You give up on the sticky feet. Why bother when you can run up to something and stab it, and it’s dead and a meal in an instant?!

As an effective hunter, you can grow quite large. Your simple leg gills become adapted to air instead of water and you need more of them, so they eventually become internal structures of very many fine folds, looking like pages. These are called “book lungs”.

The biggest of you now is about 20cm, but larger forms existed. You are a Scorpion.