If you are a little pale jumping spider, the smooth pale bark of a Poplar Gum is a pretty good hunting ground: you’re fairly well camouflaged and there is a steady parade of prey passing by (Gotcha!). But you have to remember that you might be prey yourself (like your relative here), and be sure of a place to hide.
Most of the year that’s not a problem, because there are enough bits of loose bark to hide your whole family a dozen times over. But once a year, the tree sheds all its bark to expose a fresh new layer that is as smooth and bare as Red Square (not that you know about Red Square). Where do you hide?
You make a retreat. You find a dimple in the bark and roof it over with a taut span of silk, leaving at least two inconspicuous doorways (a retreat with only one entrance can become a trap, as you know). Then you’re okay. The roof is almost the colour of the bark, and is an even better match after it collects a bit of dust, and it lasts for months.
For the record: The spider is the Flat White Jumping Spider, Arasia mollicoma, Salticidae – thanks to Rob Whyte (do not follow the link unless you like spiders!) for the ID. I see these spiders regularly on the poplar gum trunk. This retreat is by no means unusual: half a dozen are made on the bottom couple of metres of the trunk after each time the bark is shed.