Monarch butterflies (Danaus plexippus plexippus) boast a life cycle that has fascinated millions and been studied in classrooms throughout North America. Because Monarchs are easily recognizable in all the stages of their life cycle and are relatively easy to find on their host plants, Monarch eggs and larvae are commonly brought indoors and observed as they complete their metamorphosis.
Monarchs undergo what is referred to as “complete metamorphosis” because they go through four distinct life stages: egg, larva, pupa, and adult. The egg stage begins when a monarch butterfly female lays her hundreds of eggs by individually adhering each one to the underside of a milkweed leaf (Asclepias spp.). She typically places just one egg per plant, thereby scattering her eggs throughout the landscape. The monarch egg hatches after about 4 days, and a tiny (about 2 mm or less than ⅛ in) caterpillar emerges to begin the larval stage of the monarch.
The larval stage of the monarch actually has five stages, or instars, of its own. The newly hatched caterpillar (called the first instar) promptly begins to do what monarch caterpillars do best: it eats. The caterpillar first eats its own empty egg case, then proceeds to eat milkweed leaves, the exclusive diet of the monarch caterpillar. It will eat ravenously for 10 to 14 days, growing rapidly as it goes. When its body out-grows its skin, the caterpillar will shed its skin, or molt, to reveal a fresh, larger skin that can expand to accommodate the growth of the hungry caterpillar. Each newly-molted caterpillar stage is referred to as a consecutively-numbered “instar”.
When the caterpillar has reached its fifth instar, and has eaten to stretch that skin, its body will be approaching 45 mm (1¾in) long. At this point, the caterpillar will enter into its pupal stage by forming a chrysalis. To form the chrysalis, the caterpillar crawls away from its milkweed plant to find a protected, horizontal surface. Sometimes the caterpillar chooses railings on structures, limbs of trees, or even sturdy twigs. Once it has selected a spot, it spins a web-like mat called a silk pad, and hangs upside down from that pad. The caterpillar’s body hangs in a “J” shape for approximately 24 hours. Then it molts a final time, splitting its skin to reveal a bright green chrysalis underneath.
The chrysalis will hang suspended securely from its perch for another 9-15 days. All the while, the insect is metamorphosing again, changing into a butterfly. When the metamorphosis is nearly complete, the black, orange and white markings of the new butterfly will be visible through the pupa covering.
When the butterfly is ready to emerge, the chrysalis splits, and the butterfly, the adult stage of the monarch, crawls out to hang upside-down on its perch. Its wings will appear tiny and misshapen but they will quickly stretch out and flatten as the butterfly forces fluids from its abdomen through the wing veins. When its wings have dried and hardened, the butterfly will begin to open and close them, preparing for flight, and will soon take off to search for nectar.
All photos: Susan Olcott/WVDNR