Microfiber reinforcement of an arthropod cuticle

Summary 1. According to its fine structure arthropod cuticle is a laminated composite material. The exoskeleton of a spider is surveyed in an electron microscopical study as to the characteristics of its fiber reinforcement.2. Five types of fiber arrangement are distinguished, ranging from unidirectional to continuously rotating fiber orientation and with three types combining elements of both of these extreme cases. The unidirectional type is rare and its occurrence confined to the innermost part of the walking leg endocuticle and to parts of articular membranes. Quite contrary a continuous and regular rotation of fiber direction is common. In hard cuticle (exocuticle) this type of reinforcement is found exclusively.3. Lamellar width, indicating a 180° rotation of the fiber direction, varies within wide limits. Variation is due to varying numbers of fiber layers and the amount of directional change between them (ca. 0.6–24°, type E).4. The diameter of the chitin/protein microfiber amounts to ca. 35 Å. Its length measured up to 1,5 m in the preparations. A periodicity of ca. 80 Å is noted. The precision of fiber alignment within the cuticle is high.5. In the cuticle of the opisthosoma and the articular membranes endocuticular lamellae are taking a wavy course, which is interpreted to be in relation to the cuticle's softness.6. The mechanical implications of both an increasing number of fiber directions in a composite material and of the different types of fiber reinforcement found in the spider cuticle are demonstrated by a calculation of the directional properties of the moduli of elasticity in tension and shear. The predictable strong influence of both the fiber content and the binder material's E-modulus on the mechanical properties of cuticle is illustrated.7. Instead of a trajectorial design a continuous rotation of the fiber orientation is found in all exocuticles. Thus the spider does not make use of the special advantage (high strength/weight ratio) of unidirectional fiber arrangement in places where it seemed most likely.Many thanks are due to Ms. H. Tscharntke for expert assistance with electron microscopy and photographic work. Dipl.-Ing. F. Och and Dr. G. Haberl (Messerschmitt-Bölkow-Blohm GmbH, Ottobrunn) provided the calculations for Figs. 14 and 15. Their generous help is gratefully acknowledged. Dr. K. Frahm (Universität München, Fakultät f. Tiermedizin) kindly shared his densitometer. Ms. F. Althaus carefully carried out the drawings.