Bed bugs are parasitic insects. A species called Cimex lectularius, also known as the common bed bug. Eat mammals and birds. Cimex hemipterus is a tropical bed bug that mostly bites humans. Both species feed on blood while sleeping. Bed bugs are attracted to carbon dioxide and their warmth. Adult bedbugs have no wings and are about a quarter inch long. often leave bite marks on the skin Bed bug saliva contains substances that cause skin reactions.