Change into Fireant, Shield Bug and go Invincible
Start a game and press Start to pause the game and enter C, A, L, L, D, A, D repeatedly until you hear the sound of the orb weapon. Do not unpause the game yet!!! Enter C, A, L, L, D, A, D until the sound stops. Press Start and you will be a fireant. Pause again and enter the code once more, and you will be the shield bug.

This tip can be used to achieve invincibility in the bonus rounds as well. While in the bonus round, enter the code until you become the shield bug. Unpause the game and when the shield wears off you should be invincible.

Change Weapons
While playing, pause the game and press C, A, Left, Left, Down, A, Down.

Extra Life
For an extra life, pause the game and press Right, Up, B, A, Down, Up, B, Down, Up, B.

Health Refill
To refill your health, pause the game and press B, A, B, A, Left, Up.

Level Select
While playing, pause the game and press Up, Right, A, B, A, Down, Left, A, Down