Post embryonic development of insects and metamorphosis
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4. INTRODUCTION
• Postembryonic development allows some of the most remarkable examples of
the diversity and ecology success of insect
• The distinguishing between juvenile & adult phenotypes can be extreme,
allowing different life stages to occupy very different ecological niches.
• During their postembryonic growth period insects pass through a series of
stages (instars) until they become adult, the time interval (stadium) occupied
by each instar being terminated by a molt.
• Juvenile insects have become more concerned with feeding and growth,
whereas adults form the reproductive and dispersal phase.
5. HATCHING
• Most insects force their way out of the egg by exerting pressure against the
inside of the shell.
• The insect increases its volume by swallowing the extra embryonic fluid & in
some cases by swallowing air which diffuses through the shell.
• Then waves of muscular contraction pump hemolymph forwards so that the
head & thoracic regions are pressed tightly against the inside of the shell.
• In grasshoppers & perhaps in other insects these muscular waves are
interrupted periodically by a simultaneous contraction of the abdominal
segments which cause a sudden increase in pressure in the anterior region.
6. • The dorsal membrane of the neck in grasshoppers has a pair of lobes –
cervical ampullae,which are inflated by the increase in hemolymph pressure.
They serve to focus the pressure on a limited area of the shell.
• If the shell does not split, the ampullae are withdrawn & a further series of
posterior to anterior waves of contraction follows ending with anterior or
another sudden abdominal contraction.one of these sudden contraction
ultimately ruptures the shell.
• The position of the rupture generally depend on where the insect puts pressure
on the chorion.
7.
8. Grasshopper chorion split --- transversely above the ampullae
Agabus ( water beetle ) longitudinal split of chorion
Egg of Blowfly have longitudinal hatching lines running along its length.
In Acrididae,Heteroptera produce digestive enzyme & secreted directly into extra
embryonic space.
Cuticular structure known as Egg bursters aid hatching in a number of insects (
usually on head )
1.Pentatomidae T or Y shaped central tooth
2.Cimicomorpha row of spines along each side of face
3.Fleas,Mosquitos & tse tse flies --- Cuticular tooth in the membrane depression
erected by blood pressure
etc……………………..
9. •Intermediate molt
it’s process of shedding of embryonic cuticle immediately after hatching. in
this process the larva swallows air & by further pumping ,splits embryonic cuticle
over the head
10. Hatching stimuli
The stimuli that promote hatching are largely unknown & in many cases insects appear
to hatch whenever they reach the appropriate stage of development.
Suitable temperature are necessary for an insect to hatch. Temperature is species
specific
1.Cimex - 8º C
2.Milkweed bug - 13º C
3.Desert locust - 20º C
Environmental stimuli
1.Chilo partellus (stem boring moth ) ----- high light intensity need for hatching
2.Aedes ( Aquatic insect ) ---- lower oxygen greater percentage of hatching
3.Agabus (Coleoptera ) ----- high oxygen greater percentage of hatching
11. FORMS OF DEVELOPMENT
• Through insect evolution there has been a trend toward increasing functional
and structural divergence between juvenile and adult stages.
• In modern insects three basic forms of postembryonic development can be
recognized, described as
1.Ametabolous
2.Hemimetabolous
3.Holometabolous
according to the extent of metamorphosis from juvenile to adult
12. AMETABOLOUS DEVELOPMENT
• In Thysanura (and other primitive hexapods), which as adults remain
wingless, the degree of change from juvenile to adult form is slight and is
manifest primarily in increased body size and development of functional
genitalia.
• Juvenile and adult apterygotes inhabit the same ecological niche, and the
insects continue to grow and molt after reaching sexual maturity.
• The number of molts through which an insect passes is very high and variable.
• For example,
in the firebrat, Thermobia domestica, between 45 and 60 molts have
been recorded.
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14. HEMIMETABOLOUS DEVELOPMENT
• In almost all exopterygotes the later juvenile instars broadly resemble the adult,
except that their wings and external genitalia are not fully developed.
• Early instars show no trace of wings, but, later, external wing buds arise as
sclerotized, non-articulated evaginations of the tergopleural area of the wing-
bearing segments. Wings develop within the buds during the final larval stadium
and are expanded after the last molt.
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16. • Other, less obvious, changes that occur during the growth of exopterygotes
include the addition of neurons, Malpighian tubules, ommatidia, and tarsal
segments, plus the differentiation of additional sensilla in the integument.
• This mode of development is described as hemimetabolous and includes a
partial (incomplete) metamorphosis from larva to adult.
• Exopterygotes usually molt a fixed number of times, but, with the exception
of Ephemeroptera, which pass through a winged subimago stage, never as
adults.
17. HOLOMETABOLOUS
DEVELOPMENT
• Holometabolous development, in which there is a marked change of form
from larva to adult (complete metamorphosis).
• Occurs in endopterygotes and a few exopterygotes,
for example,
Whiteflies (Aleurodidae: Hemiptera), Thrips (Thysanoptera), and
male Scale insects(Coccidae: Hemiptera).
• Most obvious structural difference between the larval and adult stages of
endopterygotes is the absence of any external sign of wing development in the
larval stages.
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20. LARVAL DEVELOPMENT
• Postembryonic development is divided into series of stages, each separated from next
by a molt.
• The form that insect assumes between molts ins known as Instars/stages/stadia.
• No further molts occur once the insect is adult except in Apterygota.
• During larval development there is usually no marked change in body form, each
successive stages being similar to proceeding one.
• But the degree of change from last larval stage to adult varies considerably & may be
very marked.
• These changes is called metamorphosis
loss of adaptive features peculiar to larva & gain of features peculiar to adult.
Metamorphosis occur in the absence of peculiar level of Juvenile hormone (JH).JH is
morphostatic hormone.
21. • A key finding illuminating the nature of metamorphosis is that the gene
encoding transcription factor --- broad Complex (brc)
• brc is expressed during molting in several insects in response to ecdysteroid
molting hormones, but only in the absence of JH
• brc thus appear to be a key determinant of metamorphic developmental
progression.
22. TYPES OF LARVA
• Larvae of hemimetabolous insects essentially resemble adults they are
sometimes called Nymphs (terrestrial form) & Naiads (aquatic form) ,
distinguish them from the more radically different larvae of holometabolous
insects.
• The most conspicuous difference between hemimetabolous & holometabolous
larva is in the development of the wings
• Hemimetabolous – wings develop as external buds which become larger at each
molt, finally enlarging to adult wings
• Holometabolous – wings develop in invagination's of epidermis beneath, the
larval cuticle & so are not visible externally. The invaginations are finally
everted so that the wings become visible externally when the larva molts to
pupa.
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25. • Many different larval forms occur among holometabolous insects.
1. Oligopod larva
2. Polypod larva
3. Apodous larva
26. Oligopod larva
• The least modified with respect to the adult is the oligopod larva.
• Oligopod larva which is hexapods with well developed head capsule & mouth
parts similar to adult, but no compound eyes.
• Two forms of oligopod larva is present
1.Campodeiform larva
which is well sclerotized, dorso-ventrally flattened & is usually a
long legged predator with a prognathous head.
eg: Neuroptera,Trichoptera,Strepsiptera & some Coleoptera
28. 2.Scarabaeiform larva
which is fat with poorly sclerotized thorax & abdomen, and which
is usually short legged & inactive, burrowing in wood or soil.
eg: Scarabaeoidea & some other Coleoptera
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30. Polypod larva
• A second basic form is the Polypod larva which is abdominal prolegs in addition
to thoracic legs.it is generally poorly sclerotized & is relatively active form
living in close contact with its food.
eg: Lepidopetra,Mecoptera,Tenthredinidae
Apodous larva
• Third basic form is Apodous larva which has no legs & has a poorly sclerotized
cuticle.
eg: Diptera
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35. • Different forms can be recognized according to degree of sclerotization
of head capsule.
1.Eucephalous
2.Hemicephalous
3.Acephalous
36. Eucephalous
With a well sclerotized head capsules, found in Nematocera
( Diptera),Buprestidae & Cerambycidae ( Coleoptera) & Aculeata
( Hymenoptera).
Hemicephalous
With a reduced head capsule which can be retracted within the
thorax. Found in Tipulidae & Orthorrhaphous brachycera ( Diptera)
Acephalous
Without a head capsule. Characteristic of the maggots of Cyclorrhapha
(Diptera)
40. HETEROMORPHOSIS
• In most endopterygotes the larval instars are more or less alike.
• However, in some species of Neuroptera, Coleoptera, Diptera,
Hymenoptera, and in all Strepsiptera, a larva undergoes characteristic
changes in habit and morphology as it grows, a phenomenon known as
heteromorphosis (Hypermetamorphosis).
• Heteromorphosis is common in predaceous & parasitic insects in which
change in habit occurs during course of development.
41. • For example,
blister beetles (Meloidae) hatch as free-living campodeiform larvae
that actively search for food .At this stage the larvae can survive for
periods of several weeks without food.
• Larvae that locate food soon molt to the second stage, a caterpillar like
(eruciform) larva. The insect then passes through two or more additional
larval instars, which may remain eruciform or become scarabaeiform.
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44. TYPES OF PUPA
• The pupa is a non-feeding, generally quiescent instar that serves as a mold in
which adult features can be formed.
• The term pupa & pupal stage are commonly used to describe the entire
preimaginal instar.
• Pupae are categorized according to whether or not the mandibles are
functional and whether or not the remaining appendages are sealed closely
against the body.
1.Decticous
2.Adecticous
1.exarate adecticous
2.obtect adecticous
45.
46. Decticous pupa
• Found in more primitive endopterygotes [Neuroptera, Mecoptera,Trichoptera,
and Lepidoptera (Zeugloptera and Dacnonypha)].
• have well-developed, articulated mandibles (moved by the pharate adult’s
muscles) with which an insect can cut its way out of the cocoon or cell.
• Decticous pupae are always exarate; that is, the appendages are not sealed
against the body so that they may be used in locomotion.
• Some neuropteran pupae, for example, can crawl and some pupae of
Trichoptera swim to the water surface prior to eclosion.
47. Adecticous pupae,
whose mandibles are non-functional and often reduced, may be either exarate
or obtect.
1. Adecticous obtect condition the appendages are firmly sealed against the
body and are usually well sclerotized.
In Nematocerous, Diptera, Lepidoptera (Heteroneura), and in a few Coleoptera and
Hymenoptera, pupae are of the adecticous obtect type
2.Adecticous exarate pupae condition the appendages are not firmly sealed
against the body.
And are characteristic of Siphonaptera, brachycerous ,Diptera, most Coleoptera and
Hymenoptera, and Strepsiptera.
48.
49. REFERENCE
1.Entomology - third edition
Gillot C (2005)
2.The insects, structure & function – fifth edition
Chapman R F (2013) Cambridge University press
