Darwin’s Bark Spider

Physical Characteristics

Darwin’s bark spider (Caerostris darwini) is a remarkable orb-weaving spider known for producing the largest and strongest webs in the world.

Body:
It has a robust, oval-shaped body typical of orb-weavers, with a slightly flattened appearance. The cephalothorax is smaller than the abdomen, which is rounded and patterned.

Coloration:
Their coloration ranges from dark brown to black, often with mottled white or gray patches that provide camouflage against tree bark. This cryptic appearance gives them their name.

Legs:
They have eight long, spiny legs adapted for gripping silk threads and vegetation. The legs are banded with lighter markings that aid in camouflage.

Size:
Females are significantly larger than males, a trait known as sexual dimorphism. Females measure about 0.7–1.2 in (18–30 mm) in body length, while males are much smaller, typically less than 0.3 in (6–7 mm).

Web-Building Ability:
Darwin’s bark spiders construct orb webs that can span rivers and streams, with diameters up to 82 ft (25 m). Their silk is considered the toughest biological material known, over 10 times stronger than Kevlar.

The Darwin’s bark spider’s cryptic bark-like appearance, extreme sexual dimorphism, and record-breaking web-building ability make it one of the most extraordinary spiders in the world.

Reproduction

Darwin’s bark spider has a fascinating reproductive cycle, marked by extreme sexual dimorphism and unique courtship behaviors.

Mating and Courtship:
Males are much smaller than females and approach cautiously to avoid being mistaken for prey. Courtship involves tapping and vibrating the female’s web to signal his presence and intentions.

Copulation:
Mating is often risky for the male, as females may display aggression. The male transfers sperm using specialized structures called pedipalps. In some cases, males mutilate or sacrifice themselves, a behavior seen in other orb-weavers, possibly ensuring successful fertilization.

Egg Laying:
After mating, the female produces eggs and encases them in a silk sac. These egg sacs are typically attached to bark or vegetation near water, where they remain protected until hatching.

Hatching and Spiderlings:
After several weeks, spiderlings emerge from the egg sac. They often disperse by ballooning—releasing silk threads that carry them on the wind to new habitats.

Maturity:
Females grow much larger than males and may live several months to over a year, while males usually die shortly after mating.

Darwin’s bark spider’s reproductive cycle—highlighted by cautious male courtship, egg sacs near water, and ballooning spiderlings—supports survival in its unique river-crossing habitat.

Lifespan

Darwin’s bark spiders have relatively short lifespans, typical of orb-weaving spiders, with females living longer than males.

Lifespan in the Wild:
Females generally live about 1–2 years, long enough to build multiple webs, reproduce, and lay several egg sacs. Males live only a few months and usually die shortly after mating, either naturally or from sexual cannibalism.

Lifespan in Captivity:
They are rarely kept in captivity, but under controlled conditions, females may live slightly longer due to reduced predation and environmental stress.

Threats to Longevity:
Predation by birds, wasps, and other spiders, as well as environmental factors like flooding or wind damage to webs, shorten survival. Males, in particular, have very short lives since their primary role is reproduction.

The Darwin’s bark spider’s lifespan reflects its reproductive strategy: short-lived males and longer-lived females whose web-building and egg-laying ensure the continuation of the species.

Eating Habits

Darwin’s bark spiders are skilled orb-weavers, relying on their record-breaking webs to capture prey over rivers and lakes.

Diet:
They feed mainly on flying insects such as flies, moths, mayflies, and dragonflies. Thanks to their giant webs, they can also capture larger prey, including damselflies and even small birds or bats on rare occasions.

Feeding Behavior:
When prey becomes trapped in the sticky silk, the spider rushes to immobilize it with venomous bites and wraps it in silk for later feeding. They consume prey by liquefying the internal tissues with digestive enzymes and sucking out the contents.

Foraging Strategy:
Unlike many orb-weavers that build small webs in vegetation, Darwin’s bark spiders suspend massive orb webs across rivers, streams, and lakes. These webs can measure up to 82 ft (25 m) wide and are anchored to trees on opposite banks.

Role in the Ecosystem:
By catching large numbers of insects, especially aquatic insects emerging from rivers, Darwin’s bark spiders help regulate insect populations in freshwater ecosystems.

The Darwin’s bark spider’s use of giant river-spanning webs, paired with the strongest silk in nature, makes its feeding strategy one of the most remarkable in the spider world.

Uniqueness

Darwin’s bark spider is extraordinary among arachnids, combining record-breaking silk strength with unusual behaviors.

Strongest Silk on Earth:
Its silk is over 10 times tougher than Kevlar, making it the strongest known biological material.

Giant River-Spanning Webs:
It builds the largest orb webs of any spider, stretching up to 82 ft (25 m) across rivers and lakes—far larger than those of other orb-weavers.

Sexual Dimorphism:
Females are many times larger than males, with males less than one-third of the female’s size, a striking example of size difference between sexes.

Unique Habitat:
Unlike most orb-weavers, it builds webs over open water rather than in vegetation, exploiting abundant flying insects emerging from rivers and streams.

Recent Discovery:
Darwin’s bark spider was only formally described in 2009, making it a relatively new addition to science despite its spectacular traits.

The Darwin’s bark spider’s unmatched silk, massive webs, and unusual habitat choice make it one of the most unique and fascinating spiders in the world.