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Properties of spider silk and design in orb-web

Properties of spider silk and design in orb-web - exp...

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exp. Biol. (1976), 65, 483-506 With 14 figures Printed in Great Britain THE PHYSICAL PROPERTIES OF SPIDER'S SILK AND THEIR ROLE IN THE DESIGN OF ORB-WEBS BY MARK DENNY* Department of Zoology, Duke University, Durham, North Carolina (Received 24 March 1976) SUMMARY 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. Both types of silk show a breaking stress of approximately 1 GN/m 2 and an initial resilience of approximately 0-35. 3. The breaking extension ratio of viscid silk (A = 3*00) is much greater than that of frame silk (A = i # 2s), and the viscid silk relaxes to a relatively greater extent. 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. 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 expen- diture of material and energy. INTRODUCTION Distinct in their form and function among the great variety of the webs qf spiders are the orb-webs, the spare and elegant constructions used as aerial filters by members of the Argiopidae and Uloboridae. Though utilizing different silks these two remotely related families (Kaston, 1964) build webs with the same basic structural features. This raises two basic questions concerning the design of orb-webs. First, does the orb-web represent the simplest way to build an aerial filter? Witt (1965), Witt, Reed & Peakall (1968), and Reed (1969) have provided a preliminary answer by presenting evidence that the orb-web structure is a simple and effective method of translating a limited set of anatomical and neurological parameters into a functioning food-gathering device. Second, does the orb-web design minimize the expenditure of material neces- sary to capture flying insects? In an attempt to answer this second question tests were performed to define some of the physical properties of the silks of the webs of a representative Argiopid spider, Araneus sericatus. * Present address: Department of Zoology, University of British Columbia, Vancouver, British Columbia.
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484 M. DENNY Fig. i. A typical orb-web. Radii {A) diverge from an above-centre hub (23) to attach on the polygonal frame (C). The frame is held in position by the mooring or guy threads (JD) tying the web to surrounding structures. Note that at (E) an addition to the frame, a cord, has been placed in order that the radii will not attach to the junction of the frame thread with a guy. Superimposed on this supporting framework is the viscid spiral (i 7 ). Orb-web terminology An orb-web is a planar net exhibiting the characteristic structures identified in Fig. i. The frame, mooring guy and radial threads form the supporting framework for the completed web. A guy thread may simply be a continuation of a frame thread past its junction with another frame thread. Superimposed on the framework is the
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