The remarkable bark spiders (genus Caerostris: Araneidae) are poorly known Old World tropical orb-weavers, whose diversity, currently at 11 species, is grossly underestimated. Most species build large webs at forest edges, clearings, and gardens, but in Madagascar, probably the hot spot of Caerostris diversity, at least one species occupies a unique ecological niche: casting its web across streams, rivers and lakes, so that the orb is suspended above water and attached to substrate on each riverbank via bridgelines up to 25 m. Here, we summarize current knowledge on Caerostris natural history, and specifically focus on the remarkable web architecture and biology of the newly described Caerostris darwini n. sp. Darwin's bark spider builds its web, a regular orb suspended above water, and maintains it with daily reinforcing of bridgelines and renewal of orb for many days. Web size ranged from 900–28,000 cm², with the largest measured web of about 2.8 m² being the largest orb ever measured, to our knowledge. With anchor lines capable of bridging over 25 m, it also builds the longest webs among all spiders—a unique form of web gigantism. We report on mass capture of ephemeropteran prey items in C. darwini n. sp. webs during a single day. Webs contained up to 32 mayflies that were subsequently wrapped en masse before the spider fed on them. We also provide the first evidence of kleptoparasitism in these webs both by other spiders (Argyrodinae) and by newly documented, undescribed symbiotic flies. Caerostris display extreme sexual size dimorphism with large females and small males, which is manifested in enigmatic sexual behaviors such as mate guarding, male-male aggressiveness, genital mutilation, mate plugging, and self castration. Caerostris is thus a promising candidate for evolutionary studies, and its diversity, biology, and phylogenetic relationships all deserve a closer scrutiny.