Young nymphs of
Lygaeus kalmii are initially fully red, developing black diagonal markings on the pronotum. Adults are orange to red and gray/black, reaching in length. The head is black with a red spot in the center. The forewings are patterned with a black heart inside of an orange-red X which does not meet in the middle. The membranous portion of the forewings in the eastern subspecies
Lygaeus kalmii angustomarginatus has a narrow white margin, while that of the western
L. k. kalmii has white spots and a broader margin. The small milkweed bug is commonly confused with other black and red or orange insects, including
Oncopeltus fasciatus,
Lygaeus turcicus, and
Lygaeus reclivatus. ==Distribution and reproduction==