National Geographic : 1963 Apr
Scientists long have been intrigued by the monarch. At least two distinct populations exist in North America-one in the western states, the other throughout the Midwest and East. Separating them lie the great ridge of the Rocky Mountains and the western des erts. The eastern contingent winters in Flor ida, along the Gulf Coast of Texas, and in Mexico. Like migratory birds, wintering monarchs fly north in the spring to lay eggs and begin the life cycle anew. But a bird feeds its off spring many kinds of food; larvae of the mon arch dine only on milkweed of the genus As clepias, which is why the monarch is often called the milkweed butterfly. Using a hand lens, Bob Mitchell and I discovered numerous tiny jade-green, bullet shaped eggs, most of them cemented to the undersides of milkweed leaves (left). All around us flew the adult monarchs. Some few were faded with age, their wings torn. These old butterflies probably had win tered in Florida. With the approach of spring in the north and the promise of fresh milk weed there, they had set out on their long return trip northward. Monarch egg, 36 times life-size, lies glued to a downy milkweed leaf. Dark head of the developing larva shows within the pinhead-size shell. A mon arch female may lay more than 400 eggs, which hatch in 3 to 12 days, ac cording to the temperature. Most are laid on the undersides of leaves; the author turned this leaf over. Newly hatched caterpillar (above), also 36 times life-size, turns its black knobbed head and devours the trans parent eggshell from which it came. Soon it will start nibbling the milk weed leaf, its regular diet. Monarch Caterpillar Begins a Magical Metamorphosis Hanging head down from a twig in a figure J, a two-inch larva prepares to shed its skin for the fifth time and change into a pupa, or chrysalis. A six-pound human baby that grew at the same rate would weigh eight tons in two weeks. As caterpillar or butter fly, the boldly marked monarch needs no camouflage; for unknown reasons, parasites or predators seldom attack it.