Insect Growth and Metamorphosis

DEFINITIONS

Chrysalis	-	a butterfly pupa, not enclosed in a cocoon, a pupa without a protective cover.
Cocoon	-	a case formed partly or wholly of silk secreted by the larva and in which pupation occurs, a protective cover.
Instar	-	form of an insect between successive molts, the first instar being the stage between emerging from the egg and the first molt.
Larva	-	(pl. larvae) immature stage between the egg and pupa in insects with complete metamorphosis or, sometimes, between egg and adult in insects with incomplete metamorphosis.
Molting	-	the acting of shedding the cuticle or insect skin, shedding of the exoskeleton .
Nymph	-	immature stage of insects with incomplete metamorphosis.
Pupa	-	(pl. pupae) the third, inactive stage of insects with complete metamorphosis, the transition stage between the larval and adult stages. Usually non-mobile.
Stage	-	any definite period in the development of an insect, e.g.. the egg stage, nymphal or larval stage, pupal stage, adult stage. The number of nymphal or larval stages is variable.

Change of Form

Humans - gradual, continuous

Insects - inconstant, periodic, and constrained

Exoskeleton traits:

• fixed in size
• new exoskeleton
  • Incorporates the changes that are part of metamorphosis.
  • Initially soft and is larger than the old exoskeleton.
  • Therefore, change in body form comes about as a series of steps.
  • Human growth is a gradual process.
• stages between each molt are called instars.
  • -first stage which emerged from the egg is the first instar or nymph.
    • -- to grow must shed its skin or molt
    • -- may be four or five instars before the adult stage is reached.

In most insects, change of form is not gradual, it does not proceed slowly by degrees but by abrupt jumps or steps. The term describing the pronounced change in body form of insects as they mature, is known as metamorphosis.

Metamorphosis

Change from larval to adult form. Occurs in animals that have indirect development in which young differ drastically in form or life style from adult.

Greek word (meta - beyond, morphe - form) describing the series of changes through which an insect passes in its growth from the egg through the immature stages (nymph or larva and pupa) to the adult.

A change of form, used to describe the post embryonic changes in insects as they progress to adulthood.

Types of Arthropod Metamorphosis

Anamorphosis = typical of chilopods and diplopods and most crustaceans, additional segments added with each molt. Indefinite growth.

Ametabolous = no metamorphosis. Little or no change between the immature and adult form except in size and development of the sexual organs.

Ametabolous metamorphosis of a silverfish

Gradual (Incomplete) Metamorphosis (immature forms are often called nymphs)

• Nymphs resemble the adult in form except for being smaller and lacking fully developed wings and sexual organs.
• Partial change in wing and external genitalia development with each molt.
• Most adults and nymphs have the same habits and live in the same environment.
• Life Cycle: Egg --> nymphal instars --> adult

Incomplete (gradual) metamorphosis of a dragonfly and grasshopper

Complete metamorphosis

Immature forms are called larvae (larva, singular).

• Larvae are very different in form from the adult.
• The pupal stage is a transition stage, when the larva is transformed to the adult.
  • Pupa does not eat and movement is very restricted.
  • Great metabolic changes occur.
• Pupa molts to the adult form.
• Life Cycle: Egg --> larval instars --> pupa --> adult

Complete metamorphosis of a butterfly

What advantages does metamorphosis give an insect?

1. Allows growth
2. Allows specialization between stages

Incomplete metamorphosis is limited.

Complete metamorphosis is very pronounced.

Function of the different stages in insects with complete metamorphosis:

Larva: To eat and grow.

Adults: To reproduce.

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Last updated Dec. 30, 2000
Gary Brewer