Casting spider webs across Malagasy rivers: Extraordinary silk in giant orb webs

The giant webs spanning rivers in Madagascar are spun by ‘bark spiders’ (genus Caerostris), orbweavers that are widespread throughout the old world tropics, but extremely understudied taxonomically, ecologically and behaviorally. Most species hide during the day and spin at night, making large webs (up to 1.5 meters across) often in the forest edge or clearings. However, in Madagascar several Caerostris species have evolved into a unique ecological niche: during daylight they now cast their giant webs across rivers, suspending the orb directly above the water on bridgelines that can span 20+ meters, attached to trees on each side of the river. Although some other spiders build webs above water, no others can utilize large rivers as habitat. How do Malagasy Caerostris achieve this unique architectural feat? How do the heavy bodied spiders cross rivers? Have the thin silk threads that support these huge webs evolved novel mechanical properties, thus allowing diversification in a novel habitat? What prey do these spiders catch and how do they avoid destruction of their webs by the large flying animals that use rivers as forest highways? We propose an expedition to Madagascar to address these questions by combining traditional taxonomy, molecular phylogenetics, behavioral ecology, and material science.

Completion of this project will provide insight into how evolution can “max out” the material properties of silk and web production of spiders, and will provide the foundation for a comprehensive worldwide study of Caerostris biology. The project will contribute knowledge of biodiversity relevant to conservation, and will involve training of underprivileged Malagasy collaborators.

This project will be of interest to scientists and laymen alike as it bridges disciplines to study the world’s toughest fibers and the unique
webs they support.