Insect metamorphosis is a remarkable biological process that allows insects to grow and transform through distinct developmental stages. Because their exoskeleton is rigid and cannot expand, insects must periodically shed it — a process known as ecdysis or molting. Hormones and molecular signals tightly regulates this molting process to ensure each stage occurs at the right time and in the correct sequence. This blog post explains hormonal Control of Insect Metamorphosis in detail.
Hormonal Control of Insect Molting
A well-coordinated endocrine system controls the molting and metamorphosis of insects. Two key neuroendocrine structures near the insect brain, the corpora allata and corpora cardiaca, play essential roles in this process.
- Corpora Allata (CA): Produce Juvenile Hormone (JH), which maintains the larval or nymphal state and prevents premature metamorphosis.
- Corpora Cardiaca (CC): Store and release Prothoracicotropic Hormone (PTTH) and other neurohormones that regulate ecdysone secretion.
- Prothoracic Glands: The main source of ecdysone, the steroid molting hormone that triggers ecdysis.
Ecdysone: The Molting Hormone
When an insect is ready to molt, neurosecretory cells in the brain release PTTH, which is stored and released from the corpora cardiaca. PTTH then stimulates the prothoracic glands to secrete ecdysone into the hemolymph.
Ecdysone is initially secreted as a prohormone (alpha-ecdysone) and converted into its active form, 20-hydroxyecdysone (20E), in peripheral tissues. This active hormone binds to the ecdysone receptor (EcR) inside epidermal cells, acting as a genetic switch that activates genes responsible for molting. These genes coordinate:
- Apolysis – separation of the old cuticle
- Ecdysis – shedding of the old exoskeleton
- Formation of the new cuticle
Unlike peptide hormones such as PTTH or bursicon, ecdysone is a steroid hormone derived from cholesterol. Insects must obtain cholesterol from their diet. Its steroid structure allows it to easily penetrate cell membranes and regulate gene transcription directly in the nucleus.
Juvenile Hormone: The “Status Quo” Hormone
The Juvenile Hormone (JH), produced by the corpora allata, plays a crucial role in maintaining the larval or nymphal stage. During early development, high JH levels signal the insect to remain in a juvenile form. When a molt is triggered by ecdysone, the presence of high JH ensures that the result is simply a larger larva, not an adult.
Classic experiments show that removing the corpora allata drastically reduces JH levels. Without JH, the next ecdysone pulse triggers premature metamorphosis, resulting in a dwarf pupa. This demonstrates the essential role of JH as a repressor of adult development during immature stages.
As the insect reaches its final larval stage, JH levels drop sharply, removing this inhibition. The next ecdysone pulse then drives metamorphosis into a pupa and eventually an adult (imago), completing the insect’s life cycle.
How Molting Is Initiated: The Brain–Prothoracic Gland Axis
Molting in insects is precisely timed with growth. As the larva feeds and its exoskeleton stretches, sensory receptors detect internal pressure and send signals to the brain. When the insect reaches a critical size, the brain’s neurosecretory cells release PTTH, stored in the corpora cardiaca, which then stimulates the prothoracic glands to secrete ecdysone.
This brain–prothoracic gland axis ensures that molting occurs only when necessary for further growth and development.
Hormonal Interaction Determines Molt Outcome
Whether the insect stays juvenile or transitions to adulthood depends on balance between ecdysone and Juvenile Hormone:
- High JH + Ecdysone pulse → Molt into a larger larval instar (juvenile molt).
- Low/Absent JH + Ecdysone pulse → Molt into a pupa or adult (metamorphic molt).
This hormonal coordination allows multiple larval molts before final metamorphosis.
The Molting Process: Step by Step
- Apolysis: The epidermis separates from the old cuticle under the influence of 20-hydroxyecdysone.
- New Cuticle Secretion: Epidermal cells produce molting fluid and new cuticle layers.
- Ecdysis: The insect sheds its old exoskeleton.
- Sclerotization: The hormone bursicon hardens and pigments the new cuticle.
Conclusion
Insect molting and metamorphosis are highly coordinated physiological processes. Interaction of ecdysone, Juvenile Hormone, and PTTH controls these two processes. While ecdysone initiates molting, Juvenile Hormone determines the developmental outcome, and the corpora cardiaca ensures timely hormone release. Together, these endocrine signals regulate growth, transformation, and reproduction—allowing insects to thrive and complete their life cycles successfully.