Monarch butterflies undergo complete metamorphosis, which includes four life stages: egg, larva, pupa, and adult. The transition from the larval stage to the pupal stage is often mistakenly called “cocooning.” The protective casing is correctly called a chrysalis, not a cocoon, as only moths create a silken cocoon. The timing of this shift is governed by the caterpillar’s growth and environmental cues that signal the change into a butterfly.
The Larval Journey: Tracking Caterpillar Development
The larval stage is dedicated entirely to consumption and massive growth. Monarch caterpillars progress through five distinct growth phases, called instars, separated by molting events. After hatching, the larva eats its eggshell before continuously feeding on the host plant, milkweed. The caterpillar sheds its skin at the end of each instar to accommodate its rapidly increasing size. Over this period, the caterpillar can grow up to 2,000 times its original mass, reaching up to 45 millimeters in its final, fifth instar. The entire larval stage typically lasts between nine and 16 days under optimal summer conditions, though duration is influenced by temperature.
Signs of Transformation: When the Change Begins
The signal that the caterpillar is ready to pupate is marked by changes in behavior and appearance. Once the caterpillar completes its fifth instar and reaches maximum size, it stops feeding on milkweed. This is followed by restlessness, as the caterpillar wanders away from the host plant to find a secure, sheltered location for transformation. The caterpillar often expels its last waste, clearing its gut in preparation for metamorphosis.
After finding a suitable horizontal surface (a leaf, twig, or ceiling), the caterpillar spins a small, silken pad. It attaches its hind end to this pad and hangs upside down in the characteristic “J-shape.” This J-shape is the final indication that pupation is imminent, typically lasting 10 to 16 hours. Just before the final skin shed, the caterpillar’s tentacles may appear shriveled, a sign that fluids are being withdrawn as the pupa forms underneath the skin.
Chrysalis Formation: Shedding the Larval Skin
Chrysalis formation begins after the J-shape period. The caterpillar’s skin splits behind the head and is pushed up toward the silk pad through muscular contractions, revealing the soft, newly formed chrysalis underneath. The caterpillar must then perform a maneuver to secure the new pupal form.
It wiggles the shed skin until it is bunched at the top, then grasps the silk pad with a hook-like appendage called the cremaster. This attachment allows the chrysalis to hang securely. The final step involves shedding the last remnants of the larval skin, which drops away to reveal the fully formed, jade-green chrysalis. This hard, smooth outer shell is the actual pupa, protecting the insect during its transformation into a butterfly.
Duration of the Pupa Stage and Environmental Influences
Once the chrysalis has formed, the insect begins the internal reorganization necessary to become an adult butterfly. The pupal stage typically lasts about eight to 15 days in summer conditions. The most important factor influencing the timing of emergence is ambient temperature.
Warmer temperatures accelerate metabolic processes within the chrysalis, resulting in a shorter pupal period. Conversely, cooler temperatures slow development, causing the monarch to remain in the chrysalis for a longer time. Seasonal factors also influence the duration for the final generation of monarchs each year. These late-summer butterflies emerge with undeveloped reproductive organs and enter diapause, a period of suspended development, which allows them to survive the long migration and overwintering period.