Zapper

Win the round
Accumulate 6 eggs every round, then beat the round.

Best zapping weapon
Buy a lot of dragonflies, to get the ultimate zapping weapon.

Finding hidden passages
Listen to your friend's directions to find hidden passages.

Flying:
Win the 1st world at least 400 points

Unlocking cheats:
Accumulate all 100 fireflies in every level.

Infinite Lives
Pause the gameplay, hold L and press UP, UP, LEFT, LEFT, RIGHT, LEFT, RIGHT

Unlimited shield:
Pause the gameplay, then hold L and press Up, Down, Up, Left, Right, Down, Up

Cheats:
Accumulate all 400 fireflies in a stages to unlock cheats:

Stage 1: Insect Eye. Differen vision mode.
Stage 2: Screen Switch. Reverses stages.
Stage 3. Auto-Hop. Hold Dpad to keep Zapper moving.
Stage 4: Infinite Super Zap. Infinite Super Zap.