An arachnid that casts a web across entire rivers sounds like fiction, but it is a reality for the Darwin’s bark spider (Caerostris darwini). Discovered in 2009, this spider showcases remarkable evolutionary adaptations. Its unique combination of web architecture and silk composition sets it apart from other species, making it a subject of scientific fascination.
Habitat and Physical Description
The Darwin’s bark spider is native to the forests of Madagascar, where it exclusively inhabits areas directly adjacent to rivers and streams. This preference for riverine habitats is a core part of its survival strategy, allowing it to exploit a niche that other spiders cannot. The spiders are most commonly found in Andasibe-Mantadia National Park.
This species exhibits extreme sexual dimorphism. Females are significantly larger, with a body length between 18 and 22 millimeters, while males measure only around 6 millimeters. The female’s coloration is predominantly black with white hairs, providing camouflage against tree bark. In contrast, the male is a reddish or light brown color, also with white hairs.
The Record-Breaking Web
The web of the Darwin’s bark spider is the largest of any single spider. The central orb can reach up to 2.8 square meters (about 30 square feet), but the anchor lines are the most impressive feature. These bridge lines can span up to 25 meters (82 feet), allowing the spider to suspend its trap directly over a river or small lake.
Positioning the orb directly above the water’s surface allows the spider to capture insects like mayflies and dragonflies that use the river for travel. To build it, the spider releases a long silk thread into the wind from one side of the river. Once the wind carries it to the other side and it snags, this thread becomes the initial bridge line from which the entire structure is built.
The web’s capture area can range from 900 to 28,000 square centimeters, and some have been observed holding dozens of insects at once. The structure is strategically simplified, focusing on size and placement rather than intricate patterns. This design allows for rapid construction and repair in its unique environment.
Unparalleled Silk Strength
The silk produced by the Darwin’s bark spider is the toughest biological material ever studied. Toughness, a measure of the energy required to break a material, averages 350 megajoules per cubic meter (MJ/m3), with some fibers reaching 520 MJ/m3. This makes it more than twice as tough as any other known spider silk and over ten times tougher than a similarly sized strand of Kevlar.
This toughness results from a combination of strength and elasticity. While the silk has a tensile strength comparable to steel, it is also exceptionally stretchy. This elasticity allows the silk to absorb massive kinetic energy before it ruptures, a necessary property for withstanding the impact of large insects. The dragline silk that forms the web’s frame is particularly stretchy.
Material scientists are researching the silk’s unique molecular structure to replicate its characteristics for human applications. Potential uses include advanced textiles, durable medical sutures, and lightweight composite materials. The spider’s silk represents a blueprint for a new class of materials.
Diet and Hunting Strategy
The spider’s diet primarily consists of insects that fly over the water, such as mayflies and dragonflies. The web is highly effective at this task. For example, a single web has been found holding more than 30 mayflies at once.
Unlike many orb-weavers, the female Darwin’s bark spider does not sit in the center of her web. Instead, she waits on a branch near one of the anchor points, out of sight. This behavior likely helps her avoid predators while she monitors the web for vibrations from captured prey.
The large web also attracts other species. A type of fly engages in kleptoparasitism, which means it steals food from the spider’s web. These flies land on captured prey and begin to feed before the spider can secure its meal.