A two-system model for chitin-protein complexes in insect cuticles.

Insect cuticles consist of planes of microfibrils which may either form preferred oriented layers or rotate progressively to form a helicoid. The two types may alternate to form daily growth layers. In Hydrocyrius exo-cuticle, microfibrils of diameter 45 A rotate at angles of 7 degrees to 8 degrees to form a helicoid with a 44-to 50-fold screw axis, and have an axial periodicity of about 65 A. The sense of rotation of helicoidal systems is bilaterally asymmetrical indicating a crystallization assembly process, whereas variations in their pitch and directions of preferred oriented layers both show bilateral symmetry, indicating specific cellular control.