I don't understand

How it can be that a small spider, however well nourished she may be, can lay two bags full of eggs, when each bag is bigger than herself, at her fattest?

"Don't touch my babies!"

I found this proud mother on the backside of a plywood board leaning against the wall in my backyard, and she never so much as twitched a hair while I laid the board flat and fussed around with lamps and the camera. She is guarding her second egg sac; the first is that blur in the bottom left of the photo.

When I'd finished taking photos, I carefully put the board back in the same position. I hope the next tenants ignore her until the babies hatch.