Calvin the Bug refers to two of Calvin's alter egos:
Calvin the Bug as a fly
Calvin the Bug appeared twice as a housefly bent on transmitting contamination and disease.
First appearanceCalvin the Bug's insect form first appeared in a Sunday strip where it searched around a picnic table for a human to infect with plague. Calvin the Bug was here analogous, by virtue of using its proboscis, to the slurping noise Calvin made while drinking a soft drink at a picnic.
Second appearanceCalvin the Bug made one more appearance in its insect form in another Sunday strip. In this strip, it was not at an unspecified picnic but instead roamed freely about Calvin's house, contaminating cookies and exacerbating Calvin's parents. Calvin the Bug was here analogous to Calvin's irritating behavior around the house, and so were the parents' reactions.
In between its appearances, Calvin the Bug's insect form had changed appearance. Although its anatomy remained similar, its color scheme had changed. The eyes had gone from yellowish-orange to red, the legs and thorax had become black instead of red and blue (respectively), the abdomen, from its original green, turned to a greyish blue and the mandibles went from grey to yellow.