Katydid’ femur-tibia leg joint.
Photo and scanning electron micrographs of katydid leg joint.
High-magnification scanning electron microscopy image of katydid leg joints and its comparison with the external texture of their body.
Ultrahigh magnification scanning electron microscopy image of katydid leg joints and its comparison with the external texture of their body.Insects like katydids, grasshoppers, and crickets can jump many times further than their body size. This remarkable prowess relies on their hind legs, which have to withstand repeated flexing and extending during jumping with more reliability and durability than most mechanical devices. So how do they do it? Researchers from Texas A&M University have used a battery of materials science and engineering techniques to find out what keeps katydids on the hop [Oh et al., Acta Biomaterialia (2017), doi: 10.1016/j.actbio.2017.08.013].
“In the quest for new inspirations for engineering design, we investigated the frictional, structural, and mechanical properties of the hind leg femur-tibia joint of katydids with the hope of discovering novel insights for the development of antifriction and antiwear coatings and lubrication systems,” explains Mustafa Akbulut, who led the study.
Katydids, like other jumping insects, use specialized hind legs to propel themselves with great acceleration. The rapid extension of the back legs as the insects push off from the ground puts extreme pressure on the joints between the femur and tibia.
Using a combination of high-resolution electron microscopy, tribology, nanoindentation, and spectroscopy, the researchers found that the exoskeletal material making up the katydids’ hind leg joints possesses some unique attributes.
All insect parts, including the joints are made from a mixture of chitin and protein. But the surface of katydids’ femur joint is covered with a periodic array of cylindrical ridges covered with nanowire-like lamellar patterns and, at regular intervals, valleys decorated with hillock-like structures. The hierarchical surface texture ranges from just a few nanometers (1-10 nm) to a few hundred nanometers (100-300 nm) and right up to the micron scale. The surface of the tibia, meanwhile, is much smoother and shows no well-defined patterning.
This unique combination of hard and soft, rough and smooth gives rise to exceptional mechanical properties. Very rough surfaces produce a lot of wear when they come into contact. Very smooth surfaces, by contrast, stick together thanks to van der Waals forces, leading to adhesive wear. But the hind leg joints of the katydid generate neither smooth-on-smooth nor rough-on-rough contact. The katydid gets it just right.
The patterned surfaces in the femur-tibia joint reduce the contact area of the two moving surfaces, decreasing adhesive forces and the resulting wear. Meanwhile, the softer patterned surface of the femur moving against the harder smooth surface of the tibia cuts down on abrasive wear.
“The reduced friction implies that the muscle strength is very effectively used by katydids, without losing significant energy to the frictional losses,” explains Akbulut.
The researchers believe their findings could inform the design of more efficient and durable antiwear coatings and lubrication systems.