The Arthro in Arthropod

The root, “arthro” means joint. The root “pod” refers to an appendage, and “arthropod” means “jointed appendage”. The jointed appendages of insects are part of the insect exoskeleton. Unlike the joints of humans which are internal, the arthropod joints are on the outside of the body. These joints must possess what mechanical engineers call, “tribiological” properties. Important tribiological properties of moving joints are low friction, adequate lubrication and minimal wear.

Friction and adhesion create resistance to movement and generate heat. Both are undesirable properties of joints. Biologists can study the properties of living organisms from the point of view of the engineer to gain insights into production of synthetic materials that mimic the best properties of biological tissues.

The grasshopper joint between its femur and tibia of its hind (jumping) leg is interesting because large force is applied during the jump. These joints are resistant to wear with low friction. What gives those joints these properties? Three engineers at Texas A&M, Bassem A. Kheireddin, Toby C. Williams and Mustafa Akbulut, describe the,
“Tribological properties of femur–tibia articulation of lubber grasshopper” in the Journal, Tribiology International.

They find that the leg joint of the lubber grasshopper consists of contact and non-contact surfaces. Minimizing the area of contact, minimizes friction. The contact area consists of a smooth convex surface that fits a textured convex surface. The texturing of one of the surfaces reduces the contact area between the two surfaces. Compared with other surfaces, such as glass on glass, steel on steel or wood on wood, grasshopper joints have an ultra-low coefficient of friction. This keeps wear and heat generation to a minimum.

The textured surface is stiffer and resists deformation under pressure. This keeps the contact area from increasing under pressure. The smooth surface is softer and deforms to absorb the pressure. The combination of a more rigid textured surface opposed to a soft and smooth surface produce excellent mechanical properties in the grasshopper joint.

The Tibia-Femur joint in the hind leg of a Lubber Grasshopper

About jjneal

Jonathan Neal is an Associate Professor of Entomology at Purdue University and author of the textbook, Living With Insects (2010). This blog is a forum to communicate about the intersection of insects with people and policy. This is a personal blog. The opinions and materials posted here are those of the author and are in no way connected with those of my employer.
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