Darwin's Bark (Caerostris darwini)

Physical description and identification

Darwin's bark spider is a heavy-bodied orb weaver built for camouflage rather than display. Pressed against a branch with its legs drawn in, an adult female disappears into the bark: the outline is broken by lumps and ridges and the colours are the greys and rust-browns of lichen and dead wood. It is the web, not the spider, that people notice first.

Adults

• Size: females measure roughly 18–22 mm (0.7–0.9 in) in body length, with a leg span approaching 7 cm. Males are tiny at about 5–6 mm (0.2 in), a quarter of the female's length and a small fraction of her weight.
• Colour: females are dark brown to blackish, dusted with short white and pale-grey hairs on carapace, abdomen and legs, which gives a frosted, lichen-flecked look. Males are paler, light brown to reddish, with the same scattered white hairs.
• Body shape: the female abdomen is broad, hard-edged and irregular, and the front of the carapace carries two pairs of raised humps. These tubercles are the most useful field character in the genus and are what makes a resting Caerostris so hard to pick out against bark.
• Eyes and legs: eight eyes in the usual araneid pattern of two rows, the lateral pairs set well apart on small mounds. The legs are short and stout for an orb weaver, faintly banded, and held tightly folded at rest.
• Males: mature males are recognised by their swollen pedipalps, which carry a straight-tipped conductor and a spoon-shaped embolus. Many older males on webs are missing part or all of one palp.

Eggs and egg sac

The female builds a silk egg sac and fastens it in sheltered vegetation away from the open orb rather than carrying it. Breeding data are still thin, but as in other araneids the sac holds a single large clutch that develops inside before emerging.

Spiderlings

Hatchlings are pale and unmarked, darkening with each moult until the adult bark pattern appears. They leave the sac together, then scatter on wind-blown silk lines to build small, conventional orbs of their own in streamside vegetation. Reaching adult size takes several rounds of molting.

The web

The web is what earned this spider its scientific attention. A female releases a long, buoyant bridging thread that the wind carries across a river, stream or small lake; once the far end snags she tightens it into a load-bearing cable. Bridge lines of 10–25 m have been measured, and from the middle of that cable she drops an anchor line and builds the orb in free air above the water. Finished webs reach about 2.8 square metres (28,000 cm²) — the largest orb webs recorded for any spider.

The silk has evolved to match those demands. Testing of C. darwini dragline silk found it to be the toughest biological material yet measured: it absorbs roughly twice as much energy before breaking as any other spider silk tested, and weight for weight more than ten times as much as Kevlar. That is toughness — the total energy a fibre soaks up before it fails — not raw tensile strength, where silk and Kevlar are much closer. The distinction matters, because it is stretch combined with strength that lets a web absorb a large insect over open water without tearing. See our guide to spider webs for how orbs are built.

Where Darwin's bark spider lives

Caerostris darwini is endemic to Madagascar and has never been confirmed anywhere else. It was found in 2009 in Andasibe-Mantadia National Park in the island's eastern rainforest belt and formally described the following year, and subsequent surveys have recorded it at other Malagasy river sites. Other members of the genus — the bark spiders — are widespread through mainland Africa, including South Africa, and across parts of tropical Asia, but none of them builds webs on this scale.

The habitat requirement is unusually specific: slow-moving or standing fresh water with trees on both banks, close enough for a bridge line to span and tall enough to hold the orb clear of the surface. By day the female leaves the hub for a branch at one anchor point, where her bark camouflage does its work, and returns to the web at dusk.

Is Darwin's bark spider venomous?

Yes — like virtually all spiders it has venom glands and uses venom to subdue insects caught in the orb. There is no evidence that the venom of Caerostris darwini is medically significant to humans. It belongs to the orb weaver family, a group with no dangerous members anywhere in the world, and it is unrelated to the widow and recluse spiders behind almost all serious spider bites. No verified harmful bite from this species exists, unsurprisingly for a spider that lives over water in remote Malagasy forest.

Can Darwin's bark spider bite?

It can, as any spider with working fangs can if it is squeezed against skin, but the circumstances almost never arise: the response to a large approaching animal is to drop on a dragline or freeze against bark. On the rare occasions an araneid this size does bite, the result is a brief pinprick and some local redness that fades within hours.

Ecological importance and behaviour of Darwin's bark spider

By stringing its orb across open water, this spider exploits a gap in the food web that almost nothing else fills. Aquatic insects — mayflies, caddisflies, midges, damselflies and dragonflies — emerge as adults in enormous pulses and fly upward into exactly the airspace the web occupies. A single web can intercept dozens of insects in one emergence night, moving energy from the aquatic system into the forest.

Web maintenance: the giant bridge line is expensive to build and is reused, sometimes for weeks, while the orb below it is taken down and rebuilt far more often to restore the stickiness of the capture spiral.

Natural predators: insectivorous birds, lizards, frogs and predatory wasps take adults and juveniles, and the exposed position of the web leaves the spider with no cover during a rebuild. Webs also attract kleptoparasitic dewdrop spiders that steal wrapped prey from the edge of the orb. Our overview of what eats spiders covers these pressures.

Mating and sexual cannibalism: the size gap between the sexes makes courtship dangerous. Males approach cautiously, often waiting near the hub for a female to moult, and field observations have documented prolonged oral contact with the female's genital region, plugging of her openings, and breakage of the palp that leaves the male a functional eunuch which may then guard her. Cannibalism occurs but is not inevitable.

Relationship with humans: direct contact is rare, but the species has had an outsized scientific impact. Its silk is a standing target for materials research, and because it is known only from Madagascar and depends on intact riverside forest, it is a useful indicator of how well those waterways are protected.

Similar spiders and how to tell them apart

Quick facts

Did you know?

• The species was named for Charles Darwin because its description was completed on 24 November 2009, the 150th anniversary of the publication of On the Origin of Species.
• It was discovered in 2009 and described in 2010 by Matjaž Kuntner and Ingi Agnarsson, working in Madagascar's Andasibe-Mantadia National Park.
• The spider cannot swim the bridge line across, so a 25 m thread must be carried the whole way by air currents.
• The famous "ten times tougher than Kevlar" figure refers to toughness, not tensile strength. Confusing the two is the most common error written about this spider.
• Males frequently end up as eunuchs, with one or both palps broken off inside the female. A eunuch can no longer mate, but he can still guard her against rivals.

In summary, Darwin's bark spider is a harmless, superbly camouflaged Malagasy orb weaver that is also an engineering record-holder: the biggest orb web on Earth, hung over a river on the toughest fibre biology has produced. Look for the web first — the spider will be flattened against bark at one end of it.