Formation process of droplet At first, how droplets were arranged at a regularly spaced row on capture thread was observed at an appropriate time interval. As soon as a spider constructed an orb web, a short part of the capture thread was cut out by means of a specially devised tool, and was subsequently set on an optical microscope. Furthermore, the shape of the droplets and their average spacing were measured for both spiders used here. Surface tension A collected quantity of viscid liquid is too small to measure the surface tension by a Du Nouy meter in which a large volume of test liquid was necessary. Therefore, a new machine was constructed as shown schematically in Fig.1. As illustrated in the enlarged view, the test liquid filled between the flat cylinders③ facing each other forms a symmetrical meniscus④. If the radii of meniscus curvatures are defined as r1 and r2, the surface tension T is calculated by Laplace equation F = πr１ ２ p, p = T ( 1/r 2 –1/r1) T = F/[πr１ ２ p( 1/r 2 –1/r1)] (1). The shape of the meniscus and the applied force F was recorded by a video camera through an optical microscope. The values of r1 and r2 were measured by the radii at the necked region. The measurement was done at a humidity of about 60% and a temperature 20℃. Microstructure Since the diameter of dragline is 5μm at most even for an adult female of Nephila clavata, it is difficult to observe its cross section by an optical microscope. A swelling technique by urea was used to investigate the microstructure. In urea, the thread diameter becomes more than 10 times bolder than the original one. A mixed solution of different concentrations of CO(NH)2; 6-12M, NaOH(pH8); 0.05-1M and NaCl; 50mM was used. When threads shorter than about 5mm are placed in a swelling agent on a glass slide, they begin to swell immediately at their both ends. Furthermore, a recoil test developed by Allen [9] was used to facilitate the observation of the interior structure of the thread. Tensile test How to prepare tensile specimens using samples collected from both spiders was stated elsewhere. When the dragline is wetted by water, it contracts after sufficient time. This phenomenon was named super contraction (SC) by Work [10]. The tensile samples for SC draglines were made as shown in Fig.2. The paper flame with a tight dragline set previously at its rectangular window was folded at the central part so that its ambient length might be obtained. After submersion during sufficient period, the thread became tight and the tensile specimen with an ambient SC ratio (SCR) was made. The super contracted ratio SCR was defined as SCR = (L0-Lc)/L0 where L0 was the original length of the flame window and Lc was the shrinkage length after SC. Tensile tests were done for dry and wet draglines with ambient SCRs and capture threads. 3．EXPERIMENTAL RESULTS AND DISCUSSIONS Formation process and geometry of viscid droplet When a spider produced two core threads by a pair of flagelliform glands, viscid liquid produced by a pair of aggregate glands covered the core fibers simultaneously. Just after the capture threads were drawn out from the glands, the viscid liquid was still parallelepiped around the core threads. More than 10 minutes was necessary at least until some signs for the generation of regularly necked parts were recognized. After about 40minutes, a series of regularly spaced droplets formed completely along the core fibers. The shape of the droplet is nearly the same for both spiders used as shown in Fig.3 where the dimensions are normalized by the maximum radius of the droplet. As shown by the solid curve in the figure, its shape is approximated well by a revolution ellipsoid. Surface tension of sticky liquid The load-time curves for the sticky liquid are shown in Fig.4. The liquid between the cylinders was pulled at a constant speed until their gap reached 0.25mm(point A), then the gap was kept constant for 2
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