For folks hunting tarantula's or scorpions, I spent a couple hours over the weekend getting both. Some tips:
- Carry nothing that can be equipped except your net, but keep it stored (that way regardless of whether you press d-pad left or right you'll get your net).
- Walk! Should be obvious, but walk around with max volume so you can see and hopefully hear before it's too late.
- If it looks like they're heading for a ledge or river, don't worry about getting close - just get your net out and approach in hopes of angering them. They always hop once before they charge so you can time your swing.
- If they don't see you and aren't running for a ledge, then sneak up with your net to nab them before they see you.
Lastly, over the course of three or so hours, I saw five or six tarantulas and two or three scorpions, but rarely (if ever?) in the same areas - so I wouldn't buy into the the whole "spawning area" argument I've seen a few times.
Oh, and I found them by doing large circles around town (occasionally veering off course to follow the river). I also made sure to scare off any bugs I saw, not that I can say for sure if that helped my odds of seeing a tarantula or scorpion.