Hormones · Larvae · Life Cycle · Metamorphosis

Metamorphosis

I need to know more about this in general if I want to do an artist’s book on it so I’m gathering general information. Metamorphosis is different in different organisms so I’d like to learn what difference there are.

METAMORPHOSIS
A major change in body form that occurs after Embryonic Development. The word “metamorphosis” comes from the Greek word for transformation and is mainly studied in three different groups of organisms: Arthropods (crabs/lobsters/insects), Amphibians (frogs/salamanders) and Echinoderms (sea urchins/star fish).
Metamorphosis affects the way animals eat, breathe and move and can also change the nature of the environment they live in.

AMPHIBIAN METAMORPHOSIS
Metamorphosis prepares the organism for the transition of living in water to living on land. The digestive system changes for the new diet of animals instead of plants. Eyes and ears change to adapt to function in the air. Chemical reactions in the cells also change.

Metamorphosis in amphibians is controlled by a pair of hormones: Prolactin (a protein secreted by the pituitary gland) controls the rate of growth and suppressed metamorphosis, and Thyroxine (a modified amino acid made in the thyroid gland) causes metamorphosis to begin once the organism is large enough and environmental conditions are right. The hormones pass through the circulation and “instruct” different tissues to activate and deactivate different genes, causing growth, generation and degeneration.

These hormones are produced by other vertebrates including humans. (Thyroxine regulates metabolism and Prolactin helps millk production in nursing women)
In Fish, Prolactin is used for keeping a cell’s salt content in balance.

INSECT METAMORPHOSIS
Insects have a rigid exoskeleton (a cuticle) that supports their body mass and allows attachment of muscles for movement. This cuticle is shed periodically to allow for growth, which is known as molting, and the number of times this occurs is regulated by the insects genes.

About 10% of all insect species go through a process of Incomplete Metamorphosis. The egg hatches into a juvenile called a nymph that resembles the adult but is much smaller and incapable of reproduction. At each molt the nymph grows before the newly produced cuticle hardens, and when the signal for metamorphosis arrives, the molt produces an adult capable of reproduction. Male and female adults can then mate and produce eggs.

Complete Metamorphosis is a different process undergone by 90% of all insect species, including ants, BEES, flies, butterflies and moths. In this instance the larval form looks nothing like the adult (for example a larval butterfly is a caterpillar). Each larva undergoes a series of molts so it can grow huge amounts. When the metamorphosis signal arrives, the insect undergoes a radical change; they stop feeding and moving, anchoring themselves so something, then either spins a cocoon around itself or encloses itself in its own hardened cuticle. The emerging insect is the fully grown adult.

SILKWORM METAMORPHOSIS
Silkworms undergo complete metamorphosis and its development begins when the egg hatches and includes five larval stages. Each stage is larger than the one before as it eats several times its own body weight in food each day. At the end of the fifth larval stage, the molting that follows is very different as the larva spins a cocoon made out of silk and becomes a pupa. The next molt occurs within the pupal case when metamorphosis begins. The cocoon breaks open when the adult has formed and it then emerges to begin reproduction.

Metamorphosis involves the complete replacement of one body form with another. Inside the pupal case, the larval tissues break down and their molecules are reutilised in the construction of the cells and tissues of a very different looking animal, the adult. Certain groups of cells (imaginal disks) along with the larval brain are generally the only tissue that are not broken down in the process. Inside the cocoon at this stage is a white, milky sap and little else as the organism has been completely broken down. They literally become a soup.

The imaginal disks (round and flat sheets of cells) begin to evert (meaning to turn outwards or inside out) or telescope and for the external structures which are characteristic of the adult cuticle. There is one for each eye, for each antennae, for the wings and for each leg. These structures attach and the adult insect is constructed. This process can take a week or months.

HORMONAL REGULATION OF METAMORPHOSIS
In insects, three different hormones combine to regulate the timing of both milting and metamorphosis. Each of these hormones are produced in a different tissue and each has a different chemical structure and function. The signal to molt comes from a small group of cells within the brain in response to neural or environmental signals. The hormone produced there (PTTH – Prothoracicotropic hormone) is a mall protein that passes through the blood to all parts of the body. The Prothoracic gland responds by producing a second hormone called Ecdysone, which is the molting hormone. It is a steroid that the insect requires in its diet and must undergo some chemical changes before it becomes active and initiates molting. This is the molting hormone for insects, arthropods and many other animals.

Ecdysone’s major effect is to trigger molting by causing the hardening of the cuticle and the separation of the living cells beneath the cuticle from it. The cuticle then dries and cracks and the larva can then emerge from its old skin and grow. If PTTH production stops, the amount of ecdysone released also falls. This normally happens in the period between molts but can also occur during a molt. The pupa will remain in diapause until an environmental signal, consisting of a minimum of two weeks in the cold followed by a normal spring warming, triggers the resumption of PTTH secretion and the completion of metamorphosis.

Many genes required for larval functions are turned off in response to ecdysone, while the genes for molting and metamorphosis are turned on. This effect can be seen when imaginal disks are placed in a culture dish; the disks stop growing and begin metamorphosis.This development can produce normal looking legs floating free in the culture dish. The changes seen can be directly attributed to the function of genes turned on by ecdysone.

Ecysone not only causes metamorphosis but also triggers certain simple molts. The third hormone involved in the control of metamorphosis is responsible for the choice of whether to molt and grow or undergo metamorphosis to the adult form. This compound is called Juvenile Hormone (JH). JH is produced in a gland called the corpora allata has a different chemical structure in different species.

When a molt is triggered, the type of molt is determined by the levels of  JH present. During larval development, the amount of JH in the hemolymph is high and the tissues respond to ecydsone by undergoing a molt to become a larger larva. Later in development, the corpora allata stops producing JH and any new molt triggered by ecydsone causes the organism to proceed to the pupal stage and begin metamorphosis. Ecydsone and JH thus govern which molt occurs.

TERMS QUICK GUIDE

  • Corpora Allata: a gland in insects that synthesizes and secretes juvenile hormone (JH)
  • Ecdysone: a hormone that triggers both molting and metamorphosis in insects as well as in many other species of animals
  • Imaginal Disk: flat sheets of cells within an insect larva; these cells will change shape during metamorphosis and form the external structures of the adult
  • Juvenile Hormone (JH): a species-specific hormone which controls whether a molt will produce a larger larva or initiate metamorphosis
  • Larva: the reproductively immature feeding stage in the development of many species of animals, including those insects which undergo complete metamorphosis
  • Nymph: the sexually immature feeding stage in the development of those insects which undergo incomplete metamorphosis
  • Prothoracic Gland: the gland where ecdysone is made in insects
  • Prothoracicotropic Hormone (PTTH): a hormone made in the brain of insects which stimulates the prothoracic gland to make ecdysone
  • Pupa: the stage of insect development during which metamorphosis occurs

Sources used in this blog post:
Pelliccia, J.G. (2015) ‘Metamorphosis’, Salem Press Encyclopedia of Science

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