Download Viviparity

Insect biology
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The science that deals with insect’s individual
developmental history.
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It is divided into the study of reproduction,
embryology, post
embryology and adult
Insect Reproduction
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Most species of insects have
males and females that mate
and reproduce sexually
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Some insects reproduce
asexually, without the joining of
male and female gametes.
Types of reproduction
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Sexual reproduction
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Parthenogenesis
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Polyembryony
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Viviparity
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Paedogenesis
Facultative parthenogenesis
Obligate parthenogenesis
Ovoviviparity
Adenotrophic viviparity
Pseudoplacental viviparity
Haemocoelous viviparity
Sexual reproduction
Most insects reproduce sexually, by the joining of
male and female gametes or sex cells.
 Gametes are produced by a special kind of cellular
reproduction.
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parthenogenesis
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A form of reproduction in which the ovum
develops into a new individual without
fertilization.
Natural parthenogenesis has been observed in
the aphid.
Facultative parthenogenesis
 sporadic parthenogenesis : Silkworm/phasmatodea
Obligate parthenogenesis
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Cyclical parthenogenesis: aphids
heterogeny
Obligate parthenogenesis
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Constant parthenogenesis:
In many social insects, such as the honeybee
and the ant, the unfertilized eggs give rise to
the male drones and the fertilized eggs to the
female workers and queens.
Polyembryony
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Parasitic hymenopteran
Viviparity
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Ovoviviparity
Nutrition was supplied by egg.
Embryo finishes development in the mother’s body.
Egg hatches in the mother’s body.
Larva leaves mother’s body after hatching.
Scale, aphid, thrips, house fly, parasitic fly
Viviparity
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Adenotrophic viviparity
Pupiparity
Nutrition of embryo was supplied by egg.
Embryo finishes development in the mother’s body.
Egg hatches in the mother’s body.
Larvae leave mother’s body until they are access to
pupae.
Some species in the house fly family.
Viviparity
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Pseudoplacental viviparity
No or little yolk in egg
Nutrition was absorbed by pseudoplacenta in the
mother’ body.
Some species of aphids, Dermaptera and
cockroaches
Viviparity
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Haemocoelous viviparity
Ovary breaks when embryo finishes development.
Eggs are released in the mother’s haemocoele
Larvae feed on mother’s organs until they come out
from mother’ body before puparity.
Paedogenesis
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Paedogenesis is a form of neoteny in insects in
which the larval stage reproduces without maturing
first.
It occurs in the females of
certain beetles, Strepsiptera, bagworms, and gall
midges.
Paedogenesis is the precocious development of
sexual maturity in a larva.
Insect reproduction
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Short life cycles-most go through generation in 1-6
weeks
Large number of offspring / female-100-2,000 eggs
Fruit flies-2 week life cycle26 generations/year100
eggs / female
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In 1 year from 1 male and 1 female
if all offspring survive to breed would produce 1041
flies
Ontogenesis
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Preembryonic development: sperm/egg
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Embryonic development
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Postembryonic development: after finishing
embryonic development to adult
Egg
In most insects, life begins as an independent egg.
 This type of reproduction is known as ovipary.
 Manufactured within the female's genital system
 Released from her body through an ovipositor
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Production of eggs by the female is called öogenesis
The egg-laying process is known as oviposition.
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Each insect species produces eggs that are
genetically unique and often physically distinctive
as well -- spherical, ovate, conical, sausage-shaped,
barrel-shaped, or torpedo-shaped.
Each egg is composed of only a single living cell -the female gamete.
Some types of eggs
Mode of ovipositing
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on the object surface：insects secret gland
An ovipositing butterfly
Eggs of a tent caterpillar
Eggs of a true bug (Hemiptera)
On the plant surface
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In the host organization
 In the concealment locus
In the water
In the earth
Insects protect their eggs
Egg structure
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Egg shell-chorion
Vitelline membrane
Periplasm
Nucleus
Micropyle
Yolk
Cytoplasm
Cytoplasm reticulum
chorion
Periplasm
Vitelline membrane
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An egg's cell membrane is known as the vitelline
membrane .
It is a phospholipid bilayer similar in structure to
most other animal membranes.
It surrounds the entire contents of the egg cell,
most of which consists of yolk (food for the soonto-develop embryo).
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The cell's cytoplasm is usually distributed in a thin
band just inside the vitelline membrane (where it is
commonly called periplasm ) and in diffuse strands
that run throughout the yolk ( cytoplasmic
reticulum ).
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The egg cell's nucleus (haploid) lies within the yolk,
usually close to one end of the egg.
Near the opposite end, the öosome (a region of
higher optical density) may be visible as a dark
region in the more translucent yolk.
The egg's anterior/posterior polarity is determined
by the relative positions of the nucleus and the
öosome.
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The egg is covered by a protective "shell" of protein
secreted before oviposition by accessory glands in
the female's reproductive system.
This egg shell, called the chorion , is sculptured
with microscopic grooves or ridges that may be
visible only under an electron
microscope.
chorion
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The chorion is perforated by microscopic pores
(called aeropyles ) that allow respiratory exchange
of oxygen and carbon dioxide with relatively little
loss of water.
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The micropyle , a special opening near the anterior
end of the chorion, serves as a gateway for entry of
sperm during fertilization.
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A female receives sperm from her male partner
during the act of mating---insemination
She can store that sperm for long periods of time in
a special part of her reproductive system, the
spermatheca.
As a developing egg moves past the opening to the
spermatheca, a few sperm are released onto its
surface.
Fertilization
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The sperm swim toward the micropyle -- the first
one to reach its destination enters and injects its
nucleus into the egg.
The sperm nucleus quickly fuses with the egg
nucleus to form a diploid zygote -- a one-celled
embryo.
This event is known as fertilization.
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After the egg is fertilized, it undergoes a period of
rapid growth and development known as
embryogenesis.
EMBRYOGENESIS
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A developmental process that usually begins once
the egg has been fertilized.
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It involves multiplication of cells (by mitosis) and
their subsequent growth, movement, and
differentiation into all the tissues and organs of a
living insect.
EMBRYOGENESIS
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Cleavage and formation of blastoderm
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Formation of germ band, embryonic layer and
embryonic envelop
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Segmentation of embryo and formation of
appendages
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Formation of organizations and systems
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Blastokinesis and disappearance of embryonic
envelop
Cleavage and formation of blastoderm
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This process of nuclear division is known as
superficial cleavage
As they form, the cleavage nuclei migrate through
the yolk toward the perimeter of the egg.
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They settle in the band of periplasm where they
engineer the construction of membranes to form
individual cells.
The end result of "cleavage" is the blastoderm -a one-cell-thick layer of cells surrounding the yolk.
Formation of blastoderm
Formation of blastoderm
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The first cleavage nuclei to reach the vicinity of the
öosome are "reserved" for future reproductive
purposes -- they do not travel to the periplasm and
do not form any part of the blastoderm.
Instead, they stop dividing and form germ cells
that remain segregated throughout much of
embryogenesis.
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These cells will eventually migrate into the
developing gonads (ovaries or testes) to become
primary öocytes or spermatocytes.
Only when the adult insect finally reaches sexual
maturity will these cells begin dividing (by meiosis)
to form gametes of the next generation (eggs or
sperm).
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Germ cells never grow or divide during
embryogenesis, so DNA for the next generation is
"conserved" from the very beginning of
development.
This strategy has a clear selective advantage: it
minimizes the risk that an error in replication will
accidently be passed on to the next generation.
Formation of germ band
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Blastoderm cells on one side of the egg begin to
enlarge and multiply.
This region, known as the germ band (or ventral
plate), is where the embryo's body will develop.
The rest of the cells in the blastoderm become part
of a membrane (the serosa) that forms the yolk
sac.
Cells from the serosa grow around the germ band,
enclosing the embryo in an amniotic membrane
Formation of germ band
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At this stage of development, when the embryo is
not much more than a single layer of cells, a group
of control genes (called homeotic selector genes)
become active.
These genes encode for proteins that contain a
special active site (the homeobox) for binding with
DNA.
They interact with specific locations in the genome
where they function as switches for activating (or
inhibiting) the expression of other genes
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Basically, each selector gene controls the expression
of certain other genes within a restricted domain of
cells based on their location in the germ band.
By regulating activity within a suite of genes that
produce hormone-like "organizer" chemicals, cellsurface receptors, and structural elements, the
selector genes guide the development of individual
cells and channel them into different "career paths".
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This process, called differentiation, continues until
the fundamental body plan is mapped out -- first
into general regions along an anterio-posterior axis,
then into individual segments, and finally into
specialized structures or appendages.
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As the germ band enlarges, it begins to lengthen
and fold into a sausage shape with one layer of
cells on the outside (the ectoderm) and another
layer of cells on the inside (the mesoderm).
An important developmental milestone, called
dorsal closure, occurs when the lateral edges of
the germ band meet and fuse along the dorsal
midline of the embryo's body.
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Ectoderm cells grow and differentiate to form the
epidermis, the brain and nervous system, and most
of the insect's respiratory (tracheal) system.
In addition, the ectoderm invaginates (folds inward)
at the front and rear of the embryo's body to create
front and rear portions of the digestive system
(foregut and hindgut).
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Mesoderm cells differentiate to form other internal
structures such as muscles, glands, heart, blood,
fat body, and reproductive organs.
The midgut develops from a third germ layer (the
endoderm) that arises near the fore- and hindgut
invaginations and eventually fuses with them to
complete the alimentary canal
Developmental Fate of Insect Germ Layers
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Ectoderm: Epidermis, exocrine glands, brain and
nervous system, sense organs, foregut and hindgut,
respiratory system, external genitalia.
Mesoderm: Heart, blood, circulatory system,
muscles, endocrine glands, fat body, gonads (ovaries
and testes).
Endoderm: Midgut.
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During its early development, the embryo's body is
rather worm-like in appearance.
Individual segments first become visible near the
anterior end (the protocephalon) where ectodermal
tissue differentiates into the brain and compound
eyes.
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Bud-like swellings develop in front of the mouth
opening.
They will eventually grow to form the labrum and
the antennae.
Segments behind the mouth also develop bud-like
swellings.
Each of the first three post-oral segments form
paired appendages that become
mouthparts: mandibles, maxillae, and labium.
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The next three post-oral segments develop into the
thorax -- they form appendages that become
walking legs.
Segments of the abdomen also develop limb buds
but these soon shrink and disappear -- perhaps
they are vestigal remnants of abdominal
appendages found in more primitive arthropods
(like millipedes and centipedes).
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In general, the rate of embryonic development
depends on temperature (insects are poikilothermic)
and on species-specific characteristics of
development.
Embryogenesis ends when the yolk's contents have
been consumed: the immature insect is fully formed
and ready to hatch from the egg.
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During the hatching process (often called eclosion)
the young insect may chew its way through the
egg's chorion or it may swell in size by imbibing air
until the egg shell "cracks" along a predetermined
line of weakness.
Formations of inner layer
Three stages of
embryonic development
A protopod B polypod C oligopod
Embryonic development of
tobacco hornworm
.Manduca sexta eggs.
M. sexta embryo 19
M. sexta egg
showing micropyle hours after fertilization
M. sexta embryo 57
hours after fertilization
M. sexta embryo 115
hours after fertilization.
M. sexta embryo 37
hours afterfertilization
Newly emerged larva
showing the head
Egg hatching
Insect eggs close to hatching showing embrios with
red "eye-spots".
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Once the hatchling emerges, it is called a first
instar nymph (or larva).
As it grows, it will continue to develop and
mature.
These post-embryonic changes are known as
morphogenesis.
MORPHOGENESIS
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Once an insect hatches from the egg it is usually
able to survive on its own, but it is small, wingless,
and sexually immature.
Its primary role in life is to eat and grow.
If it survives, it will periodically outgrow and replace
its exoskeleton (a process known as molting).
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In many species, there are other physical changes
that also occur as the insect gets older (growth of
wings and development of external genitalia, for
example).
Collectively, all changes that involve growth,
molting, and maturation are known as
morphogenesis.
Instar
Timeline of MORPHOGENESIS
Molting
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The molting process is triggered by hormones
released when an insect's growth reaches the
physical limits of its exoskeleton.
Each molt represents the end of one growth stage
(instar) and the beginning of another
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In some insect species the number of instars is
constant (typically from 3 to 15), but in others it
may vary in response to temperature, food
availability, or other environmental factors.
Molting stops when the insect becomes an adult -energy for growth is then channeled into production
of eggs and sperm.
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An insect cannot survive without the support and
protection of its exoskeleton, so a new, larger
replacement must be constructed inside the old
one -- much like putting an overcoat under a
sweater!
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The molting process begins when epidermal cells
respond to hormonal changes by increasing their
rate of protein synthesis.
This quickly leads to apolysis -- physical
separation of the epidermis from the old
endocuticle.
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Epidermal cells fill the resulting gap with an inactive
molting fluid and then secrete a special lipoprotein
(the cuticulin layer) that insulates and protects
them from the molting fluid's digestive action.
This cuticulin layer becomes part of the new
exoskeleton's epicuticle
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After formation of the cuticulin layer, molting fluid
becomes activated and chemically "digests" the
endocuticle of the old exoskeleton.
Break-down products (amino acids and chitin
microfibrils) pass through the cuticulin layer where
they are recycled by the epidermal cells and
secreted under the cuticulin layer as new procuticle
(soft and wrinkled).
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Pore canals within the procuticle allow movement
of lipids and proteins toward the new epicuticle
where wax and cement layers form.
When the new exoskeleton is ready, muscular
contractions and intake of air cause the insect's
body to swell until the old exoskeleton splits open
along lines of weakness (ecdysial sutures).
The insect sheds its old exoskeleton (ecdysis) and
continues to fully expand the new one.
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Over the next few hours, sclerites will harden and
darken as quinone cross-linkages form within the
exocuticle.
This process (called sclerotization or tanning)
gives the exoskeleton its final texture and
appearance
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An insect that is actively constructing new
exoskeleton is said to be in a pharate condition.
During the days or weeks of this process there may
be very little evidence of change.
Ecdysis, however, occurs quickly (in minutes to
hours).
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A newly molted insect is soft and largely
unpigmented (white or ivory).
It is said to be in a teneral condition until the
process of tanning is completed (usually a day or
two).
Summary of Molting
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Step
Step
Step
Step
Step
Step
Step
Step
Step
1:
2:
3:
4:
5:
6:
7:
8:
9:
Apolysis -- separation of old exoskeleton from epidermis
Secretion of inactive molting fluid by epidermis
Production of cuticulin layer for new exoskeleton
Activation of molting fluid
Digestion and absorption of old endocuticle
Epidermis secretes new procuticle
Ecdysis -- shedding the old exo- and epicuticle
Expansion of new integument
Tanning -- sclerotization of new exocuticle
Exoskeleton traits
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fixed in size
new exoskeleton
Incorporates the changes that are part of
metamorphosis.
Initially soft and is larger than the old exoskeleton.
Therefore, change in body form comes about as a
series of steps.
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Stages between each molt are called instars.
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first stage which emerged from the egg is the first
instar or nymph.
-- to grow must shed its skin or molt
-- may be four or five instars before the adult stage is reached
Cicada Ecdysis
An adult cicada (Homoptera) just after
molting
Metamorphosis
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In nearly all insects growth involves a
metamorphosis, that is, a transformation in form and
in way of life, especially from larva to adult
Types of Metamorphosis
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anamorphosis
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epimorphosis
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prometamorphosis
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Incomplete metamorphosis
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hemimetamorphosis
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paurometamorphosis
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hyperpaurometamorphosis
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complete metamorphosis
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hypermetamorphosis
anamorphosis
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Abdomen segments added with each molt: 9-12
Little or no change between the immature and adult
form except in size and development of the sexual
organs.
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Protura
epimorphosis
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In a few very primitive, wingless insects (such as
the silverfish) there is no metamorphosis.
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The insect emerges from the egg as a miniature
adult and the only futher changes are in size and in
maturation of the reproductive organs.
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Collembola, diplura, thysanura
Prometamorphosis
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Nymphs of mayflies have 1245 aquatic instars, and wings are visible in older nymphs.
 Between the nymph stage and adult stage there is a
subimago
 The subimago is fully winged and flying
Mayfly
nymphs
Incomplete metamorphosis
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Incomplete, or gradual, metamorphosis is seen in
members of less advanced orders (such as locusts and
their relatives and the true bugs).
The larva, often called a nymph (or, if aquatic, a naiad)
is usually similar in form to the adult, but lacks wings.
The wings begin as external bumps on the larva, and
the adult emerges from the last molt without having
undergone a pupal stage.
Incomplete Metamorphosis
3 Insect Stages
Eggs
 Larvae
• Body form resembles adult
• No wings
 Adults
• No increase in size
• Reproduction
• Wings fully grown if present
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Example: Squash Bug
HOMOPTERA / HETEROPTERA (THE TRUE
BUGS: SPITTLE BUGS, APHIDS, ETC.)
paurometamorphosis
hemimetamorphosis
Of an insect with aquatic young undergoing
incomplete metamorphosis in which the young does not
resemble the adult
 The young stage is called naiads
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hyperpaurometamorphosis
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Thysanoptera are hemimetabolous
Their final immature stage is quiescent, non-feeding,
and enclosed in a silken cocoon.
This developmental stage, called a "pupa’’
Thrips represent an "intermediate" stage between
hemi- and holometabolous development.
Male scale
Complete metamorphosis
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Complete metamorphosis is characteristic of over
80% of all insect species
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The wingless, wormlike larva is completely unlike
the adult, and its chief activities are eating and
growing.
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After several molts the larva enters a quiescent
stage called the pupa; the pupa does not eat and
usually does not move, but within the exoskeleton a
major transformation occurs that involves the
reorganization of organ systems as well as the
development of such adult external structures as
wings and compound eyes.
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In some insects the pupa is enclosed in a protective
case, called the cocoon, built by the larva just
before pupation.
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When the transformation is complete the final molt
occurs: the adult emerges, its wings fill with blood
and expand, and the new exoskeleton hardens.
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The chief function of the adult is propagation; in
some species it does not eat.
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Do small butterflies grow up to be big butterflies?
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Do small butterflies grow up to be big butterflies?
Complete Metamorphosis
4 Insect Stages
– Eggs
– Larvae
– Pupae
• Transformation from larva to adult
• True legs, wings, antennae are formed
– Adults
• No increase in size
• Reproduction
• Short Life span
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LEPIDOPTERA
(BUTTERFLIES AND MOTHS)
COLEOPTERA (BEETLES)
DIPTERA (FLIES)
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Adult and immature insects with complete
metamorphosis feed on the different food
egg 1st 2nd
instar larva
3rd
pupa
adult
The Emergence Of A Monarch
The Monarch chrysalis is one of nature's most beautiful creations.
This butterfly wears a crown of gold on jade green.
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About 24 hours before
the emergence of the
adult butterfly, the
chrysalis
becomes completely
transparent, revealing
the new butterfly
inside.
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Breaking free of the
chrysalis, a Monarch
greets the world.
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After struggling free of
the chrysalis, the
Monarch immediately
begins to inflate its
wings with a reservoir of
blood contained in its
swollen abdomen.
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As the wings inflate,
the body of the
butterfly attains its
normal proportions.
When the wings are
fully inflated, the insect
expels any excess fluid
and rests.
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In a few hours, with its
wings dried and hardened,
the Monarch will take
wing on its first flight.
Hypermetamorphosis
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Meloidae or blister
beetles, undergo
what is referred to as
hypermetamorphosis.
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In this situation, growth
is by complete
metamorphosis, but there
are distinct changes in
external form and habits
at each
successive larval molt