1. 1.

    Determinations were made of several physical properties of the viscid and frame silks of the orb-webs built by the spider Araneus sericatus (Cl.).

  2. 2.

    Both types of silk show a breaking stress of approximately 1 GN/m2 and an initial resilience of approximately 0.35.

  3. 3.

    The breaking extension ratio of viscid silk (λ = 3.00) is much greater than that of frame silk (λ = 1.25), and the viscid silk relaxes to a relatively greater extent.

  4. 4.

    These properties are correlated with the degree of crystallization of the protein from which the silk is made, this factor being controlled by the sequence of amino acids in the protein.

  5. 5.

    The physical properties of the viscid and frame silks allow them to function effectively as shock absorbers and structural elements, respectively; and allow the orb-web to function as an aerial filter with a minimum expenditure of material and energy.

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