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Pupa

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.^[1]

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1 Position in life cycle
- 1.1 Duration
- 1.2 Emergence
2 Structure
3 Defense
4 Chrysalis
5 Cocoon
6 References
7 See also

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 potential host.

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. 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 they have few defenses. A common feature is concealed placement. Some species of Lycaenid butterflies are protected in their pupal stage by ants. Some species of pupae are capable of making sounds or vibrations to scare potential predators. A few species use chemical defenses including toxic secretions.

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.^[2] This sudden and rapid change from pupa to imago is called metamorphosis.

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. Butterfly larvae do not spin a cocoon; their pupa is called a chrysalis.

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.^[3]

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

^ Borror, D. J. Dwight M. DeLong and Charles A. Triplehorn. An introduction to the study of insects. New York: Holt, Rinehart & Winston. Sixth Edition.
^ AMNH Accessed December 2006
^ Malcolm J. Scoble. 1992. The Lepidoptera: form, function and diversity. Oxford: Oxford University Press.