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Final Report Summary - WEBSTRUCT (A comparative study of the structural and dynamical forces in orb webs during prey impact and under wind-loading)

The original focus of the WEBSTRUCT project, to measure forces and deformations in a range of orb spider species and relate these to the energetic costs of the web-building process, changed somewhat during the project. In the course of conducting experiments on web-deformations in the wind-tunnel on a range of orb spiders, the Marie Curie fellow noticed that the non-sticky spiral in webs of Nephila edulis are under high tension and seems to play a mechanical role during wind-loading. As this observation had the potential to explain the old mystery of why nephilid spiders, unlike all other orb spiders, retain the non-sticky spiral in their finished webs, the fellow, after consultation with his supervisor, decided to focus on this question. To answer it the research was split up into two separate but interdependent studies.

The nature of the non-sticky spiral. This study consisted of a detailed study of the non-sticky spiral in Nephila edulis webs. First, geometry data of the web and the zigzag pattern of the non-sticky spiral was obtained from photographs and scanning electron images (see figure), which revealed that the non-sticky spiral in Nephila webs differs from the non-sticky spirals of orb webs, not only by being retained in the finished web, but also by showing several modifications including a zigzag shape and a strengthened junction where the non-sticky spiral wraps around the radius like a helix (see figure). Second, mechanical properties of the silk of Nephila webs where obtained from uni-axial tensile tests of threads taken directly from the web, which showed that stiffness, strength and elasticity of the silk in the non-sticky spiral did not differ from the silk of the radial, frame and anchor threads, but that the fibre diameters of the former were significantly smaller than those of the latter. Third, the obtained geometry and material data was then used to construct a finite element model from which it was shown that the zigzag pattern was generated by the spider by adding large pre-stresses (up to 20 times as high as the pre-stresses in the radii) during construction of the non-sticky spiral.

The function of the non-sticky spiral. This study built upon the previous study by focusing on the question of why the Nephila spiders retain the non-sticky spiral in their webs despite it causing a reduction in effective capture spiral area and resulting in diminished sensitivity to vibrations generated by prey impacts. Preliminary observations by the Marie Curie fellow led to the suspicions that the non-sticky spiral was vital for minimising the deformations in high winds of these large and dense webs. This hypothesis was confirmed by measuring the deformation of intact areas of webs placed in a wind-tunnel and then comparing that to the deformation of the same areas after the non-sticky spiral had been cut. In addition it was shown that the non-sticky spiral confers mechanical stability to the web during movement of the heavy spiders in the webs, by comparing a first prey capture event in the intact web with a second prey capture in the same web after the non-sticky spiral had been cut. Finally, a finite element model was built of a simplified web under wind-loading by using the geometry and material data obtained in the previous study. This confirmed the results from the experiments and in addition showed that the zigzag pattern of the non-sticky spiral and the pre-tensile forces are vitally important for its function

In addition to the main project on the non-sticky spiral in Nephila webs, the Marie Curie Fellow also participated in a range of smaller research projects related to the original objectives of the WEBSTRUCT project.

Behaviour of ecribellate and a cribellate webs in windy conditions. Wind-tunnel experiments were conducted on webs built by juvenile and adults of two ecribellate (orb spiders using gluey sticky spirals) species (Araneus diadematus and Nephila edulis) and one cribellate (primitive orb spiders using multiple small fibres to generate van der Waals forces in their sticky spirals) species (Uloborus plumipes). The results showed that the latter webs fail at much lower wind-speeds and fail in the radial and capture spiral threads, whereas the ecribellate species fail, when they fail - which they do less than half the time even at the highest wind-speeds possible to generate in the wind-tunnel, in the anchor threads that connect the webs to its surroundings. However, some additional finite element studies using actual digitized webs are still needed before any firm conclusions regarding the relative importance of silk properties vs. geometry.

Wind effects on prey capture behaviour. A small wind tunnel was used to generate different wind speeds in which the prey capture behaviour of the spider Araneus diadematus was tested. The results revealed that it ran more slowly towards entangled fruit flies, which had longer escape times in more windy conditions. For the spider, this indicates a lower capture probability and diminished overall predation efficiency at higher wind speeds. Thus the spider’s behaviour of taking down its web as the wind picks up may therefore not be a response only to possible web blow-out at high wind speeds.

Cues that spiders use to build orbs: From observations of web-building behaviour, it was possible to demonstrate that spiders, in addition to the 5 cues already known, use two additional cues, related to the distance between the non-sticky spiral (NS) and the inner loop of the sticky spiral (IL), to accurately construct their sticky spiral. A combination of direct observations of spider movements, analyses of complete and partially complete webs, and responses to experimental modifications of the web of two species in different families, Micrathena duodecimspinosa (Araneidae) and Leucauge mariana (Tetragnathidae), indicate that both the NS-IL distance itself, as well as short-term memory of the change in NS-IL compared with that on other recently encountered radii, correlate with sticky spiral spacing. When the distance from the NS is large, the spider apparently ceases to attend to other cues based on the IL location.

Finite element modelling of silk cocoons. The researcher used his finite element skill obtained during the project to build models of three silk worm cocoons differing in geometry and in material properties of the silk and sericine composite making up the cocoon shell. This project in still ongoing, but the preliminary results of the finite element model suggests that the geometry of the cocoon plays a significantly larger role for the deformation and damage tolerance of the cocoon, and hence for the fitness of the silkworm, than the material properties of the cocoon shell.

Impact. In addition to the basic scientific outcome of the project, disseminated at 4 international conferences and meetings and expected to result in a minimum of 6 scientific publications, two undergraduate research projects were undertaken, one on-going and one resulting in a manuscript currently in review, within the scope of the WEBSTRUCT project. The project also provided useful training for the future career of the Marie Curie fellow by enhancing his supervision, teaching and project management skills as well as giving him new scientific skills including the theory and practice of mechanical tensile testing and finite element analysis. Furthermore, the project attracted public interest, which resulted in an invitation for the Marie Curie fellow to publish a short popular science article on spiders and their webs in the booklet ‘Findings on Elasticity’ published by the Dutch PARS Foundation and participation in a documentary on spiders and their webs produced by the French production company ZED to be aired on Arte and perhaps the National Geographic Channel. Finally, the results on the function of specific silk structures in the orb web and on the importance of geometry for deformations in silkworm cocoons may potentially be of biomimetic interest for designers of tensile structures and for polymer engineers.

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