The horns of some species are long, while the horns of others are reduced to the point they cannot be seen. Together, hoods and horns are referred to as the corona. Flowers of milkweeds are interesting because they have an third whorl of five hoods each of which encloses a horn (modified filaments of the anthers). Like other flowers, milkweeds have floral whorls of sepals (collectively referred to as the calyx) and petals (collectively called the corolla). Leaves and stems have features shared across many groups and lack unique combinations of features for species identification. Affinities among closely related species can be shown because floral morphology is conservative. Some northern species of milkweed contain almost no toxins while others seem to contain so much of the toxins that they are even lethal to monarch caterpillars.įlowers are significant in plant identification because flowers are intricate structures with many features that can be assessed, counted, and measured. There is considerable variation in the amount of toxins in different species of plants. Vertebrate predators may avoid monarchs because they learn that the larvae and adults taste bad and/or make them vomit. They sequester these compounds in their exoskeletons, making the larvae and adults toxic to many potential predators. When monarch larvae ingest milkweed, they also ingest the plants' toxins, called cardiac glycosides. Most species are toxic to vertebrate herbivores if ingested due to the cardenolide alkaloids contained in the leaves and stems.
The name "milkweed" refers to the milky latex contained within the leaves. In the Midwest, milkweeds were historically common and widespread on prairies, but habitat destruction has reduced their range and numbers. Milkweeds are perennial plants, which means an individual plant lives for more than one year, growing each spring from rootstock and seeds rather than seeds alone. Most members of the genus Asclepias are tropical however, there are more than 100 species of Asclepias and several genera of viny milkweeds in North America and monarch larvae have been observed feeding on many of these. This family name is derived from Asklepios, the Greek god of medicine and healing. Monarch larvae appear to feed exclusively on milkweeds (Asclepidacae).
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