on my bike-chain with beetle-knee lubricant. I wonder how you milk beetle’s knees?

PTFE (Teflon) is poly-tetra-flouro-ethane. It’s a nasty fluorinated plastic. The beetle-knee lubricant is protein based – biodegradable. A bio-inspired solution which is definitely better for the environment!

References

Nadein K, Kovalev A, Thøgersen J, Weidner T, Gorb S. 2021 Insects use lubricants to minimize friction and wear in leg joints. Proc. R. Soc. B 288: 20211065. https://doi.org/10.1098/rspb.2021.1065