The case of the shrinking spider

A month ago I noticed an attractively patterned spider hanging in its orb web in my front garden. It reminded me immediately of a few Golden Orb-weavers which I found in Western Queensland last Easter (photos here). Those spiders were fully adult females and smaller than I thought they ought to be, but this one was a little smaller still – not quite as big as the familiar St Andrew’s Cross spider. Here it is:

colourful spider in web
The skein of prey debris in the web is absolutely typical of Golden Orb-weavers

No adult female Golden Orb-weaver should be this small – they are normally among our biggest spiders, with a reputation for eating birds, for goodness sake! Here, to remind you if you didn’t know them, are adult females of our common species: Nephila plumipes and Nephila pilipes.  Could this be a male? I didn’t think so, because the males are tiny brown and black critters like this.

Baffled, I turned to Rob Whyte of Arachne.org, sending him the photo above and two close-ups, front and back views (below). Rob told me the photos

…are of a male, and look to me like Nephila pilipes. The sub adult male can have semi-developed palps and may be bigger than the final moult version. …

Interestingly in species like Nephila which get eaten by the female there is no need for size. A large male may even be a threat. The [evolution of] sexual dimorphism doesn’t seem to be affected by the size of the male, the females seem to be on a different size path, so male genes apparently don’t contribute to that.

Males need to be pretty big up to sub-adult, to be fit for foraging and moulting, but then they turn into a lean mean sex machine able to nimbly navigate the web and other males and the mood of the female (if they try and she is not ready she just eats them).

So there we have it: my spider is an adolescent (in human terms) boy, and when he grows up he will be only about two thirds of this size. I can’t think of any other kind of animal which ‘grows down’ instead of ‘growing up’ – can you?

spider underside
As many orb-weavers do, it was hanging head down with its back to the vegetation
same spider from behind.
View from behind and slightly below

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