ANCIENT ALTERNATIVE FOR ADHESION

Abstract

It's one of the oldest scenes in the `Horror' repertoire; a spider's web brushing a passing face! But spiders and their webs have been around longer than humans have been afraid of them. Today, most spiders' webs are coated in a sticky glue to ensnare flies and other victims. But a few ancient species stuck to an older, drier alternative. They produce cribellar threads;thousands of minute filaments coating a supporting thread that snag insects by their setae, like Velcro hoops over loops. Remarkably, the silk also sticks to glass and metal sheer surfaces through van der Waals molecular forces. But not all dry webbed spiders produce the same types of microscopic fibril. Some species produce lumpy fibrils, while the most ancient member of the cribellar family, the rare Hypochilidae, produce smooth fibrils. Intrigued by the differences between both types of silk, Brent Opell and his student Anya Hawthorn noticed that the lumpy fibre's adhesive properties improved under damp conditions, while the smooth fibres didn't. Wondering whether the knobbly fibres picked up water and adhered through hygroscopic forces, the team modelled the fibre's adhesive qualities under dry and damp conditions(p. 3905).First they needed fresh spider silk, but Hypochilus refused to spin its smooth fibred webs in the lab, even though Hawthorn and Opell built them cosy cliffs to make their homes on. So the team headed south to the spider's home in North Carolina to gather the unusual silk. Fortunately, Hyptiotes was more cooperative, contented to construct their knobbly silk webs in plastics boxes in the lab.With a steady supply of web silk, the team began measuring both fibres'stickiness as the humidity rose. Although the `sticky-o-meter' was straightforward to use in the field, operating it inside a humidity-controlled chamber was tedious, but Hawthorn persevered. As the humidity rose, the knobbly silk became stickier, while Hypochilus' remained the same.But was the increased stickiness due to humidity, or some other physical property? Hawthorn and Opell modelled the effects of van der Waals and hygroscopic forces on the smooth and lumpy silks. After measuring the area of both fibres that pealed away from the sticky-o-meter's plate, and calculating the resulting van der Waals and hygroscopic forces, the team realised that the increased humidity was giving Hyptiotes the sticky edge.Which probably gives Hyptiotes the mechanical edge too. When the conditions are right, their silk becomes stickier by freeloading on the moist fibre's hygroscopic attachment. But if Hypochilus want to increase their threads' stickability, they have to coat their threads with more smooth fibrils, at a significant metabolic cost. Which probably explains why most dry webbed spiders have upgraded to hygroscopic knobbly fibres as a low-cost alternative adhesive, leaving the Hypochilidae as the sole surviving link to the spiders' silky past.