Juvenile hormone controls development in insects

The Rhodnius decapitation experiments yielded a curious result: Regardless of the instar used, the decapitated bug always molted directly into an adult form. Additional experiments by Wigglesworth demonstrated that a hormone other than those responsible for molting determines whether a bug molts into another juvenile instar or into an adult.

Because the head of Rhodnius is long, it was possible to remove just the front part of the head, which contains the brain and the corpora cardiaca, while leaving the rear part intact. That rear part contains two endocrine structures called the corpora allata (singular, corpus allatum). When fourth-instar bugs that had been fed a week earlier were partly decapitated, leaving the corpora allata intact, they molted into fifth instars, not into adults.

This experiment was followed up by more experiments using glass tubes to connect individual bugs. When an unfed, completely decapitated, fifth-instar bug was connected to a fourth-instar bug that had been fed and had had only the front part of its head removed, both bugs molted into juvenile forms. A substance coming from the rear part of the head of the fourth-instar bug prevented the expected result that both bugs would molt into adult forms.

We now know that the substance responsible is juvenile hormone and that it comes from the corpora allata. As long as juvenile hormone is present, Rhodnius molts into another juvenile instar. The corpora allata normally stop producing juvenile hormone during the fifth instar. When the bug stops producing juvenile hormone, it molts into an adult.

The control of development by juvenile hormone is more complex in insects that, like butterflies, undergo complete metamorphosis. These animals undergo dramatic developmental changes between instars. The fertilized egg hatches into a larva, which feeds and molts several times, becoming bigger and bigger. Then it enters an inactive stage called a pupa. It undergoes major body reorganization as a pupa, and finally emerges as an adult.

An excellent example of complete metamorphosis is provided by the silkworm moth, Hyalophora cecropia (Figure 42.4). As long as juvenile hormone is present in high concentrations, larvae molt into larvae. When the level of juvenile hormone falls, larvae molt into pupae. Because no juvenile hormone is produced in pupae, they molt into adults.

Endocrine cells produce brain hormone (yellow) which is transported to the corpus cardiacum where it is released.

Prothoracic gland

Brain hormone stimulates the prothoracic gland to secrete ecdysone (pink).

As juvenile hormone wanes the larva molts into a pupa.

The pupa does not produce juvenile hormone, so it metamorphoses into an adult.

Brain

Endocrine cells produce brain hormone (yellow) which is transported to the corpus cardiacum where it is released.

Prothoracic gland

Brain

Brain hormone stimulates the prothoracic gland to secrete ecdysone (pink).

Pupa And Adult And Prothoracic Gland

As juvenile hormone wanes the larva molts into a pupa.

The pupa does not produce juvenile hormone, so it metamorphoses into an adult.

Adult

42.4 Complete Metamorphosis Butterflies and moths undergo complete metamorphosis, in which the feeding larvae (caterpillars) bear no resemblance to the reproductive adult. Three hormones control molting and metamorphosis in the silkworm moth Hyalophora cecropia.

Adult

42.4 Complete Metamorphosis Butterflies and moths undergo complete metamorphosis, in which the feeding larvae (caterpillars) bear no resemblance to the reproductive adult. Three hormones control molting and metamorphosis in the silkworm moth Hyalophora cecropia.

42.1 Principal Hormones of Humans

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Responses

  • DOUGLAS
    What hormone in insects is responsible for molting?
    8 years ago
  • keijo
    Which of the follwoing is one of the hormones that controls molting in insects?
    7 years ago

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