Wind induces variations in spider web geometry and sticky spiral droplet volume

Abstract

SUMMARYTrap building by animals is rare because it comes at a substantial cost. Using materials with properties that vary across environments maintains trap functionality. The sticky spiral silks of spider orb webs are used to catch flying prey. Web geometry, accompanied by compensatory changes in silk properties, may change across environments to sustain web functionality. We exposed the spider Cyclosa mulmeinensis to wind to test whether wind-induced changes in web geometry are accompanied by changes in aggregate silk droplet morphology, axial thread width or spiral stickiness. We compared: (i) web catching area, (ii) length of total silks, (iii) mesh height, (iv) number of radii, (v) aggregate droplet morphology and (vi) spiral thread stickiness, between webs made by spiders exposed to wind and those made by spiders not exposed to wind. We interpreted co-variation in droplet morphology or spiral stickiness with web capture area, mesh height or spiral length as the silk properties functionally compensating for changes in web geometry to reduce wind drag. Wind-exposed C. mulmeinensis built webs with smaller capture areas, shorter capture spiral lengths and more widely spaced capture spirals, resulting in the expenditure of less silk. Individuals that were exposed to wind also deposited larger droplets of sticky silk but the stickiness of the spiral threads remained unchanged. The larger droplets may be a product of a greater investment in water, or low molecular weight compounds facilitating atmospheric water uptake. Either way, droplet dehydration in wind is likely to be minimized.

Diet-induced covariation between architectural and physicochemical plasticity in an extended phenotype

“…We then calculated the droplet volume across a measured 0.5 mm length of thread (DV/0.5 mm) (Opell andHendricks, 2007, 2009). We calculated the surface area of the droplets (DSA) using the formula (Opell and Hendricks, 2009;Wu et al, 2013):…”

“…For instance, the properties of gluey silks and spiral threads within orb webs may co-vary with web architectures when humidity, wind speed or dietary regimes change (Townley et al, 2006;Opell et al, 2013;Wu et al, 2013;Blamires et al, 2014;Stellwagen et al, 2014). To identify the adaptive benefits associated with extended phenotypic plasticity we thus need to discern whether the physicochemical properties of silks co-vary with web architectural properties as part of an overall shift in foraging strategy.…”

Exploration behaviour and behavioural flexibility in orb-web spiders: A review

“…), suggests a high degree of plasticity in the construction rules. Clear examples of behavioural flexibility additionally include the matching of size and geometry to the available silk supply (Eberhard, 1988b), the increase in the distance between capture spiral turns at lower temperatures to decrease web-building time and/or to better target the larger insects still active and the decrease in web size and overall thread length and increase in stiffness and distance between spiral turns that reduces drag in wind-exposed webs Liao et al, 2009;Wu et al, 2013).…”

Humidity-mediated changes in an orb spider's glycoprotein adhesive impact prey retention time

“…Differences in diet and prey type cause striking differences in both a web's architecture and the properties of its threads (Blamires et al, 2016;Herberstein and Tso, 2011;Scharf et al, 2011;Townley et al, 2006;Tso et al, 2007), as can environmental factors such as wind (Wu et al, 2013). After a web has been spun, its frame and axial threads and its capture threads respond to daily changes in environmental conditions, with humidity having the greatest impact (Agnarsson et al, 2009;Sahni et al, 2011;Opell et al, 2011Opell et al, , 2013Stellwagen et al, 2014Stellwagen et al, , 2015Amarpuri et al, 2015).…”