A note on metamorphosis in insects.

1. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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Metamorphosis In Insects
Metamorphosis is the transition (change) from a larval stage to an adult stage. In many
species that undergo metamorphosis, a large proportion of the animal's structure
changes, and the larva and the adult are unrecognizable as being the same individual.
Direct Developers: Animals (including humans) whose young are smaller versions of the
adult (they don’t undergo metamorphosis).
Indirect Developers: Animals in which embryonic development includes a larval stage
with characteristics very different from those of the adult organism, which emerges only
after a period of metamorphosis.
Insects have three types of developmental processes:
1. Holometabolous
It is a type of development in which there is complete metamorphosis. The juvenile
hatches from the egg and is called a larva (caterpillar, maggots, etc.). The larva molts
and gets bigger and bigger after each molt. These stages of larva (that get bigger) are
called instars. The larva then transforms into an inactive pupa which does not move or
eat, this pupa transforms into the adult stage or imago which tears out of the pupa shell,
i.e. in butterflies, moths, beetles, etc.
2. Hemimetabolous
It is a type of development in which there is half metamorphosis, i.e. in grasshoppers
and bugs. After spending a very brief period as a pronymph (whose cuticle is often shed
as the insect hatches), the insect looks like an immature adult and is called a nymph.
The rudiments of the wings, genital organs, and other adult structures are present and
become progressively more mature with each molt. At the final molt, the emerging
insect is a winged and sexually mature adult, or imago.

2. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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3. Ametabolous
It is a type of development in which there is no metamorphosis. A few insects, such as
springtails, have no larval stage and undergo direct development, these insects have a
pronymph stage which looks like a small adult; it grows larger after each molt but is
unchanged in form.
Figure: Ametabolous, Hemimetabolous, and Holometabolous development in insects.
Life Cycle Stages of Insects
There are several life cycle stages of insects:
 Larva: It is the immature stage that hatches from eggs, it does not resemble the
adult form. It feeds excessively.

3. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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 Instar: The larva undergoes molting and increases in size and is then called an
instar. The instar after the first molt is called first instar, the instar after the
second one is called a second instar, and so on.
 Pupa: It is the dormant stage of insects, it is enclosed in a cocoon and does not
move or eat. It converts into the adult form inside its cocoon.
 Imago: It is the adult stage of the insect that tears its way through the cocoon.
 Pronymph: It is a larval stage of insects that show hemimetabolous or
ametabolous development, it resembles the adult form but is much smaller in
size.
 Nymph: It is a bigger form of the pronymph (after molting).
Mechanism of Insect Metamorphosis (Holometabolous)
In holometabolous insects, the transformation from juvenile into adult occurs within the
pupal cuticle. Most of the larval body is degenerated by programmed cell death
(apoptosis), while new adult organs develop from undifferentiated imaginal cells. Thus,
within any larva, there are two distinct populations of cells: (1) larval cells, which are
used for the functions of the juvenile insect; and (2) imaginal cells, which can
differentiate. There are three main types of imaginal cells:
 Cells of Imaginal Discs: these will form the cuticular structures of the adult, including
the wings, legs, antennae, eyes, head, thorax, and genitalia.
 Histoblast Nests: these are clusters of imaginal cells that will form the adult abdomen.
 Clusters of imaginal cells in each organ: these will proliferate to form the adult organ
as the larval organ degenerates.
1. Specification & Proliferation
Specification of the general cell fates (i.e., that the disc is to be a leg disc and not a wing
disc) occurs in the embryo. The more specific cell fates are specified in the larval

4. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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stages, as the cells proliferate. The type of leg structure (claw, femur, etc.) generated is
determined by the interactions between several genes in the imaginal disc. These
genes code for paracrine factors that direct the development of surrounding cells.
2. Differentiation
The mature leg disc in the third instar of Drosophila does not look anything like the adult
structure. It is determined but not yet differentiated; its differentiation requires a signal,
in the form of a set of pulses of the molting hormone 20-hydroxyecdysone (20E).
3. Determination of Wing Imaginal Discs
The largest of Drosophila's imaginal discs is that of the wing. The wing discs are
distinguished from the other imaginal discs by the expression of the vestigial gene (wing
specific gene). When this gene is expressed in any other imaginal disc, wing tissue
emerges.
Hormonal Control of Insect Metamorphosis
Insect molting and metamorphosis are controlled by two effector hormones: the steroid
20-hydroxyecdysone (20E) and the lipid juvenile hormone (JH).
 20-Hydroxyecdysone initiates and coordinates each molt and regulates the changes
in gene expression that occur during metamorphosis.
 Juvenile Hormone prevents the ecdysone-induced changes in gene expression that
are necessary for metamorphosis, i.e. it ensures that the result of that molt is
another larval instar, not a pupa or an adult.
1. Releasing & Effect of 20-Hydroxyecdysone
The molting process is initiated in the brain, where neurosecretory cells release
prothoracicotropic hormone (PTTH) in response to neural, hormonal, or environmental
signals. PTTH is a peptide hormone that stimulates the production of ecdysone by the
prothoracic gland. Ecdysone is modified in peripheral tissues to become the active

5. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
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molting hormone 20-Hydroxyecdysone. Ecdysone is released in pulses, these pulses of
20-Hydroxyecdysone commit and stimulate the epidermal cells to synthesize enzymes
that digest the old cuticle and synthesize a new one
2. Releasing & Effect of Juvenile Hormone
Juvenile hormone is secreted by the corpora allata (endocrine gland). As long as
Juvenile hormone is present, the 20-Hydroxyecdysone-stimulated molts result in a new
larval instar. In the last larval instar, however, the production of JH is inhibited, and
there is a simultaneous increase in the body's ability to degrade existing JH. Both these
mechanisms cause JH levels to drop below a critical threshold value, triggering the
release of prothoracicotropic hormone (PTTH) from the brain. PTTH, in turn, stimulates
the prothoracic gland to secrete a small amount of ecdysone. The resulting pulse of
20E, in the absence of high levels of JH, commits the epidermal cells to pupal
development (it converts the larva to pupa).
3. Pulses of Ecdysone
There are two major pulses of 20E during Drosophila metamorphosis. The first
ecdysone pulse during the last larval instar triggers the processes that inactivate the
larva-specific genes and initiates the morphogenesis of imaginal disc structures. The
second pulse transcribes pupa-specific genes and initiates the molt. At the imaginal
molt, when 20E acts in the absence of juvenile hormone, the imaginal discs fully
differentiate and the molt gives rise to an adult.