Ok, here's what I've found based on all of my scorp/rantula hunting:
They "aggro" (or become aware of you) if you walk or run anywhere near them, meaning they'll sit there looking at you. Get any closer than that and they'll leap at you.
If you see one in advance, remain still but ALWAYS keep pointed toward it via small taps on the D-pad (small taps are absolutely key).
They seem to head south most of the time. Sneak behind it using small taps. If it's facing your direction, don't move. If you never move, they seem not to aggro on you.
If it has already aggroed on you, walk away and creep back in.
They seem to move, pause, move, pause. Via small D-pad taps, you should be able to get close enough to net them so long as they haven't seen you.
I'm not sure what happens if you let one touch you when you haven't actually aggroed it but I assume it will bite you.
That's all I've learned so far.