Carrai Cave Spiders
The Carrai Cave Spider is known only from the moist forests of the Carrai Plateau in northern New South Wales.
It is common in the limestone caves of that region, although it probably also occurs among the rocks and tree trunks of the surface. Two related species are known from the forests of southern Victoria and northern Queensland respectively. (NSW)
This species is important as the first web builder to be discovered within a family of some of the earliest (basal) araneomorph hunting spiders ("basal" because they still show "primitive" features not present in more "advanced" families present today). This helped to establish the idea that all spiders have evolved from ancestors that build cribellate webs.
The Carrai Cave Spider is characterised by the enlarged tarsal claws on the front legs and their small, unique prey catching web. Body length is 8 to12 mm and the legs are long and slender. The spiders are a glossy fawn brown in colour, somewhat darker on the head and jaws region. The abdomen has several light grey chevron markings.
Ecology/Way of Life:
These spiders are common in the limestone caves of the Carrai Plateau, although they probably also occur among rocks and tree trunks on the surface. In caves, the webs of these spiders are usually seen around entrance regions or areas where bat guano is deposited. Such sites are high energy zones where food animals like beetles, flies, moths and crickets are plentiful.
The web of these spiders and their prey catching behaviour is unusual. An upper network of threads attached to the rock walls supports a small, slanting, ladder-like platform of cribellate catching silk which is guyed just above the ground by two parallel supporting, silk lines; the platform measures about 25 by 6 mm. Cribellate silk comes from a specialised, flattened silk spinning organ called the cribellum which has thousands of tiny silk producing spigots (see spider structure). Each cribellate silk thread is made up of thousands of very fine silk fibrils that are drawn from the cribellum by the calamistrum, a row of bristles on the last legs. Webs made from this type of catching silk are very good at tangling and holding prey. The spider positions itself head down on the platform, and extends its front legs until the enlarged claws are just above the ground. Sensory hairs and slit organs on its legs monitor web and air vibrations caused by prey animals walking below. When a moth or beetle wanders within range, the spider lunges quickly downwards with its hooked legs and scoops the victim up onto the cribellate silk platform. The sticky clinging cribellate silk wraps itself around the struggling prey, helping to hold it as the spider clasps and bites it. The enlarged claws on the front legs, which have evolved as part of this web-based prey catching method, are also present and used for grasping prey in the many ground dwelling, non-web building relatives of Progradungula.
Interaction with Humans/Threats:
None have been identified as yet. Much of the known distribution of this species is protected within national parks. However, changes to the localised environment in which it lives could affect this species markedly.
This species was named by R.R. Forster and M.R. Gray in 1979; the genus name comes from the Latin: pro = before, gradus = step, and ungula = claw, a reference to this species' species ancient ancestry and its enlarged front leg claws; the species name reflects is presence on the Carrai plateau.
Forster, R.R., Platnick, N.I. and Gray, M.R. (1987). A review of the spider Superfamilies Hypochiloidea and Austrochiloidea (Araneae, Araneomorphae). Bulletin of the American Museum of Natural History 185 (1):1-116
Gray, M. (1978). Silk, Spinnerets and Snares. Australian Natural History 19: 226-230.
Gray, M.R. (1983). The male of Progradungula carraiensis Forster and Gray (Araneae, Gradungulidae) with observations on the web and prey capture. Proceedings of the Linnean Society of New South Wales 107: 51-58.
Topics:Gondwana Spider structure Prey capture Spider webs Karst systems Silk Guano
Text & map by Mike Gray; photograph by Mike Gray, courtesy of the Australian Museum.
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