23 April 2009

Platform Spider - Corasoides species

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Platform Spider - Corasoides sp.

Currently in the Stiphidiidae family and also known as a Trampoline Spider due to the large fine-meshed, flat web that can span over 60 cm (2'), not including off-shoots between gaps in surrounding vegetation. At one end of the web there is a tunnel leading to an underground chamber where the spider retreats. The web is not sticky and relies on insects falling into it from overhanging shrubs, or flying insects after colliding with the many overhead intertwining supporting silken threads. As the prey is not held fast, the spider must rush out and subdue it with a bite before it can escape, this it can do with remarkable speed. To encourage an occupant to show themselves, I very lightly touched the web, with a length of grass, I then inadvertently blinked and when my eyes reopened, the spider had already grasped the grass, having covered 40 cm (1'4") in a fraction of a second.

These spiders are nocturnal, so the web is empty during the day, even when insects fall into it the spider will not be drawn out. So these spiders are rarely seen unless you go with a torch at night and know exactly where the web is. This is because the web is so fine, it is virtually invisible unless you look very carefully, or when the morning/evening dew is clinging to it. It must take the spider several days to construct these large elaborate webs with their incredibly fine weave, so they build them away from animal tracks to keep trampling damage to a minimum. The webs of the above species are usually within 30 cm (1') of the ground and constructed a metre or so (3') off a track and with the tunnel backing into dense taller shrubbery. The height of the platform from lower vegetation is variable, but usually between 5 cm to 15 cm (2"-6").

I presume only one spider occupies a single web, but take-overs do occur. The photo of the 6 legged male spider above occupied a web for some time, but was suddenly replaced (and has not returned), by a fully intact spider, although it was slightly smaller in overall size. Whether this impaired spider was driven off, or had gone seeking females I cannot say, but a change-over did occur.

From observations of the males and females, there is not much difference in their size, which range from 1.5 to 2 cm (1/2"-3/4") in body length. Although of reasonable dimensions they are still difficult to see at night, as their dark coloration blends into the normally twiggy background. Despite their less than obvious nature, these spiders and webs are quite common and easily found (if sought) off most walking tracks.

As a post script, I believe these spiders are being reviewed and may be placed into a different family due in part to their eye arrangement, which more closely relate to spiders of the Amaurobiidae family.

Update 4 April 2010
Two photographs added. These are of a couple of female Platform Spiders that came visiting, having decided to move or been driven out by a predator last September. This was immediately after the cold winter period when these spiders would have been cocooned from the elements in their burrows and were now becoming active, their webs would probably also be a bit tatty and require repairs. Being easier to photograph away from their webs, it was possible to show the enormous chelicerae and fangs of these spiders, and bear in mind these are adults spiders and would have been around 75 mm (3”) in length (including the legs), so a bit scary looking and not one for arachnophobes.