Calvin In Mutant Fly Form is a minor alter ego of Calvin's. He appeared in a single weekday strip. In it, an unspecified experiment had "gone horribly wrong", replacing Calvin's head with a fly's and giving him insectoid wings. Although still sentient like a human, the mutant had fly-like preoccupations, and followed the odor of decomposing food.
Calvin's appearance and referral to the "experiment" are an obvious nod to The Fly.