Opportunities to use the word aposematism do not present themselves often. Today, however, it is warranted because Monarchs provide a textbook example of the phenomenon of warning coloration. The bold stripes are a clear signal to predators to stay away. And, for good reason. The primary food of this instar is Milkweed, a plant which contains cardiac glycosides, compounds which are highly toxic. The pharmacodynamics of glycosides, especially the cardenolide steroids, are widely known. Digitalis or digoxin, perhaps the best known of these compounds, is derived from Foxglove. In humans, the compound increases the force of heart muscle contraction and reduces heart rate. It has been used, clinically, as an antiarrhythmic. Such agents are therapeutic at low doses and can be lethal at higher ones.