Consequences of electrical conductivity in an orb spider's capture web
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- Vollrath, F. & Edmonds, D. Naturwissenschaften (2013) 100: 1163. doi:10.1007/s00114-013-1120-8
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The glue-coated and wet capture spiral of the orb web of the garden cross spider Araneus diadematus is suspended between the dry silk radial and web frame threads. Here, we experimentally demonstrate that the capture spiral is electrically conductive because of necks of liquid connecting the droplets even if the thread is stretched. We examine how this conductivity of the capture spiral may lead to entrapment of charged airborne particles such as pollen, spray droplets and even insects. We further describe and model how the conducting spiral will also locally distort the Earth's ambient electric field. Finally, we examine the hypothesis that such distortion could be used by potential prey to detect the presence of a web but conclude that any effect would probably be too small to allow an insect to take evasive action.
KeywordsSilkElectrostatic chargesAirborne particlesAerosolEarth electric field
The typical spider's orb web is suspended by silk frame threads and consists of a capture area where sticky silk threads form a spiral in a wheel of silk ‘spokes’ radiating from a central hub where the spider sits. In cribellate spiders, the capture threads are dry gossamer and consist of hundreds of finest filaments hackled into a wool-like consistency that holds the insect prey using electrostatic charges (Opell et al. 2011a). In the ecribellate spiders, like our garden cross spider, the capture spiral carries a coating of wet silk droplets swollen with water and loaded with a surprisingly strong solution of somewhat unexpected compounds such as GABAmide (γ-aminobutyramide), choline, isethionic acid, N-acetyltaurine and betaine as well as traces of cysteic acid, lysine, serine, glycine, KH2PO4 and KNO3 (Vollrath et al. 1990); finally, there is a good supply of the glycoprotein N-acetylgalactosamine to provide the adhesive glue (Vollrath and Tillinghast 1991). The complex composition makes the coating exceptionally hygroscopic (Vollrath et al. 1990), which functionalizes it to attract water from the atmosphere. While a thread is being spun by the spider, this coating is deposited as a thin and even film (Edmonds and Vollrath 1992). Ambient water rapidly accumulates and leads to an immediate swelling of the coating cylinder, which in turn (via a Plateau–Rayleigh instability) leads to the formation of elongated aqueous droplets distributed evenly along the threads. The droplets have an important double function: they act as functional component providing the windlass mechanism that give this capture thread its very special properties (Vollrath and Edmonds 1989), and they also carry the glue that allows prey to be retained (Opell et al. 2011b; Vollrath and Tillinghast 1991).
A water droplet moving through air charges up electrostatically (Gott 1933), and the ionic charges are highly mobile in the aqueous solution (Richens 1997). A droplet suspended in moving air by a good insulator would charge up equally, and silk is an excellent insulator as has been known (and experimentally used) since Faraday's time. Importantly, airborne particles, including aerosols, pollen and even insects, also become highly charged when moving/flying through the air (Es'kov and Sapozhnikov 1976).
Here, we hypothesize that any charged particle as it approaches the capture thread of a spider's web attracts the charge of opposite sign and repels the charge of the same sign from the glue droplet chain in its vicinity. The charged body and the web would then attract each other electrically.
This phenomenon has practical implications for the web and spider. For example, ecribellate orb webs are exceptionally good at harvesting micron-sized airborne particles (Rachold and Heinrichs 1992) and that could include dangerous pesticide aerosols (Samu et al. 1992). On the other hand, individual web capture threads are clearly attracted to an approaching insect, and the silk will literally ‘spring’ towards it as the insect gets very near (Ortega-Jimenez and Dudley 2013). The mechanism behind this phenomenon relies partly on the charges of both insect and silk droplet and partly on the mechanical properties of the thread, which combines an extremely low modulus with very high extensibility (Vollrath and Edmonds 1989). Because the capture thread is both very soft and extremely stretchy (Köhler and Vollrath 1995, Opell et al. 2008), it requires very little force to distort it, which means that very small electrostatic forces could already have a dramatic effect.
Importantly, the individual droplets on the typical capture thread remain interconnected by narrow necks of liquid (Vollrath and Edmonds 1989; Edmonds and Vollrath 1991). Thus, the aqueous coating, if indeed it was conductive along the length of the capture spiral, could lead to local distortions of the Earth's electrostatic field. We examine this hypothesis theoretically in order to better understand the dynamism of the entire system and its ramifications.
In order to measure the conductivity of individual spiral threads, we constructed a small Faraday ‘cage’ to fit under a microscope and to hold a 1-cm-long section of capture thread suspended between two metal holders insulated against the cage. These holders could be stretched manually by means of a linear extender. Each metal holder was attached to an ohmmeter capable of measuring the electrical resistance (in gigaohms), and the current flowing was measured as a thread was extended and relaxed while the droplets were simultaneously filmed for later measurements of droplet behaviour and dimensions.
In order to correlate the conductivity of the threads to characteristics of the web, we conducted experiments whereby a metal sphere of 5 mm diameter connected to high DC voltage was slowly approached towards a web or single strands suspended between insulators.
Results and discussion
This demonstrated that in Nature not only individual web capture threads but also whole webs (by the combined action of many threads) would be attracted equally no matter what the sign of the charge on an approaching particle. Thus, in a very real sense, the capture spiral becomes active rather than remains passive and extends to entangle charged prey. Future studies will have to demonstrate (or refute) whether the charges might actively assist the underlying glycoprotein glue dollops circling the core capture spiral deep inside each droplet (Vollrath and Tillinghast 1991) as they stick to insect prey more or less firmly (Opell et al. 2011).
The maximum charge on an airborne particle is limited by the breakdown or ionization of the air that surrounds the particle in the electric field created by its charge (Feynman et al. 2013). Ionized air conducts electricity, which will then discharge the particle. Dry air breaks down in an electric field of about Emax = 3 × 106. The application of Gauss's law connects the charge density σ on the surface of a conductor and the electric field E just outside it such that σ = ε0εRE. If the airborne particle is represented by a conducting sphere, the maximum charge density becomes σmax = 2.655 × 10−5 cm−2 so that its total charge becomes q = 4πb2σmax. To obtain estimates for the maximum effect of the electrical interaction, this maximum charge will be assumed for the airborne particles (Jaworek et al. 1998).
To calculate realistic interactions, we assume a fixed glue droplet radius of a = 10 μm based on actual measurements (Edmonds and Vollrath 1992). For an aerosol particle with a radius b = 10 μm and initial drift velocities Vx of 0.01, 0.05 and 0.1 ms−1, the effective enhancement factors were calculated to be 85.3, 31.9 and 23.1, respectively, based on the equations given above. For an aerosol particle of radius b = 20 μm with the same three initial velocities, the effective enhancement factors would become 143.3, 56.1 and 35.4, respectively. This means that the electrical charge on the aerosol particle, interacting with the opposite charge that it induces on the glue droplet, has enhanced the collecting efficiency of each of the glue droplets by these factors.
Importantly, the collecting efficiency of the capture spiral of A. diadematus for airborne aqueous droplets with a distribution of radii up to 50 μm falling under gravity was estimated to be a factor of about 20 greater than that expected from the purely mechanical cross section of the droplets (Samu et al. 1992). Stokes' law predicts (see above) that spherical water droplets of radius 10 and 20 μm in air will fall under gravity with velocities of about 0.012 and 0.048 ms−1. When account is taken of the assumptions made in our theoretical estimates, including that of maximum charge, and the uncertainties due to the distribution of sizes of the aerosol particles in the experiment, the electrical interaction is clearly capable of explaining the enhanced effectiveness of the conducting capture spiral as a collector of small charged airborne particles.
The concept of possible distortion by a conducting web of the Earth's natural electrostatic field was also modeled from the first principles. Our experiments with charged spheres demonstrated that the capture spiral threads extend far outwards towards a charged object depending both on the level of charge and the distance between the two. This suggests that flying insects—typically highly charged from active movement through the air (Warnke 1976)—approaching a web are at danger of being actively captured by threads that actively ‘jump’ forward before active contact (Ortega-Jimenez and Dudley 2013). However, the accumulated charges on the web may have another effect. Importantly, because all droplets are interconnected by narrow necks of liquid, the capture thread is conducting along its length. Thus, it is not only attracted to a charged insect but might also carry a warning if an insect could sense the presence of charges in the environment, as some insects apparently can (Warnke 1976; Greggers et al. 2013; Clarke et al. 2013).
Under stable sunny conditions with clear skies at the surface of the Earth, there exists an electric field of approximately EE = 100 V m−1 pointing downward. During thunderstorms, the electric field may be very much stronger and may even reverse in direction. A spider's orb web with its conducting capture spiral may be considered as a vertical conducting circular disk. Such a conducting disk placed in a previously uniform electric field will distort the electric field. In particular, the static electric field tangential to the surface of any conductor must be zero because the tangential component of an electric field at the boundary between a dielectric material and a conductor is conserved across the boundary, and all static electric fields within the conductor are zero (Bleaney and Bleaney 1989).
We consider the circular web as a thin vertical conducting disk of radius a in the (x,y)-plane of a set of Cartesian coordinates with the origin at the centre of the web and such that the y-axis points vertically downward. The z-axis then forms the axis of the disk through the origin and normal to the plane of the disk while the x-axis is in the plane of the disk and parallel to the surface of the Earth. The electric field of the Earth EE acts vertically downward along the direction of the y-axis. Let us consider an insect flying along the z-axis toward the disk. At a great distance from the disk, the charges induced on the surface of the disk by the downward-pointing electric field do not appreciably change the field EE sensed by the insect. However, as the insect approaches the disk, the downward Earth's field it experiences is reduced by the electric field of the charges induced in the surfaces of the disk until when close to the disk the downward field is reduced to zero. If the insect is capable of sensing the vertical electric field, it could detect the presence of the spider web because it distorts the Earth's field even although it cannot see the web.
Summary and conclusions
We have now demonstrated that the capture spiral threads of certain orb spiders are conductive and remain so even when stretched. In more generic terms, our observations and calculations suggest that particular spider webs can be very efficient collectors indeed of aerosol particles. In practical terms this means that harvesting webs for chemical analysis may prove a useful technique in assessing the extent of ‘pollution‘ by airborne particulates. This strongly supports the data and conclusions of experimental work by Samu and collaborators on pesticide aerosols (Samu et al. 1992) and by others on solid airborne particles (Rachold and Heinrichs 1992). We may conclude that droplet-bearing orb webs can be excellent indicators of air pollution including the presence of heavy metals (Xiao-li et al. 2006). And if such webs are ingested by the spider for recycling (which is common in some spiders), then the animals themselves will become accumulators of such pollutants.
We further demonstrated the natural (biological) relevance to the spider of droplet charging. The highly extensible capture threads carrying a string of mobile charges always in opposition to any approaching object enhance the effectiveness of the threads and thus spring forward when perhaps the insect is so close as to actually see, or otherwise sense, the web. Whether the charges also enhance the capture efficiency of the thread by affecting the glue remains to be demonstrated, but it is not inconceivable that the slapping action of the forward-springing thread may at least provide for a closer initial bond of glue and insect. So far, the measurements by Ortega-Jimenez and Dudley demonstrate that the charges may indeed have significant implications for the capture efficiency of ecribellate (droplet bearing) orb webs by enhancing its active reach (Ortega-Jimenez and Dudley 2013).
Finally, the hypothesis that any web-induced distortions of the Earth's electrostatic field might be sensed by insect prey clearly now needs to be tested empirically. Recent observations and measurements by two independent groups suggest that even very small disturbances of an electrostatic field can be used by flying insects with Greggers et al. (2013) showing that honey bees may use electric field to learn abstract cues, while Clarke et al. (2013) demonstrate that bumblebees use electric fields as floral cues. Clearly, now it remains to be seen whether these insects might also be able to use their e-sensors in order to avoid webs and thus becoming ‘dinner’.
We thank the Science and Engineering Research Council of the UK for funding the original study in 1985 and the Air Force Office of Scientific Research (FA9550-12-1-0294) and European Research Council (324607) for funding the recent follow-up analysis by FV. FV also thanks a patient editor, four excellent anonymous reviewers and Sebastien Neukirch for helpful comments. Donald Edmonds sadly passed away last year after a long illness.