It is a generally accepted concept that certain secondary plant metabolites, the cyanoge-nic glycosides, the cardiac glycosides and the pyrrolizidine alkaloids, are the main chemical compounds providing certain Lepidoptera - the Heliconiinae, Acreinae, Ithomiinae, Danaiis plexippus (Linnaeus, 1758) and the day-flying Zygaenidae with a potent chemical defense against their avian predators. In this paper the validity of the theory of Aposematism in butterflies is challenged: the concept of chemical defense in butterflies, is considered a misconception; it is argued that insectivorous birds are not capable of tasting the plant metabolites believed to provide butterflies with a potent chemical defense, and thus the concept that beak marks on the wing of a butterfly are a proof of taste rejection of the prey by the avian predator is not valid. It is another misconception. The foraging behavior of an insectivorous bird is discussed in light of the Optimal Foraging Theory (Stephen & Krebs, 1986). For a bird to survive, the energy gained during foraging has to be higher than the energy lost. Prey selection depends first and foremost on energetic profitability. Thus a palatable but energetically unprofitable butterfly will be avoided, regardless of its apparently apo-sematic color pattern. There is a positive correlation between energetic profitability and the morphological and flight characteristics of butterflies. Heliconiinae and Itomiinae are avoided under natural conditions not because they have the protection of a chemical defense and conspicuous considered aposematic color patterns but because they are not energetically profitable to pursue and eat. It is argued that our knowledge of the purported chemical defense of the monarch butterfly [D. plexippus (L.)] provided by the cardiac glycosides (CDGs), sequestered from the host plant during the larval stage of development of the butterfly, is based solely on specially designed and controlled laboratory experiments creating conditions that do not exist in the natural environment. A revision of the currently accepted theories and concepts is suggested.