Question

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Name the hormone which brings about metamorphosis in silk moth.
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Answer 1

ANSWER :

$\: \: \: \: \: :\implies \boxed{\sf{Ecdysone}}$

MAIN REASON :

Ecdysone is a steroidal pro hormone of the major insect molting hormone 20-hydroxyecdysone, which is secreted from the prothoracic glands. Insect molting hormones are generally called ecdysteroids.

MORE ABOUT IT :

• A silkworm moth has a complex life cycle in which the immature animal—the larva—grows and molts (sheds its exoskeleton) several times in succession. These larval stages are called instars. The last larval instar then becomes a pupa, a stage in which the animal transforms, or metamorphoses, into an adult.

• Two hormones control the transitions of the moth from a juvenile into an adult. One of these hormones, called juvenile hormone, is produced and released from endocrine cells within structures called corpora allata, located at the back of the brain.

• The other developmental hormone, called ecdysone, is produced and released from the developing animal's prothoracic gland. Although juvenile hormone is continually released in the larva, ecdysone is released episodically. ... The levels of both ecdysone and juvenile hormone control the development of the moth.

• Ecdysone release is triggered by the presence of another hormone, called PTTH (prothoracicotropic hormone). PTTH is produced by neurosecretory cells in the brain. These neurons have axons that extend into or near the corpora allata, where their axon terminals release PTTH. The neurosecretory cells, in turn, are controlled by other brain neurons that integrate information from environmental, neural, and hormonal stimuli.

• The levels of both ecdysone and juvenile hormone control the development of the moth. Early on, the levels of juvenile hormone are high, but steadily decline and disappear by the time the animal reaches the pupal stage. In contrast, ecdysone levels peak once during every larval instar.

• Ecdysone circulates in the insect and becomes activated. Its primary affect is on the epidermis, the layer of living tissue just inside the exoskeleton. The hormone triggers epidermal cells to secrete enzymes that loosen their connection with the old exoskeleton, allowing it to be shed. Then the epidermal cells synthesize a new, larger exoskeleton. In other words, a molt occurs. Each peak of ecdysone triggers a molt. Before this molt, the larva grows until it is nearly too big for its tight exoskeleton. After this molt, the first instar larva has become a second instar larva.

• In addition to triggering a molt, ecdysone also promotes metamorphosis. However, high levels of juvenile hormone have the opposite effect and inhibit metamorphosis. Thus, after each molt during which juvenile hormone is present, the larva remains a larva, albeit a bigger larva.

• When the larva reaches the fifth instar stage, its levels of juvenile hormone have declined dramatically. These low levels of juvenile hormone are not able to inhibit metamorphosis. The larva becomes a pupa. During the pupal stage, the animal undergoes metamorphosis and then emerges as an adult.

CONCLUSION :

• A number of insects, including moths, butterflies, beetles, and flies, undergo complete metamorphosis. Each of these animals hatches from an egg into a larva (caterpillar, grub, or maggot), and then proceeds through several rounds of molting and growth.