Problem 10 - Biology: The Dynamic Science

Question

Insect development is regulated by: a. ecdysone, a peptide secreted by the brain. b. juvenile hormone, a terpenoid secreted by the corpora allata near the brain. c. molt-inhibiting hormone, a steroid secreted by the prothoracic glands. d. prothoracicotropic hormone, a steroid secreted by the hypothalamus. e. melatonin, a peptide secreted by the brain in the larval stage.

Step-by-Step Solution

Answer

The correct answer is option 'a' with an amendment: ecdysone is a steroid secreted by the prothoracic glands.

Step 1: Understanding the question

The exercise is a multiple choice question asking about which substance regulates insect development and where it is secreted from.

Step 2: Identify correct hormone and secretion source

Research on insect development reveals that ecdysone is the hormone responsible for regulating molting and metamorphosis in insects. It is a steroid hormone, not a peptide, and is secreted by the prothoracic glands, not the brain.

Step 3: Evaluate the given options

Option 'a' incorrectly labels ecdysone as a peptide and incorrectly states it is secreted by the brain.Option 'b' correctly identifies juvenile hormone as a terpenoid and its source as the corpora allata near the brain, but this hormone regulates the maintenance of larval or juvenile characteristics, not directly regulating overall development.Option 'c' incorrectly describes molt-inhibiting hormone as a steroid; it is actually a peptide and does not regulate development on its own.Option 'd' incorrectly labels prothoracicotropic hormone as a steroid secreted by the hypothalamus; it is indeed involved in the stimulation of ecdysone production but is a peptide secreted by the neurosecretory cells of an insect's brain.Option 'e' is incorrect because melatonin is involved in the regulation of circadian rhythms and not in regulating insect development as required in the question.

Key Concepts

Ecdysone

One of the key regulators of insect development is ecdysone, which plays a crucial role in the process called molting, enabling the insect to grow and eventually metamorphose. Ecdysone is a type of steroid hormone produced in the prothoracic glands of insects.

Ecdysone triggers the shedding of the old exoskeleton, a necessary step for the insect to increase in size or to transition to a new developmental stage, such as changing from a larva to a pupa or from a pupa to an adult. Contrary to the exercise's suggestion, ecdysone is not a peptide, nor is it secreted by the brain. Instead, this hormone is chemically complex, similar to the steroids found in vertebrates, and its production is a significant event that leads to dramatic changes in an insect's life cycle.

Juvenile Hormone

Another vital hormone in insect development is the juvenile hormone. It is a terpenoid, which is a class of organic chemicals derived from five-carbon isoprene units. The juvenile hormone is secreted by the corpora allata, a pair of glands located near an insect's brain.

This hormone is essential for maintaining the insect's larval characteristics during early stages. It works in balance with ecdysone—the higher the level of juvenile hormone, the more pronounced the maintenance of juvenile features, delaying the onset of metamorphosis. As the insect approaches the end of its larval stages, the levels of juvenile hormone drop, allowing ecdysone to induce the transformation to the next life stage. Contrary to a direct role in regulating the developmental process, as suggested in the textbook exercise, the juvenile hormone instead might be viewed more as a modulator that dictates if the insect will continue as a larva or move towards metamorphosis.

Prothoracic Glands

The prothoracic glands are endocrine glands within insects responsible for the secretion of ecdysone. They are located near the thorax, hence their name. The role these glands play is akin to the role endocrine glands play in vertebrates; they secrete hormones that regulate growth and development.

Upon receiving the signal from the prothoracicotropic hormone, which is a neurohormone produced by the neurosecretory cells in the insect brain, the prothoracic glands synthesize and release ecdysone. This steroid hormone then circulates through the insect’s body, reaching target cells and initiating the molting process. The prothoracic glands are, therefore, a pivotal component in the endocrine system of insects, controlling the timing and process of development through their hormone output.

Corpora Allata

The corpora allata are a pair of small endocrine glands found in insects, located close to the brain. These glands produce the juvenile hormone, which is instrumental in controlling the growth and development of the insect.

Throughout the various stages of an insect's life cycle, the fluctuating levels of juvenile hormone released by the corpora allata determine whether an insect will stay in its larval form or initiate the process of metamorphosis. If the corpora allata are highly active, juvenile hormone is secreted in greater quantity, preserving the larval state. As the insect matures and the corpora allata's activity diminishes, decreasing levels of juvenile hormone allow ecdysone to take on a more pronounced role in stimulating metamorphosis. Thus, the corpora allata, through the juvenile hormone, are essential in the timing of an insect's transition from larva to adult.