Northern St Andrew’s Cross spider

orange-brown spider in web
Northern St Andrews Cross spider with water views

Our swimming pool is (ideally) so clean that it doesn’t attract insects or anything that preys on them but this optimistic spider strung up her orb web across one corner of it a couple of weeks ago and is still there. Either she is very patient, and very hungry, or she knows more about pool ecology than we do.

She looks like the St Andrew’s Cross spider familiar to gardeners everywhere in Australia but she is in fact Argiope picta, properly known as the Northern St Andrew’s Cross spider.  Rob Whyte at Arachne.org says:

It is very similar to the St Andrews Cross spider Argiope keyserlingi found in southern and eastern areas, the two ranges overlapping. Zig-zag ribbons of bluish-white silk form a full or partial cross (stabilimentum) through the centre of the orb web. This cross gives the spider its common name.

The differences between the two species are so slight they are difficult to describe as well as to notice. Broad white bands across the abdomen broken either side of the centre are my primary way of identifying a female as A picta rather than A keyserlingi. Differences in overall coloration between the species are much slighter than the differences each species shows as it matures, going from greyish, to orange with lighter stripes, to a very dark brown with yellow and white stripes.

And that’s only the females; “the males … of both species are much smaller and essentially indistinguishable from each other to the naked eye,” to quote Whyte again. Fortunately, picking the difference doesn’t matter much to any creature in the world except the spiders themselves – and I’m sure they know!

I will just mention two related posts before hitting “Publish”:

  • Fishing spider talks about another spider in my pool – a water-loving species, so the location is less peculiar.
  • Two spiny spiders compares another pair of near-twin species, the spiny (southern) Christmas Spider and its northern cousin.

There’s always something new to discover.

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