What do the Pikmin do when you get all 201 treasures?

1) You'll just have to wait and see!

2) You should be able to see them; it's probably safer to walk around without the Pikmin to find where the enemies are. Remember, there's no time limit in Caves and it saves between each Sublevel - so take your time, there's no rush.

3) The Snagret's head flies off when you destroy it.

P.S. The Titan Dweevil's on Sublevel 14 of Dream Den - this is how I beat him:
- It doesn't really matter how many Pikmin you have here as long as they're Yellow.
- It's quality not quantity for this boss.
- So use Olimar to take your Yellows (20's good) over to Louie and reveal the big spider.
- Have the Prez with the other Pikmin in a corner somewhere.
- Use the Yellows to take out the Monster Pump (Water) first.
- It'll take a while to knock out the Pump so if Titan uses the water/poison/fire attack swarm the Yellows together.
- If he uses Electricity just keep attacking.
- Now he can't shoot bubbles of water anymore and he can't drown any of the Pikmin or the ones with the Prez.
- Now go after the Comedy Bomb (Poison) in the same manner.
- It'll take a while and if he uses the poison/fire attack swarm the Yellows together or keep attacking if it's electricity.
- Next get the Flare Cannon (Fire) then the ever so easy Shock Therapist (Electricity).
- If you have some of that Red Ultra-Spicy Spray (D-Pad Down) use it because it's really helpful and useful here.

When you get all treasures, you get a bonus scene that shows Olimar leaving and the pikmin start glowing. Yes, the titan dweevil blows up. (DIE FIEND!DIEEEEEEEEEEEEEEE!) the snagret's head probably flys off, because it's body blows up.