Blundering into a sticky spider's web is unpleasant enough, even when you are large and can tear free. But for smaller creatures, impacting a web is a death sentence. Brent Opell and colleagues from Virginia Tech, USA, explain that the stickiness of a web depends on the stretchiness of the adhesive globules that are dotted along the spiral thread. ‘When an insect flies into an orb web, its legs, wings and body present different textures to the web's sticky threads and the insect's struggle to escape generates forces whose magnitude and direction change rapidly’, says Opell. Previous lab-based experiments had shown that the stickiness of web glue could be optimised to the humidity of the spider's natural environment, as the stretchiness and adhesion of the droplets changes as the humidity alters. But what could this natural variation mean in practice for snared insects; could variations in local humidity affect the survival chances of a thrashing fly?
Having recorded that the humidity in forests inhabited by marbled orb-weaver spiders (Araneus marmoreus) ranged from 67% to 76%, Opell and his colleagues then collected portions from spider webs in the same locations to test the effects of different humidities on the sticky adhesive droplets. Back in the lab, Opell and Mary Hendricks kept the sticky threads at 20–90% humidity and found that the adhesive became stretchier and contacted a larger area on the surface of trapped victims as the humidity increased to 72%, before declining as the air became moister. And when they compared the strength of the adhesive attachment, it was greatest at 72% humidity. The spiders’ webs seemed to be optimised to hold fast at the humidity found in their home territory.
But what would this mean in practice for insects fighting to free themselves from peril? Stabilising intact web portions at 37%, 55% and 72% humidity before gently placing an anaesthetised fly across the sticky threads and filming its bid for freedom, undergraduate students Katrina Buccella, Meaghan Godwin and Malik Rivas recorded that all of the flies escaped in periods ranging from 3.3 to 34.7 s. However, the flies that were detained on the web at 72% humidity had the toughest time escaping and were trapped for 11 s more than the flies that encountered the drier webs.
So, humidity clearly affects the length of time it takes for a trapped insect to break free of its bonds and Opell says, ‘This difference … is ecologically significant as the short time after an insect strikes a web, and before a spider commences wrapping it, is the insect's only opportunity to escape’. Opell also points out that these observations agree with other studies, which have shown that evolution has fine-tuned the stickiness of spider webs to the humidity of the environment by changing the compounds that attract water to the sticky glue droplets, allowing webs to hold on hardest to hapless victims at the spider's home humidity.
