Fatal Attraction: Does Static Help Spiders Catch Prey?
It has probably happened to you: you pull a sweater over your head or take a load of freshly dried clothes out of the dryer, and all of the sudden you are a victim of static electricity. Your hair defies gravity, unwanted items cling to your clothes, your eyes feel dry, and you may even experience minute electrical shocks. This can be annoying and potentially awkward, to be sure . . . yet, as recent research conducted by scientists at the University of California at Berkeley shows, it could be worse, especially if you are an insect approaching the charged threads of a spider web.
Insects often become “charged” as a result of walking over electrostatically charged surfaces or encountering charged particles in the air as they fly. This creates a profound predicament for insects that pass by spider webs: the silk threads made by spiders are not that different from the threads of a nice silk shirt. While it might be annoying to have an errant sock stuck to the back of your blouse, it is probably not as inconvenient as being a bee and suddenly finding yourself in the grip of an electrical force pulling you towards a deadly predator’s trap.
This combination of factors—the mild electrical charge present on spider silk, and the fact that insects can be come “charged” during their daily activities—has potential implications regarding the utility of spider webs for ensnaring prey. Recently, two researchers from the University of California at Berkeley designed an experiment to test how spider silk interacts with charged prey items. They sought to determine whether spider silk would “deform” towards positively charged insects—in effect, whether the webs would use electrical attraction to reach out and nab a meal for the spider.
The results, recently published in Scientific Reports (Ortega-Jimenez & Dudley 2013), inspire fresh awe for the sophistication underlying even simple spider webs. When the researchers dropped positively charged insects and charged water droplets near the uncharged webs of cross spiders (Araneus diadematus), the strands displayed “rapid and substantial” attraction to charged objects. Web deformations were not observed when uncharged insects were exposed to the webs.
When charged insects were dropped near a web, the silk would deform (ie, move toward the object) by distances of up to 2 mm. This may sound like a modest stretch, but it corresponds to the mesh spacing of many spider webs, allowing the electrostatic attraction between silk and potential prey to essentially close the gaps in the web. This likely increases the capture success for the hapless, electrified insects, although further studies will be needed to validate that effect in the wild.
A few interesting side notes also came out of this experiment. For example, several species of insect were used in this study: honeybees, bottle flies, fruit flies, and aphids. The researchers measured the voltage of each individual insect, and found that although there was little variation between conspecifics, each species as a whole seemed to carry a characteristic charge that differed from the other species. For example, a honeybee carries a voltage of 0.5 kV, while a fruit fly carries only about 0.06 kV, nearly an order of magnitude lower. This may have to do with the relative mass of the insects, as the researchers found that the charges of the water droplets used in the study did vary with size. Again, the implications of this size-charge connection for capture success in the wild will need to be tested with further studies.
The present study measured web deformation in only two dimensions, but the researchers noted that spiral threads are known to be even more extensible than radial ones. The webs used in this experiment showed qualitative changes in thread movements based on where an insect passed relative to both radial and spiral components of the structure. (See this page for an excellent explanation and illustration of how webs are built and how the radial and spiral threads are organized). In addition, the webs used in the study were “grounded” so that they carried no significant charge. In reality, there is evidence that spider webs in the wild may be negatively charged, meaning that their electrostatically induced responses to positively charged prey items could be even stronger than those demonstrated in this study.
It will be fascinating to follow this line of work if it is extended to field conditions. I immediately wondered whether seasonal variations in temperature and/or humidity affect these static interactions, and thus may influence spiders’ capture success of different prey species at different times of year, or across the geographic range of a given spider species. Ortega-Jimenez and Dudley opened a crucial door with this study, and it will be interesting to see where that leads us in the future.
Ortega-Jimenez VM, & Dudley R (2013). Spiderweb deformation induced by electrostatically charged insects. Scientific reports, 3 PMID: 23828093