Effect of spider's weight on signal transmittance in vertical orb webs

Abstract

Spider orb web is a sophisticated structure that needs to fulfil multiple roles, such as trapping prey and transmitting web-borne signals. When building their web, heavier spiders tend to increase the pretension on the web, which seems counterintuitive since a tighter web would decrease the chances of stopping and retaining prey. In this article, we claim that heavier orb-weaving spiders increase tension on the web in order to reduce the attenuation of the vibratory signal coming from the bottom part of the web. We support our claim by first building a detailed spider web model, which is tuned by a tension-adjusting algorithm to fit the experimentally observed profiles. Then, the effects of the spider weight and the web tension on the signal transmittance properties are investigated using state-of-the-art finite element analysis tools.

Keywords: orb web; sensing; vibration.

Figure 1.

Spider web model used in this study with geometrical parameters. Solid cyan lines show possible positions where the spider can detect incoming signals, which are called leg nodes. A fly can be captured at any point on the solid magenta line, which is called a signal node.

Figure 2.

Tension adjustment algorithm flowchart.

Figure 3.

Displacements and tension change values near the hub, shown by the red and blue colours, respectively, caused by the signal source located at the red dot. Web’s response to transverse excitation is shown at (a) and to lateral excitation at (b). (i) Spider web with 490 μN downward force applied to the hub but no inertia and (ii) 50 mg spider at the hub. For transverse waves, tension changes on the radii are tens of nanonewtons at maximum and do not show any significant change for different cases. The displacement is affected by the addition of inertia, and positional information for the signal location becomes significantly better compared to the no inertia case. Similar observations are present for the tension change values due to the lateral waves.

Figure 4.

Prestresses in the web with ${\displaystyle T_{rad}=100}$ μN: (a) no spider present at the hub, (b) 490 μN and (c) 980 μN weight applied at the web centre, representing two different spider weights in y direction. The numbers present the pretension values of radii close to the frame. All the values shown, including the colour bar, have units of μN.

Figure 5.

Average displacement and tension change of leg nodes on two radii closest to the signal location is shown with changing spider mass for ${\displaystyle T_{rad}=100}$ μN. The signal source is located at the bottom region, shown by the red dot on the spider web model (a), and the wave types of the vibration are transverse, longitudinal and lateral for (b–d), respectively. For transverse waves, the displacement decreases as the mass of the spider increases. For lateral and longitudinal cases, tension change is not observable for 75 and 100 mg cases, and the increase in displacement comes from the silk that sags and is not under any tension.

Figure 6.

Contour map of signal amplitude with respect to the spider weight and radii pretension ($T_{rad}$) for the vibration generated at the bottom part of the web, with white lines showing the equivalue points. Three points on the map are shown as web models. The signal source is shown as the red dot, and the displacements on the radii are shown with red solid lines, where a larger distance from the hub represents a larger value on the radial.

Figure 7.

Contour maps for (a) longitudinal and (b) lateral waves. The signal amplitude is represented as tension change on radii, with colour bar representing the values with units of μN, plotted with respect to spider weight and radii pretension ($T_{rad}$). Model representations corresponding to the areas marked on the contour map are shown in (c) for longitudinal and (d) for lateral waves. The signal source is shown as the red dot, and the tension change on the radii is shown with blue solid lines, with a greater distance from the hub representing a larger amplitude. The signal amplitude created by the longitudinal waves does not seem to change much with changing tension (a); however, when the tension on the radii becomes too low, the signal is completely lost. Lateral waves also lose signal transmission below a certain (radii tension)/(spider weight) ratio threshold as seen in (b). Unlike the longitudinal excitation, for the lateral excitation case, higher tension of the radii negatively affects signal transmission, and the locational information becomes less precise above a certain value.