Carrai Cave Spider

Introduction

The Carrai Cave Spider is important because it has helped to establish the idea that all araneomorph spiders have evolved from web building ancestors.

Identification

Gradungulidae is one of the few araneomorph families with two pairs of book lungs. Progradungula carraiensis is recognised by the presence of enlarged front tarsal claws and by making small but unique prey catching webs, it is only known from the Carrai Plateau in northern New South Wales. 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, the legs are long and slender.

Habitat

These spiders are found in the limestone caves of the Carrai Plateau, although they probably also occur among rocks and tree trunks on the surface. In caves their webs 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.

Distribution

The Carrai Cave Spider is known only from the moist forests of the Carrai Plateau in northern New South Wales. A second species of Progradungula, P. otwayensis, occurs in Victoria.

Spiders of the family Gradungulidae are found only in eastern Australia and New Zealand. The giant Tasmanian Cave Spider Hickmania troglodytes was recently transferred to the same family. The closest relatives of the group live in South America. These spiders are relics of ancient gondwanan faunas.

Feeding and diet

Beetles, flies, moths and crickets are the main prey of the Carrai Cave Spider. The web and the prey catching behaviour of these spiders is unusual. An upper network of threads attached to the rock walls supports a small (about 25 by 6 mm), slanting, ladder-like platform of cribellate catching silk just above the ground, to which it is guyed down by two parallel, supporting, silk lines.

Cribellate silk comes from a specialised, flattened silk spinning organ called the cribellum which has thousands of tiny silk producing spigots. 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 efficient tanglers of prey. The spider positions itself head down on the platform so that its extended front legs with their enlarged claws are just above the ground. The legs, sensory hairs and slit organs 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 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 evolved as part of this web-based prey catching strategy, have been retained for prey grasping in the many ground dwelling, webless relatives of Progradungula.

Evolutionary relationships

This species is important as the first web builder to be discovered within a family of basal araneomorph hunting spiders ("basal" because they retain "primitive" features not present in more "advanced" families). This helped to establish the idea that all araneomorph spiders have evolved from web building ancestors.

References

• 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.