How Do a Pupa & Cocoon Differ?

Butterfies, moths and their insect relatives

Many insects follow the same developmental stages: from egg to larva to pupa to adult. That these stages are not universally known, teachers of children and teens would suggest, is the phonic similarity between "pupa" and, alas, "poop."

"Pupa" is a general term describing both a developmental stage in the life of an insect and the shelter the insect builds around its body to enable that stage to occur. Many insects make the transition from larval to adult stage through a pupal stage, among them mosquitoes and beetles. In many types of beetles, the shape of the pupa is nearly identical to the shape of the adult insect. What a casual observer may believe is a dead beetle may be the case incubating what will become a live one.

The cycle

Development of an insect begins with egg-laying, often on the leaves of a carefully-chosen hospitable plant. Eggs become larvae, hence the importance of hospitable leaves: the goal of larvae is to eat and eat. (Insect-eradication programs for mosquitoes target the egg- and larval-stages so as to kill the pests before they become widespread.)

The layman's most frequent acquaintance with insect-stages involves either finding a pupa or watching one being created. Moth and butterfly caterpillars possess the ability to spin silk-like filaments from their bodies to create cases in which they will evolve from crawling worm to flying insect. The larval stage of one type of moth, the silkworm, spins actual silk for this casing.

For laymen, this is the stage at which the "pupa/poop" problem arises. Arbitrarily, then, we call the pupa-stage casing of moths "cocoons" and the pupa-stage casing of butterflies "chrysalises." (This verbal distinction has contributed greatly to the even and continuing pace of more children's science classes than anyone other than teachers can imagine!)

What happens next

Scientists can describe the development that occurs during the pupal stage, but share the amazement of laymen at what occurs. Inside the casing, the body of the caterpillar undergoes all the differentiation that is manifested in the body of the adult moth, butterfly, mosquito or beetle. When it is complete, the adult insect breaks though the case to be "born." In the case of some butterflies, like Monarchs and Painted Ladies, this gestation takes a few weeks. In the case of some moths, cocoons must winter-over, with emergence of the adult occurring the following spring. Whenever it happens, the change from larva to pupa to adult remains a source of amazement for all lucky enough to see it happen.