Repair of microdamage caused by cyclic loading in insect cuticle.

Abstract

It is well known that repeated loading cycles can reduce the strength of a material and cause eventual failure by the gradual build-up of damage. Previous work has shown that mammalian bone is able to extend its life almost indefinitely by continuously repairing microdamage, preventing the development of macroscopic cracks. However, no study has been conducted until now to investigate repair of microdamage in any other biological material. We applied cyclic bending loads to the hind tibiae of desert locusts (Schistocerca gregaria). We observed a significant decrease in the elastic stiffness (Young's modulus) of the cuticle during the five applied loading cycles, indicating that microdamage had been induced. The tibiae were then left to rest for various time periods: 1 hr, 24 hr, 1 week, and 4 weeks. When tested again after up to 24 hr, there was still a significant decrease in stiffness, showing that some microdamage remained. However, in the samples left for 1 week or 4 weeks before retesting, this decrease in stiffness had disappeared, indicating that the microdamage had been repaired. This is the first ever indication that insects are capable of repairing microdamage. It is a highly significant finding-insects such as locusts rely on the stiffness and strength of their hind legs for jumping. This study suggests that, within a time period of order of a few days, the insect can fully restore the mechanical function of an overloaded leg and thus return to normal activities.