Jet Force Gemini

Rainbow Blood mode:
Collect 100 ant heads to unlock the "RainbowBlood" option on the cheat menu.

Jet Force Kids mode:
Collect 200 ant heads to unlock the "Jet Force Kids" option on the cheat menu. Enable this option to unlock the kid version of the game's characters.

Ants Into Pants mode:
Collect 300 ant heads to unlock the "Ants Into Pants" option on the cheat menu.
Enable this option to get stick figures with large pants as enemies. (Information from Cheat Machine)