The transformation of moths and butterflies is known as complete metamorphosis. This cycle involves four distinct stages: the egg, the larva, the pupa, and the adult insect. The larval stage, commonly recognized as the caterpillar, is solely dedicated to consumption and rapid growth, often increasing the organism’s mass by thousands of times. Once this tremendous growth phase is complete, the caterpillar transitions into the non-feeding pupal stage, where the dramatic internal reorganization occurs before the final emergence as a winged adult.
Cocoon Versus Chrysalis
The terminology surrounding the pupal stage is often confused, as the word “cocoon” is frequently used incorrectly to describe the structure protecting a developing butterfly. The pupa is the non-feeding, immobile stage itself, and the structure protecting it differs significantly between butterflies and moths. A chrysalis is the specific name for a butterfly’s pupa, which is typically a hard, smooth, and exposed casing. It is formed by the caterpillar shedding its final larval skin, revealing the hardened pupal exoskeleton beneath.
In contrast, a cocoon is a protective, silken wrapper spun by the caterpillar of most moth species around its pupa. Moth caterpillars use silk glands to weave this protective outer layer, sometimes incorporating materials like leaves, hair, or debris for camouflage and added strength. Therefore, while all butterflies form a chrysalis, only moths create a cocoon to house their pupa.
Internal and External Signals for Pupation
The decision for a caterpillar to stop feeding and enter the pupal stage is governed by internal physiology and external environmental cues. Internally, the primary signal is a shift in the insect’s hormonal balance, specifically the reduction of juvenile hormone (JH). High levels of JH during the earlier instars prevent metamorphosis, ensuring the caterpillar simply molts to a larger larval stage. Once the larva has reached its maximum size, JH levels drop significantly, allowing the hormone ecdysone to trigger the final molt into the pupa.
External factors act as “token stimuli,” which signal the appropriate timing for this internal change. Decreasing photoperiod, or shorter daylight hours, and cooling temperatures are the most common signals indicating the approach of winter. If the caterpillar receives these cues, it is induced to enter a state of developmental arrest called diapause once it becomes a pupa. Diapause is a prolonged, dormant state that allows the pupa to survive unfavorable conditions, such as the cold winter. Conversely, long days and warm temperatures signal a favorable season, prompting the caterpillar to proceed with a quick, active pupal development lasting only a few weeks.
The Physical Transition into the Pupa Stage
Once the internal hormonal changes are complete, the caterpillar seeks a suitable location to prepare for its transformation. This location is often concealed (under a leaf, in a crevice, or buried underground) to protect the vulnerable pupa. Butterfly caterpillars usually spin a small silk pad and attach their posterior end to it using a structure called a cremaster, often hanging downward in a characteristic “J” shape for several hours.
The actual transition begins with the final shedding of the larval skin. The old larval skin splits along the back of the thorax, and the caterpillar’s body writhes and pushes the skin upward toward the silk pad. As the skin is pushed off, the underlying pupal cuticle is revealed and quickly hardens, forming the chrysalis. Moth caterpillars that spin a cocoon will first construct the silken enclosure and then pupate inside, shedding their larval skin within the protective walls of the structure.
How Long the Pupal Stage Lasts
The duration of the pupal stage is highly variable, influenced by environmental conditions and the species’ survival strategy. For most species during active, warm seasons, the pupal stage is relatively short, typically lasting between one to three weeks. For example, a monarch butterfly pupa may complete its metamorphosis and emerge as an adult in as little as eight to fifteen days.
However, if the caterpillar pupates in response to the environmental signals that induce diapause, the duration is extended significantly. The pupa will enter a deep dormancy, suspending its development to survive the cold winter months. This overwintering stage can last for six to nine months, and in some extraordinary cases, pupal diapause can extend for multiple years. It often requires a period of cold temperature exposure before rising temperatures in the spring signal the end of diapause and the resumption of adult development.