Laurence Harvey was already being treated for the stomach cancer which would kill him in 1973, the year after this episode aired. To convincingly depict his character's pain and agony, he briefly stopped taking the painkillers prescribed by his doctor.
Laurence Harvey insisted on playing the role of Steven Macy in "The Caterpillar" as he thought that Rod Serling's script was one of the best that he ever read.
The premise of "The Caterpillar" is based on fact. The larvae of the New World screwworm fly (Cochliomyia hominivorax) has, in a few instances, been laid inside the human host ear. The fly is a notorious livestock pest that seeks out pets, zoo animals, and occasionally humans as hosts. A pregnant female screwworm fly seeks an open wound on the skin of a warm-blooded animal to lay her eggs. Within 24 hours, the eggs hatch into tiny larvae that feed on living tissue and bodily fluids, according to the Food and Agriculture Organization of the United Nations.
Also for Universal, Laurence Harvey would make one of his final TV appearances as the killer in The Most Dangerous Match (1973).