15 February 2009

Araneidae - Argiope protensa

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Araneidae - Argiope protensa

A spider widely distributed across southern Australia and closely related to the St. Andrew's Cross Spider, Argiope keyserlingi. This species has an elongated body that is officially recorded as being to 1.5 cm (little over 1/2") in length (excluding legs), but in this region the spiders I encountered were commonly 2 cm (3/4") in length, with a particularly large specimen closer to 3 cm (1"). They all occur on the same granite outcrop, but in different habitats with probable different prey, which may help explain the difference in nutrition and growth rates.

Another variation is the stabilimentum (dense band of silk built by the spider into the web), all webs other than one, had none at all and interestingly the one that did, was made by the smallest spider (1.5 cm in body length), but resided at the base of a Hakea clavata shrub Proteaceae - Hakea clavata on bare rock. This barren habitat would likely have fewer visiting prey species, which in turn would possibly affect the spider's growth development. An interesting study of the stabilimentum (Behavioral Ecology), suggest it is intended to warn day-time flying birds of the web's presence and so avoid it. This would seem reasonable in the circumstance, as birds both insectivorous and honeyeaters do periodically visit these isolated plants, so would need to be made aware of the web and so avoid it.

The photos above labelled male, female and the egg-sac, belong to a spider family residing on an isolated clump of sedge. The male who was about a third the size of the female, had his own web, although much smaller than the one occupied by the female (both were within 50 cm or around 1'6"of the ground). The male caught a flying insect while I was watching; he bit it first, then removed it from the web and carried it back to the web center, where he then wrapped it (most spiders usually wrap the spider where caught and bite them later). The egg-sac was a few centimetres off the ground and had the unusual feature of a lid to permit the spiderlings escape.

A very interesting spider that was new to me and I thank Ron Atkinson from the University of Southern Queensland for his identification assistance.