Darwin's Bark Spider

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.

The Darwin's bark spider exhibits a rich repertoire of sexual behaviors, most attributed to other spider taxa with extreme sexual dimorphism in size. These behaviors include sexual cannibalism, male preference for teneral females, binding the mate with silk, genital mutilation, plugging of female genitalia by the male, and self-emasculation. Non-typically, C. darwini males engage in oral sexual encounters, rarely reported outside mammals. Irrespective of female's age or mating status males salivate onto female genitalia pre-, during, and post-copulation. While the adaptive significance of sexual oral contact in spiders is elusive, it is hypothesized to signal male quality or reduce sperm competition.