I have a vague memory of having read, at some point, about a kind of spider that can do its visual processing only one tiny pixel at a time. It will go to the top left corner of a branch and look out, and see a little spot that is dark. Then it will move down slightly, and see a little spot that is a bit less dark. Eventually it will have scanned an entire object, and then it can finally begin to respond. (I don’t have the description of the spider exactly right. It’s a very old memory, and I don’t remember where it came from. I hope someone who knows will read this and tell me.)
The kind of spider in question is likely Portia. Peter Watts’ description sounds similar.
Yes, that description is definitely Portia. I have more links for the interested.