pupa [key], name for the third stage in the life of an insect that undergoes complete metamorphosis, i.e., develops from the egg through the larva and the pupa stages to the adult. A complete metamorphosis is characteristic of members of the orders Coleoptera (beetles), Diptera (flies, mosquitoes, and gnats) and Lepidoptera (moths and butterflies). Before entering the pupa stage the insect is an active larva, usually wormlike in form. The pupa is a resting stage in which the insect is transformed into an adult. It does not feed or increase in size, and typically it is outwardly inactive and covered by a hard integument. Internally, however, a great deal of metabolic activity occurs. Some larval organs are destroyed and some adult organs are initiated during this stage. Other adult organs develop from structures already present in the larva. At the end of the pupa stage, the integument is shed and the imago, or adult form, emerges. Pupae of moths usually have an additional outer covering, called a cocoon, built by the larva (called a caterpillar) just before it enters the pupa stage. Cocoons may be made of bits of woody material held together by silk strands, or woven entirely of silk. Some cocoons are formed on or under the ground, some under tree bark; others are suspended from branches or twigs. Some moths form cocoons by wrapping leaves around themselves and gluing them together with silk. Cocoon building occurs in other insects, e.g., wasps; the material and design of the cocoon vary greatly from one group to another. Very few butterflies make cocoons, but the butterfly pupa, called a chrysalis, is usually suspended by a silk thread, and its integument is often sculptured and brightly colored. The chrysalis of the monarch butterfly is soft green with gold spots. A few insects, e.g., the mosquito, have active pupae. The duration of the pupa stage varies in different insects from a few days to several months. Many insects pass the winter in the pupa stage, and the imago emerges in the spring.
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