Question

In insects, the effect of the juvenile hormone is to
A. Potentiate effect of ecdysone
B. Oppose the effect of ecdysone
C. Effect maturation
D. Prevent maturation

Answer 1

Hint: Ecdysone is the moulting hormone in insects and a juvenile hormone is a group of hormones which ensures the growth of larva while preventing metamorphosis. They are secreted by a pair of glands known as corpora allata.

Complete answer:
The two classes of hormones in insects are bursicon and brain hormone(BH). Through axon transport, they reach corpora cardiaca and remain stored in axon terminals. Under appropriate stimulation, they are released to blood from axon terminals. Through blood circulation, bursicon reaches body tissues and stimulates cuticle formation. BH reaches the prothoracic or ecdysial gland and stimulates it. Ecdysial gland secretes ecdysone or moulting hormone. Ecdysone induces moulting in the larval growth, promotes the formation of adult tissues and organs and initiates metamorphosis. Simultaneously, the corpora allata of the brain secretes a juvenile hormone or neotenin, which inhibits the action of ecdysone and thereby suppress metamorphosis at each intermoult. This prevents premature metamorphosis. Consequently, a larval moult follows without metamorphosis. This may be repeated many times and towards the end of larval life, corpora allata becomes least active. As a result, little or no JH is produced. This removes inhibition of ecdysis.
So, the correct answer is ‘D. Prevent maturation’.

Additional Information:
- In insects moulting, development and metamorphosis are under hormonal control.
- moulting is the shedding of the exoskeleton.
- metamorphosis is the transformation or change in shape through cell growth and differentiation.
- The two different hormones of insects are bursicon and brain hormone(BH).
- Bursicon is a proteinaceous hormone and BH is a steroid hormone.
- BH stimulates ecdysial gland to secrete ecdysone while what actually stimulates corpora allata to produce JH is not definitely understood.
- Some secretions of neurosecretory cells of the brain called allantotropins are believed to stimulate corpora allata to produce JH.

Note: The insects have a well-developed endocrine system, composed of neurosecretory cells, corpora cardiaca, corpora allata and parathoracic or ecdysial gland. Their hormones regulate metabolism, growth, reproduction and development. JH is also important for the production of eggs in female insects.