Darwin's bark spider

Darwin's bark spider (scientific name Caerostris darwini) is an orb-weaver spider that produces the largest known orb webs, ranging from 900 to 28,000 square centimetres (140 to 4,340 sq in), with bridge lines spanning up to 25 metres (82 ft). The spider was discovered in Madagascar in the Andasibe-Mantadia National Park in 2009. Its silk is the toughest biological material ever studied, over ten times tougher than a similarly-sized piece of Kevlar. The species was named in honour of the naturalist Charles Darwin, with the description being prepared precisely 150 years after the publication of The Origin of Species, on 24 November 2009. Darwin's bark spider (Caerostris darwini) is an orb-weaver spider – a member of the family Araneidae. Like other spiders of the genus, they display extreme sexual dimorphism, with large females and small males. Females vary in total body length from about 18 to 22 mm (0.7 to 0.9 in), males being around one third or less of this length, at about 6 mm (0.2 in). In a captive bred population, females were 14 (1.3–195) times heavier and 2.3 (1–4) times larger than males. 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 palpal bulb has a large conductor with a straight tip, and a longer embolus with a spoon-shaped end than other species from the region. Caerostris darwini was first described by Matjaž Kuntner and Ingi Agnarsson in a 2010 publication. The species name honours the naturalist Charles Darwin, with the description being prepared precisely 150 years after the publication of The Origin of Species, on 24 November 2009. Prior to the description of C. darwini, only 11 species of Caerostris were recognized; Kuntner and Agnarsson regarded this as an serious underestimate, with perhaps up to seven species cohabiting in the region of Madagascar where C. darwini was found. Six more species, four of these from Madagascar, were described in 2015. A molecular phylogenetic study of 12 of the species of Caerostris, including C. darwini, produced the phylogenetic tree shown below. The spider's silk is the toughest biological material ever studied, over ten times tougher than a similarly-sized piece of Kevlar. The average toughness of the fibres is 350 MJ/m3, and some are up to 520 MJ/m3, making the silk twice as tough as any other spider silk known. The web of Darwin's bark spider is remarkable in that it is not only the longest spanning web ever observed, but is the largest orb web ever seen, at an area of up to 2.8 square metres (30 sq ft). Nephila komaci, discovered in 2009, and some other Nephila species also make webs that can exceed 1 m (3 ft 3 in) across. The spider's web occupies a unique biological niche: 'They build their web with the orb suspended directly above a river or the water body of a lake, a habitat that no other spider can use'. This position allows the spiders to catch prey flying over the water, with webs observed containing up to 32 mayflies at a time.