Definitions

pupa

pupa

[pyoo-puh]
pupa, 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.

Inactive, nonfeeding stage in the life of insects that undergo complete metamorphosis. In a protective covering (cocoon or chrysalis), the larva is transformed into an adult. During pupation, a process controlled by hormones, larval structures break down and adult structures form; wings appear for the first time. The adult emerges either by splitting the pupal skin and chewing its way out or by secreting a fluid that softens the cocoon.

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A pupa (Latin pupa for doll, pl: pupae or pupas) is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago. (For a list of such insects see Holometabolism).

The pupae of different groups of insects have different names such as chrysalis in the Lepidoptera and tumbler in mosquitoes. Pupae may further be enclosed in other structures such as cocoons, nests or shells.

Position in life cycle

In the life cycle of an insect the pupal stage follows the larval stage and precedes adulthood (imago). It is during the time of pupation that the adult structures of the insect are formed whilst the larval structures are broken down. Pupae are inactive, and usually sessile (not able to move about). They have a hard protective coating and often use camouflage to evade potential predators.

Duration

Pupation may be brief, for example 2 weeks as in monarch butterflies, or the pupa may enter dormancy or diapause until the appropriate season for the adult insect (in temperate climates pupae usually stay dormant during winter, in the tropics pupae usually do so during the dry season).

Emergence

Insects emerge (eclose) from pupas by splitting the pupal case, and the whole process of pupation is controlled by the insect's hormones. Most butterflies emerge in the morning. In mosquitoes the emergence is in the evening or night. In fleas the process is triggered by vibrations that indicate the possible presence of a suitable host. Prior to emergence, the adult inside the pupal exoskeleton is termed "pharate". Once the pharate adult has eclosed from the pupa, the empty pupal exoskeleton is called an "exuvium" (or exuvia); in most hymenopterans (ants, bees and wasps) the exuvium is so thin and membranous that it becomes "crumpled" as it is shed.

Structure

In some insect orders the appendages such as legs and proboscis are free and visible in the pupal stage. Such pupae are termed as exarate and examples are seen in the Hymenoptera. In many others the pupa is a tight and compact shell with all the appendages tightly packed within and these are termed as obtect. The familiar lepidopteran chrysalis is obtect. Another form has the appendages visible, but covered within a shell. In some cases the covering is formed by the integument of the last larval instar also known as the puparium. Such pupae are termed as coarctate and are found in many of the diptera. Some exarate pupae such as those of the neuroptera also have movable mandibles attached to the head. Such pupae are termed decticous. In most other insects the mandibles are immovable and such pupae are termed adecticous.

Defense

Pupae are usually immovable and are largely defenseless. To overcome this, a common feature is concealed placement. There are some species of Lycaenid butterflies who are protected in their pupal stage by ants. Another means of defense by other species of pupae is the capability of making sounds or vibrations to scare potential predators. A few species use chemical defenses including toxic secretions. The pupae of social hymenopterans are protected by adult members of the hive.

Chrysalis

A chrysalis (Latin chrysallis, from Greek χρυσαλλίς = chrysallís, pl: chrysalides) or nympha is the pupal stage of butterflies. The term is derived from the metallic gold-colouration found in the pupae of many butterflies referred to by the Greek term χρυσός (chrysós) for gold.

Because chrysalids are often showy and are formed in the open, they are the most familiar examples of pupae. Most chrysalids are attached to a surface by a Velcro-like arrangement of a silken pad spun by the caterpillar and a set of hooks (cremaster) at the tip of the pupal abdomen.

Like other types of pupae, the chrysalis stage in most butterflies is one in which there is little movement. However, some butterfly pupae are capable of moving the abdominal segments to produce sounds or to scare away potential predators. Within the chrysalis, growth and differentiation occur. The adult butterfly emerges (ecloses) from this and expands its wings by pumping haemolymph into the wing veins. This sudden and rapid change from pupa to imago is called metamorphosis.

When the butterfly emerges from the chrysalis, usually it will sit on the empty shell in order to expand and dry its wings. However, if the chrysalis was near the ground (such as if it fell off from its silk pad), the butterfly would find another vertical surface to rest upon and dry its wings (such as a wall or fence).

Moth pupae are usually dark in color and either formed in underground cells, loose in the soil, or their pupa is contained in a protective silk case called a cocoon.

Aurelia is an old synonym of chrysalis from which is derived the term aurelian; one who studies the emergence of butterflies from chrysalids.

Cocoon

A cocoon is a casing spun of silk by many moth caterpillars and numerous other holometabolous insect larvae as a protective covering for the pupa.

Cocoons may be tough or soft, opaque or translucent, solid or meshlike, of various colors, or composed of multiple layers, depending on the type of insect larva producing it. Many moth caterpillars shed the larval hairs (setae) and incorporate them into the cocoon; if these are urticating hairs then the cocoon is also irritating to the touch. Some larvae attach small twigs, fecal pellets or pieces of vegetation to the outside of their cocoon in an attempt to disguise it from predators. Others spin their cocoon in a concealed location - on the underside of a leaf, in a crevice, down near the base of a tree trunk, suspended from a twig or concealed in the leaf litter.

Insects that pupate in a cocoon must escape from it, and they do this either by the pupa cutting its way out, or by secreting fluids that soften the cocoon. Some cocoons are constructed with built-in lines of weakness along which they will tear easily from inside, or with exit holes that only allow a one-way passage out; such features facilitate the escape of the adult insect after it emerges from the pupal skin.

References

See also

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