Caerostris darwini in Madagascar: river-spanning orb webs up to 25 metres wide

Darwin’s bark spider has drawn scientific attention as studies explain why only adult females produce the toughest spider silk ever recorded. The species, found in Madagascar, is noted for building river-spanning orb webs reaching up to 25 metres in width.

Darwin’s Bark Spider (Caerostris darwini): 

Dimension	Key Details
Common name	Darwin’s bark spider
Scientific name	Caerostris darwini
Family	Araneidae (orb-weaver spiders)
Type	Large orb-weaving spider
Related species	Caerostris kuntneri
Habitat	Forests and wetlands in Madagascar
Distribution	Endemic to Madagascar
Web location	Builds orb webs directly above rivers or lakes
Web size	Largest orb webs ever recorded, reaching up to 25 metres in width
Adult female characteristics	Females are largely black in colour, with white hairs
Male characteristics	Males are redder and lighter brown in colour
Sexual dimorphism	Adult females are significantly larger than males
Silk production	Only large adult females produce exceptionally tough silk
Silk properties	Silk outperforms steel and most human-made fibres in both strength and toughness
Silk uses by the spider	Used as safety lines, anchor threads, and escape ropes
Threats	Habitat disturbance from river/wetland degradation and infrastructure projects (e.g., hydroelectric dams); water pollution reduces prey availability and impacts silk quality
Ecological significance	Large river-spanning webs help control insect populations, especially flying insects