Micromechanical properties of consecutive layers in specialized insect cuticle:: the gula of Pachnoda marginata (Coleoptera, Scarabaeidae) and the infrared sensilla of Melanophila acuminata (Coleoptera, Buprestidae)

Insect cuticle is a highly adaptive material that fulfils a wide spectrum of different functions. Cuticle does not only build the exoskeleton with diverse moveable parts but is also an important component of a stunning variety of mechanosensory receptors. Therefore, the mechanical properties of these specialized cuticular systems are of crucial importance. We studied the different cuticular layers of the head part (gula) of the head-to-neck ball articulation of Pachnoda marginata and of the photomechanic infrared (IR) sensilla of Melanophila acuminata on the basis of cross sections. In our study, we combined histological methods (i.e. detection of the different types of cuticle by specific staining) with measurements of hardness (H) and reduced elastic modulus (E-r) by nanoindentation technique. In the gula of Pachnoda we found an unusual aberrance from the well-known layering. Between the epi- and exocuticle, two meso- and one endocuticular layers are deposited which are softer and more elastic than the underlying exo- and mesocuticular layers. The hardest of all examined materials is the cuticle of the exocuticular shell of the internal sphere of the Melanophila IR sensillum with H=0.53 GPa whereas the inner mesocuticular core of the sensillum represents the most elastic and softest layer with values of H=0.29 GPa and E-r=4.8 GPa. Results are discussed with regard to the proposed functions.