Electrostatic forces have been previously proposed to play an important role in silk adhesion14,15,16, although experimental evidence indicates that only non-electrostatic adhesive properties pertain to cribellar silk15. Our experiments show clearly that positively charged insect bodies induce rapid attraction of silk threads in the webs of cross-spiders, indirectly supporting a prior hypothesis that static charges of insects increase the prey capture success of orb-webs15. Risk of capture for a free-flying insect may correspondingly be enhanced given that the induced deformations observed here are comparable to the average mesh spacing for cross-spider webs17 (~2 mm). Observed deformations also suggest that greater charges and associated web displacements may characterize larger insects and are more likely to induce deformation of multiple radial and spiral threads (Table 1). By contrast, length-normalized deformations suggest non differences among insects (Figure 2; see also Table 1 and supplementary Table S1 online). The greater charge typically accumulated on larger test insects (see Table 1) reflects its direct dependence on both electric potential (as induced here by the Van de Graaff generator) and their capacitance. Electrostatic charge acquired by insects in free flight will similarly reflect these factors, but charge will principally be acquired through interaction between the flapping wings and the surrounding air, along with particular atmospheric conditions that promote charge accumulation (e.g., lower relative humidity).

Figure 2 Web deformation produced by a charged honeybee (a), a fruitfly (b) and a water drop (c). Images are three sequential video frames (filming speed: 1500 frames s−1). Bee, fruitfly and drop size are 12 mm, 3 mm and 1.5 mm, respectively. Image gamma was increased to 1.5. Full size image

The substantial variance in web deformation data (Table 1) likely reflects the effects of variable body position and orientation with respect to silk threads, as must characterize prey captured by spiderwebs in nature. Because electrostatic force varies with the inverse square of distance, substantial differences in thread deformation for charged objects passing through the web can be expected. Deformation data presented here, moreover, refer only to two-dimensional motions and will systematically underestimate total thread displacement. The extensibility of web spiral thread is higher than that for radial thread1 and we observed qualitative differences in thread deformation according to the position of charged fruit flies falling relative to these two distinct web elements (Supplementary Movie S5 online). It is also well known that spiral threads of ecribellate spiders are more extensible than those of cribellate species. A recent computational study found that increased elasticity of spiral threads reduces the energy absorption and dissipation of the entire web, but in contrast is mostly unchanged for individual spiral threads, despite reduction in strength18. These results suggest that comparable capture effectiveness for spiral threads can be obtained with high elasticity and low strength (and that require lower energetic investment to produce). Accordingly, charged insects or small particles may elicit greater deformations from spiral threads of ecribellate taxa.

Insects can easily acquire electrostatic charge by walking over charged surfaces or by flying in an airstream of charged particles9,10,11. Honeybee workers11 during wintertime conditions can acquire a positive charge of up to 537 pC, whereas honeybees those in non-winter weather can reach up to 200 pC, values comparable to those used here experimentally (Table 1). Positively charged bumblebees can even detect floral electric fields, which ability enhances their foraging success19. Despite the well-recognized role of electrostatic charge in pollination12, the charge magnitudes on insects in nature are largely unknown, but are expected to be higher in low humidity and in dusty conditions.

Spider-orb webs are aerial traps specialized to catch flying insects of different sizes and even occasionally birds20. Mechanical properties of the web silk dissipate the kinetic energy and impulse acting to the web produced during insect impact1. The stickiness and elasticity of ecribellate threads are mediated by their water coating3,4, which has a ~80 times higher value for relative permittivity than air21. Wet silk threads may thus be more easily polarized by an electrostatic field than are dry threads. In calm weather, air contains predominantly positive ions, in contrast to vegetation that is typically negatively charged22. Although charges of spider webs under natural conditions have never been reported, accumulation of negative charge may result in even greater deformation in response to positively charged insects. Charge accumulation may also increase deposition rates of electrically charged particles floating in the air, such as pollen and fungal spores, which are actually consumed by juvenile cross-spiders23. However, adverse dust deposition and associated web degradation may influence daily patterns of web reconstruction24, to which end charge accumulation may be a previously unrecognized contributing factor. Charged rain drops may also induce web damage or adhesion of adjacent threads; field measurements during spring rainfalls indicate both positive and negative charges on rain with values of up to 100 pC25.

In conclusion, we have experimentally demonstrated that electrostatically charged insects and water drops can induce rapid and comparably sized deformations in threads of the cross-spider's orb web. Such deformations likely increase the risk of capture for free-flying prey.