Darwin’s bark spider (Caerostris darwini) is an orb-weaver spider found in Madagascar that produces one of the largest-known orb webs, spanning upto 82 feet. Its silk is the toughest biological material ever studies. They build their webs with the orb suspended directly above a river or the water body of a lake, allowing the spiders to catch prey flying over the water. The weavers of the largest Darwin’s bark spider webs are almost always female.
Females are largely black in colour, with white hairs on the cephalothorax (prosoma), abdomen and appendages. The upper surface of the cephalothorax has two sets of “humps”, one at the sides and one towards the back. The first leg is about 35 mm (1.4 in) long. Unlike other Caerostris species from the region, C. darwini has clearly separated epigynal chambers with a pair of hooks on the posterior (rather than anterior or medial) part of the epigynal plate. The spermathecae and the ducts leading to them are strongly sclerotized (hardened). Males are redder and lighter brown in colour, again with white hairs on the cephalothorax, abdomen and parts of the appendages further from the body. The femora of the legs are red and hairless. The first leg is about 15 mm (0.6 in) long. The papal has a large conductor with a straight tip, and a longer embolus with a spoon-shaped end than other species from the region.
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