The butterfly is especially noted for its lengthy annual migration. Monarch butterflies make massive southward migrations during August through October. A northward migration takes place in the Spring. Female Monarch butterflies deposit eggs for the next generation during these migrations. The population east of the Rocky Mountains overwinters in Mexico, and the Western population overwinters in various sites in central coastal California, notably in Pacific Grove, California and Santa Cruz, California. The length of these journeys far exceeds the lifetime of any given butterfly. How the species manages to return to the same overwintering spots over a gap of several generations remains a mystery.
This is one of the few insects to manage transatlantic crossings. A few Monarchs turn up in the far southwest of Great Britain in any year when the wind conditions are right.
The life cycle of a Monarch butterfly includes a complete change of form called complete metamorphosis. This process goes through four radically different stages: First, as mentioned above, the eggs are laid by the females during migration. Second, the eggs hatch, revealing a worm-like larva, (or caterpillar). The caterpillars consume their egg case, then feeds on milkweed, and sequesters substances called cardenolides, related to the cardiac glycoside digitalis: the amount accumulated depends on the level present in the milkweed. This accumulation makes the adult butterfly distasteful and poisonous to Blue Jays and other would-be predators, and many such animals avoid consuming it: this has resulted in the evolution of mimics, which are colored like the monarch to ward off animals but are not themselves poisonous. During the caterpillar stage, the Monarchs store energy in the form of fat, and nutrients to carry them through the non-feeding pupa stage. The third stage of the Monarch life cycle is as a pupa, (or chrysalis). The caterpillar attaches itself to a lump of material secreted on a twig or leaf. It hangs upside down in the shape of a ‘J’ and in a molting, encases itself in an articulated green exoskeleton. At this point, hormonal changes occur, leading to the development of a butterfly. Fourth, the mature butterfly emerges after about two weeks.
The monarch butterfly is the state insect of Texas.