When and where is the best time to catch these please

The tarantula is out between June and August and the scorpion is out between July and September. Both are available between 7:00 P.M. and 4:00 A.M. Just wander around town during those times to increase your chances of coming across one. Also, don't take your net out until you see one, or they could attack you before you have time to react.

Indeed, in my experience, tarantulas and scorpions appear to just appear randomly. It's annoying when they're too close to a body of water - they tend to escape in there. I second 707's statement that you should definitely put your net away until you see them. You can whip it out real fast with the control pad when you need to.

Thanks guys, Ive managed to catch a tarantula but had no luck with a scorpion.  Do you know if they still appear when the firework display is on?

Yes, they should.

I've also learned that if you stand still, even with your net out, you will not get bitten.

Really? I've not tested this myself in CF, but I can say for certain that it does NOT work in Wild World

I'm fairly certain that the standing still method works if the bug hasnt sighted you. I finally caught a scorpion using the method.

After years of trying to catch both of the dreaded beasts (I'm more terrified of approaching them in City Folk), I finally caught them last month. Not in the same night, of course. For the scorpion, I used a method called "The Bus Stop Method", meaning that you get the scorpion to chase you and once you get to the bus stop, run up it. It didn't follow me on, so I swung my net and caught it. The tarantula was just wandering around, so I slowly walked up to it. I honestly thought I was going to miss when I swung and get bit, but I didn't.

Here's a video if anyone needs help: Tarantula and Scorpion Catching