Although the diversity of spider orb web architectures is impressive, few lineages have evolved orb webs larger than 1m in diameter. Until recently, such web gigantism was reported only in a few nephilids and araneids. However, new studies on bark spiders (Caerostris) of Madagascar report a unique case of web gigantism: Darwin’s bark spider (C. darwini) casts its webs over substantial water bodies, and these webs are made from silk whose toughness outperforms all other known spider silks. Here we investigate C. darwini web architecture and provide data to begin to answer two intriguing questions to explain these extraordinary web characteristics: 1) Are C. darwini webs specialized to subdue unusually large, perhaps even vertebrate, prey? 2) Do these large, riverine webs allow the spiders to capitalize on catching numerous small semi-aquatic insects? During fieldwork in Madagascar, we studied C. darwini web architecture and ecology, as well as interactions with prey. We characterize C. darwini webs as having relatively simple capture areas with very open sticky spirals and few radial lines. We also compare web features in several sympatric Caerostris species, among which C. darwini represents the most extreme case of web gigantism, with the largest orbs up to 2.76 m2 and longest bridge lines reaching 25.5 m. While preliminary, current data suggest that C. darwini webs are effective snares for semi-aquatic insects such as mayflies and dragonflies, while vertebrate prey were never observed. We suggest that mass emergence of aquatic insects may function analogously to the capture of rare, large prey that recent studies suggest are critical for reproduction in orb weaving spiders.