All members of the order Lepidoptera, the butterflies and moths, progress through a four-stage life cycle, or complete metamorphosis. Each stage – egg, larva, pupa, and adult – serves a purpose in the insect's development and life.
Egg - Embryonic Stage:
Once she has mated with a male of the same species, a female butterfly or moth will deposit her fertilized eggs, usually on plants that will serve as food for her offspring. This marks the beginning of the life cycle.
Some, like the monarch butterfly, deposit eggs singly, scattering their progeny among the host plants. Others, such as the eastern tent caterpillar, lay their eggs in groups or clusters, so the offspring remain together for at least the early part of their lives.
The length of time required for the egg to hatch is dependent on the species, as well as environmental factors. Some species lay winter-hardy eggs in the fall, which hatch the following spring or summer.
Larva - Larval Stage:
Once development within the egg is completed, a larva hatches from the egg. In butterflies and moths, we also call the larvae (plural of larva) by another name – caterpillars. In most cases, the first meal the caterpillar eats will be its own eggshell, from which it gains essential nutrients. From then on, the caterpillar feeds on its host plant.
The newly hatched larva is said to be in its first instar. Once it grows too big for its cuticle, its must shed or molt. The caterpillar may take a break from eating as it prepares to molt. Once it does, it has reached its second instar. Often, it will consume its old cuticle, recycling the protein and other nutrients back into its body.
Some caterpillars look just the same, only bigger, each time they reach a new instar. In other species, the change in appearance is dramatic, and the caterpillar may seem to be an entirely different kind. The larva continues this cycle – eat, poop, molt, eat, poop, molt – until the caterpillar reaches its final instar and prepares to pupate.
Caterpillars readying for pupation often wander from their host plants, in search of a safe place for the next stage of their lives. Once a suitable site is found, the caterpillar forms a pupal skin, which is thick and strong, and sheds its final larval cuticle.
Pupa - Pupal Stage:
During the pupal stage, the most dramatic transformation occurs. Traditionally, this stage has been referred to as a resting stage, but the insect is far from at rest, in truth. The pupa does not feed during this time, nor can it move, though a gentle touch from a finger may yield an occasional wiggle from some species. We call butterflies in this stage chrysalides, and refer to moths as cocoons.
Within the pupal case, most of the the caterpillar body breaks down through a process called histolysis. Special groups of transformative cells, which remained hidden and inert during the larval stage, now become the directors of the body's reconstruction. These cell groups, called histoblasts, initiate biochemical processes which transform the deconstructed caterpillar into a viable butterfly or moth. This process is called histogenesis, from the Latin words histo, meaning tissue, and genesis, meaning origin or beginning.
Once the metamorphosis within the pupal case is completed, the butterfly or moth may remain at rest until the appropriate trigger signals the time to emerge. Changes in light or temperature, chemical signals, or even hormonal triggers may initiate the adult's emergence from the chrysalis or cocoon.
Adult - Imaginal Stage:
The adult, also called the imago, emerges from its pupal cuticle with a swollen abdomen and shriveled wings. For the first few hours of its adult life, the butterfly or moth will pump hemolymph into the veins in its wings to expand them. The waste products of metamorphosis, a reddish liquid called meconium, will be discharged from the anus.
Once its wings are fully dried and expanded, the adult butterfly or moth can fly in search of a mate. Mated females lay their fertilized eggs on appropriate host plants, beginning the life cycle anew.