Resilin is one of the five rubber-like solids used by animals to provide elasticity in biological systems; the others are elastin, abductin, spider silk, and ColP. Resilin is found in insects and other arthropods, and does not appear in mammals (mammals use elastin).
Resilin is best known as the material used in insect
wing tendon. It is extremely good at storing energy, about 97% efficient, and insects use resilin to decelerate the wing at the end of the stroke and immediately feed that energy back into the wing on the return stroke; in a sense much of insect flight is powered by 'bouncing' the wings up and down, saving on energy use and muscle mass. Resilin can also be distorted slowly and released suddenly to concentrate work; this is how
fleas jump and
cicadas chirp so loudly. It's also the stretchiness of resilin that allows a
tick to expand its abdomen when feeding without having to shed its exoskeleton.
Resilin has proven comparatively simple to produce artificially ('comparatively simple' is still not simple; E. coli was genetically modified to produce resilin, which could then be collected and cast into larger strips), and synthetic resilin, known as rec1 resilin, has greater resiliency than polybutadiene (maintaining its ~97% resiliency), can be stored over long periods, and can be extended up to three times its resting length, and compressed to 1/3 of its resting length, without loss of resiliency.
Elastin, resilin, and the other elastomeric proteins all provide their elasticity largely because pulling on these structures result in pulling against molecular entanglements, rather than on chemical bonds; they are also aided by covalent bonds between long chain molecules, in the same was as plastic polymers are.