Caerostris

Physical description and identification

Caerostris is a genus of about twenty large, heavily built orb weavers in the family Araneidae, universally known as bark spiders. They are built for disguise rather than display: the carapace and the shoulders of the abdomen carry blunt horns and tubercles, the whole animal is coloured in bark greys, browns and blacks, and the legs fold into grooves against the body so that a resting female reads as a broken twig.

Adults

• Size: females typically run 15–30 mm in body length and males only 5–10 mm. In Darwin's bark spider the gap is extreme, with females around ten times the mass of their mates — one of the most pronounced size differences in any orb weaver.
• Colour: mottled black, charcoal, grey and brown, often with a scatter of white or silvery hairs on the abdomen that mimic lichen. Males are usually plainer, in reddish brown or tan.
• Eyes: eight, in two rows, the lateral pairs pushed out onto raised mounds at the corners of the carapace. Like other night-building araneids these spiders operate almost entirely by touch.
• Distinctive features: the tubercles. Two rounded humps sit on the front of the abdomen and further knobs project from the head region, breaking the outline in exactly the way a wood knot does. The legs look bare at a glance, but they are not hairless — the hairs are simply short, dense and flattened against stout, spiny segments.

Eggs and egg sacs

The female lays a batch of round eggs and wraps them in a silk sac fixed to vegetation or bark close to her daytime resting place; in warm conditions the young emerge after roughly two to three weeks. She guards the sac but does not open it or feed the spiderlings.

Spiderlings

Hatchlings remain briefly around the sac while they complete their first moult, then disperse. Popular accounts describe an extended stay with the mother, but what happens is short and passive — there is no feeding, no shared web and no cooperative behaviour. Within days the young are drifting off on silk lines and building their own small orbs.

The web

This is where the genus becomes remarkable. A Caerostris web is a proper vertical orb, not the untidy tangle older summaries describe, and it hangs from an anchor line that can be extraordinarily long. Darwin's bark spider (Caerostris darwini) of Madagascar strings bridge lines of up to 25 metres straight across rivers and lake margins and suspends an orb of up to about 2.8 square metres beneath them — the largest orb webs recorded for any spider. The bridging silk is also the toughest biological material measured, absorbing roughly twice the energy before breaking that ordinary spider dragline silk does. Our guide to spider webs covers how an orb is assembled.

Where Caerostris spiders live

The genus is Old World and tropical. Its centre of diversity is Madagascar, and it is well represented across sub-Saharan Africa — Kenya, Tanzania, Uganda, Nigeria and South Africa among them. It is not confined to Madagascar and south-eastern Africa, as older versions of this page stated: Caerostris sumatrana and Caerostris indica are Asian species, ranging through India, Sri Lanka, Southeast Asia and out towards New Guinea. The spider pictured on this page as C. sumatrana is itself an Asian animal.

Habitat follows the web. These spiders need two solid anchor points with a wide, uncluttered gap between them, which in practice means riverbanks, lake edges, forest clearings, tracks and the margins of swamps. The bark disguise ties them to woody vegetation, so a typical site is a wooded bank with open water or open air in front of it. They are not aquatic or semi-aquatic spiders; they simply build across water because that is where the gaps and the insects are.

Are Caerostris spiders venomous?

Yes, as nearly all spiders are, and only in a way that matters to insects. Caerostris venom subdues the mayflies, dragonflies, moths and beetles that hit the orb, and it has no medical significance for people or pets. There is no antivenom for any orb weaver because none has ever been needed, and no serious envenoming or death has been attributed to the genus anywhere in its range. These spiders are not related to the widow, recluse or funnel-web groups that do warrant medical attention.

Can Caerostris spiders bite?

They can, though records are few. A bark spider that is touched relies first on stillness, because its entire defence is being invisible; if that fails it drops on a dragline or runs along the branch. Biting requires the spider to be pressed against skin, typically by a hand landing on a branch it was resting on.

The reported effect is sharp local pain roughly comparable to a bee sting, with a small red mark and perhaps some swelling that settles within a day. There are no systemic effects. Wash with soap and water, use a cold pack for the pain, and keep the area clean. Seek medical advice only if swelling spreads well beyond the bite, if the wound later looks infected, or if there are signs of an allergic reaction such as hives or difficulty breathing. If you can, photograph the spider rather than crushing it — identification settles the question faster than anything else.

Ecological importance and behavior of Caerostris spiders

Caerostris works a strict two-shift day. The orb is built or repaired in the evening, the spider waits on it or on a signal line through the night, and around dawn it retreats to a branch, folds its legs into their grooves and becomes bark again. Prey is bitten and wrapped, and large items are stored on the web and fed on over hours. The diet is made up of flying insects — mayflies, caddisflies, dragonflies, damselflies, moths, beetles and bugs. Older accounts describing a diet of small aquatic insects are half right: the insects come from the water, but they are taken in flight, not from it. In Caerostris darwini, webs strung over rivers routinely catch mayflies in numbers and occasionally prey as large as dragonflies.

Natural predators: birds are the main threat to a spider that sits exposed on a branch all day, which is precisely why the camouflage is so elaborate. Lizards and small mammals take adults, parasitoid and mud-dauber wasps hunt orb weavers to provision nests, and ants raid egg sacs.

Prey-predator dynamics: by spanning open water the genus exploits a feeding space almost no other spider can reach. A single web positioned over a river intercepts an insect flyway that would otherwise pass untouched, which makes bark spiders locally significant consumers of emerging aquatic insects — see what spiders eat for how orb weavers sit in that food web.

Seasonal behaviour and mating: activity peaks in the wet season, when insect emergence over water is heaviest. Mating in Caerostris darwini is unusually well studied and unusually violent: the tiny male damages the female's genital opening during copulation, which functions as a mating plug and limits her remating, and he performs prolonged oral contact with her genitalia — behaviour documented in few other spiders.

Relationship with humans: negligible and entirely benign. Bark spiders do not enter buildings, do not defend their webs and are so cryptic that most people walk past them. Their outsized contribution has been scientific: the silk of C. darwini is a reference material for researchers designing tough synthetic fibres.

Species in the genus and similar spiders

Quick facts

Did you know?

• The genus was erected in 1868 by Tamerlan Thorell, the Swedish arachnologist who named thousands of spiders; older summaries credit an unnamed "Swedish arachnologist", and this is who they mean.
• Caerostris darwini was described in 2010 and named for Charles Darwin — the paper appeared on the 150th anniversary of the publication of On the Origin of Species.
• Its dragline silk absorbs roughly twice as much energy before breaking as the silk of other orb weavers, making it the toughest biological material yet measured and a benchmark for synthetic fibre research.
• A bridge line 25 metres long has to be laid on the wind, released as a filament and allowed to drift until it catches on the far bank — the spider cannot see where it is going.
• The camouflage is behavioural as well as structural: the spider settles along the branch rather than across it, and folds each leg into a groove so no outline remains to give it away.

In summary, Caerostris is a genus of large, harmless, superbly camouflaged orb weavers spread across Africa, Madagascar and tropical Asia. They are almost impossible to spot by day and responsible, through Darwin's bark spider, for two records no other spider has come close to: the biggest orb web and the toughest silk.