ENTMLGY 4000 Growth and Metamorphosis

Metamorphosis

Course Overview

  • Course: ENTMLGY 4000

  • Instructor: Dr. Ellen Klinger, Department of Entomology, The Ohio State University

Learning Objectives

  • Understand hormonal control in insect growth.

  • Define and understand the process of apolysis and ecdysis.

  • Describe the role of imaginal discs in metamorphosis.

  • Compare determinant versus indeterminate insect growth.

  • Explain different degrees of metamorphosis with insect examples.

  • Discuss evolutionary advantages of holometaboly.

  • Familiarize with vocabulary: histolysis, histogenesis, organogenesis, pharate, nymph, larva, instar.

Insect Growth and Molting

Growth Process

  • Insects must molt (shed their exoskeleton) to grow.

  • The molting process involves:

    • Apolysis: Separation of the old exoskeleton from the epidermis.

    • Ecdysis: The actual shedding of the old exoskeleton.

Insect Hormones

  • **Types of Insect Hormones:

    • Ecdysteroids:** Hormones that stimulate ecdysis.

    • Juvenile Hormone (JH): Inhibits maturation and regulates molting stages.

  • Inka Cells play a role in hormone production.

Process of Molting

Steps in Molting

  1. Ecdysis: Shedding of the old exoskeleton.

  2. Epicuticle production: New epicuticle forms.

  3. Histolysis: Breakdown of old tissues by hemocytes.

  4. Histogenesis: Formation of new adult tissues.

  5. Organogenesis: Development of adult organs.

Phases of Molting

  • Pharate Stage: Insect with two exoskeletons during the molting process.

  • Hormones control the timing and process of each molt.

Hormonal Control of Metamorphosis

Key Hormones

  • Ecdysone: Triggers molting when levels are high.

  • Prothoracicotropic Hormone (PTTH): Stimulates ecdysteroid secretion.

  • Juvenile Hormone (JH) influences whether an insect molts into a larger immature form or transitions into a mature adult.

Stages of Development

Instars and Life Cycle

  • Life cycle stages of insects: eggs, various larval instars, pupae, and adults.

  • Distinct ecdysis events occur throughout these stages.

Degrees of Metamorphosis

1. Ametabolous Metamorphosis (Monophasic)

  • Involves nymphal instars and wingless adults.

  • Example: Silverfish (Zygentoma).

2. Hemimetabolous Metamorphosis (Diphasic)

  • Includes nymphs and an adult stage with wings.

  • Types:

    • Paurometabolous: Nymphs and adults inhabit the same environment.

    • Hemimetabolous: Nymphs (naiads) are aquatic, while adults are terrestrial.

  • Example: Hemipterans.

3. Holometabolous Metamorphosis (Triphasic)

  • Transitions from larval instars to pupal instar to adult stage with wings.

  • Examples: Diptera, Coleoptera, and Lepidoptera.

4. Hypermetabolous Metamorphosis (Polyphasic)

  • Features multiple larval instars with varying lifestyles.

Evolutionary Advantages of Metamorphosis

  • Separate evolutionary pathways for larvae and adults enhance survival.

  • Allows distinct adaptations for different habitats: larvae and adults can occupy and exploit different ecological niches.

Conclusion of Lecture

  • Review of objectives and key concepts presented regarding insect growth, hormonal control, and metamorphosis.