Ultrastructural architecture of the microtrichia of the insect cuticle

The ultrastructure of the microtrichia (MT) of the insect cuticle was studied using scanning electron microscopy (SEM). After dissolving the protein matrix of chitin-protein microfibrils with NaOH, the orientation of the axial chitin fibers was three-dimensionally demonstrated. Microfibrils of the outermost exocuticular lamella lie parallel to the slope of the cone surface of the MT and rotate slightly on the top of the MT. Microfibrils of the external lamella of the transitional area between planar cuticle and conical cuticle of the MT corresponded to the shape of the surface within one lamella and preferred directions of MT's microfibrils in the successive lamella rotated. In the deeper layers of the cuticle, the rotation of both the microfibrils and successive lamella results in the twisted straw architecture of the microfibrils' composition within the MT. The deepest microfibrils that are located close to the axis of the MT form compact clusters of fibrils (1.0-1.5 μm length). The twisted-straw architecture of microfibrils in the MT is derived from principles of development of extracellular fibrous composites. It is suggested, however, that this architecture has an additional functional significance as a strategic design with particular mechanical properties. J. Morphol. 234:1-10, 1997. © 1997 Wiley-Liss, Inc.