You know... I think there's still some confusion on the difficulty settings for this one.
In Frog Bog (and some others), there's no such thing as "disc" mode. Rather, in this game, you can choose between a Day time game and a Night time game. In the night time game, you can continue slurping bugs until the moon goes down. In the daytime game, you will progress through the times of day until ending on the night time scene. In this mode, once all the bugs are consumed or have flown away, the game is over.
So, those are the two main game "modes." When selecting the day time game mode from the title screen, controls will automatically default to "easy" or, Fixed Arc. When selecting the night time game mode from the title screen, your controls will default to "medium" which is where you must control the arc while the tongue flick remains automatic.
Complicating matters... you can change the skill level (fixed arc, medium or full manual) at any time in any game mode (night or day)
It's confusing as hell, but I'm 99% certain this is accurate.