Pupa

Pupa of the Cockchafer (Melolontha melolontha)

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]

Contents

Position in life cycle

In the life 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 while 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.

Pupa 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 climate pupae usually stay dormant during winter, in the tropics pupae usually do so during the dry season). Pupation may last weeks, months or even years. Anise Swallowtails sometimes emerge after years as a chrysalis.

Emergence

Insects emerge (eclose) from pupae 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.

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 pupae of other species 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

Common crow (Euploea core) chrysalis illustrating the Greek origin of the term : χρυσός (chrysós) for gold

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.

When the caterpillar is fully grown, it makes a button of silk which it uses to fasten its body to a leaf or a twig. Then the caterpillar's skin comes off for the final time. Under this old skin is a hard skin called a chrysalis.[2]

Because chrysalides are often showy and are formed in the open, they are the most familiar examples of pupae. Most chrysalides 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.[3] This sudden and rapid change from pupa to imago is called metamorphosis but metamorphosis is really the whole series of changes that an insect undergoes from egg to adult.

When the butterfly emerges from the chrysalis, usually it will sit on the empty shell in order to expand and harden 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 harden 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 chrysalides.

It is important to differentiate between pupa, chrysalis and cocoon. The pupa is the stage between the larva and adult stages. The chrysalis is a butterfly pupa. A cocoon is a silk case that moths, and sometimes other insects, spin around the pupa.

Cocoon

The tough brown cocoon of an Emperor Gum Moth

A cocoon is a casing spun of silk by many moth caterpillars,[4] 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.[5]

The silk in the cocoon of the silk moth can be unravelled to get silk fibre which makes this moth the most economically important of all Lepidopterans. The moth is the only completely domesticated Lepidopteran and does not exist in the wild.

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.

An Emperor Gum Moth caterpillar spinning its cocoon.

Luna moth cocoon and pupa.

Assortment of Luna moth cocoons.

Luna moth emerging from silk cocoon.

Luna moth pupa removed from cocoon.

Chrysalis of Gulf Fritillary in Georgetown, South Carolina

Pupation of Inachis io

Monarch Butterfly chrysalis

References

  1. ↑ Borror, D. J. Dwight M. DeLong and Charles A. Triplehorn. An introduction to the study of insects. New York: Holt, Rinehart & Winston. Sixth Edition.
  2. ↑ Darby, Gene (1958). What is a Butterfly. Chicago: Benefic Press. p. 19. 
  3. ↑ AMNH Accessed December 2006
  4. ↑ Darby, Gene (1958). What is a Butterfly. Chicago: Benefic Press. p. 41. 
  5. ↑ Malcolm J. Scoble. 1992. The Lepidoptera: form, function and diversity. Oxford: Oxford University Press.

See also