gotcha. that makes sense. well now, I am going to have to play around with this idea yet.
You don't HAVE to wait till spring to start collecting inverts, if you really want some. This November I was getting water boatmen(I bet they'd be interesting!) and dragonfly naiads in NJ. Not sure about Ranatra
If you do wait till spring, I suggest going out late at night(past midnight would be good) by a pond, setting up a white sheet with a black light shining on it. Then sit back, crack open a beer, and watch all sorts of insects hit the sheet! I've heard of giant water bugs being collected this way and would not be surprised to see water scorps come too. In fact, I've seen mine follow the glow of my cell phone!
I should also say I'm not really digging housing these guys with fish. I was offering crickets to my water scorp last night and my green sunfish took one off the tweezers the first offering, and yanked it out of the insect's grasp the second time! Third time was a charm, thankfully. Oh yea that's another thing...I forget if I said this before, but just because they're aquatic predators doesn't mean that fish should make up a large part of their diet. If anything fish should be offered very sparingly.