Meet the spider spinning webs across rivers 25 metres wide

Spotting a spider web stretching from one riverbank to the other sounds impossible, yet it is exactly what Darwin’s bark spider (Caerostris darwini) does in the rainforests of Madagascar. This remarkable species spins the largest known orb webs on Earth, suspending them above rivers and lakes to catch swarms of flying insects with relatively little competition from other web-building spiders.

According to Keio University, the spider’s anchor thread can span up to 25 metres, while the sticky orb itself can cover as much as 2.8 square metres — the largest known web built by a single spider. Discovered in Madagascar in 2009, the species occupies a niche unlike almost any other spider: rather than building between nearby branches, it stretches a single bridge line across open water first, and only then constructs the rest of the web. How it manages to lay that first thread across such a wide gap is still not fully understood; the leading theory is that it releases silk into the wind until the strand catches vegetation on the opposite bank, then reinforces the line.

Building a bridge over a fast-flowing stream calls for a material that is both very tough and extremely flexible, and that is exactly what the spider’s silk delivers. Scientists have identified it as the toughest biological material ever studied, combining exceptional strength with remarkable elasticity. Research led by Dr Ingi Agnarsson and Dr Matjaž Kuntner found the silk’s average toughness runs around 350 megajoules per cubic metre, with some samples reaching 520 MJ/m³ — more than twice as tough as any other spider silk studied and over 10 times tougher than Kevlar of comparable size.

A 2021 review on PubMed notes that spider silk’s advantage lies in combining high tensile strength with exceptional extensibility, letting it absorb energy before breaking instead of snapping suddenly. Unlike synthetic materials, which are usually either strong or flexible, spider silk manages both at once.

Scientists at MIT, led by Markus J. Buehler along with Steven Cranford, Anna Tarakanova and Nicola Pugno, discovered that the silk’s strength is not the whole story. Under light loads like wind, the web stays stable, but during a stronger local impact — a large insect hitting the web, or falling debris — the silk temporarily softens and then stiffens again, keeping the damage contained to a small area instead of bringing down the whole structure.

Researchers now hope that studying this spider’s silk could help engineers design lighter protective gear, stronger medical sutures and tougher textiles, more than a decade after the species was first identified in Madagascar’s forests.

Wikimedia Commons/by Agnarsson, Kuntner & Blackledge