Insect Cuticle and Moulting

Structure Of Insect Cuticle
@Bhubanananda
Adhikari
 Insect body wall is called as
Integument or exoskeleton
 Ectodermal in origion
 Variously modified (rigid, flexible,
lighter, stronger)
 Body wall consists of
a) Basement membrane
b) Epidermis
c) Cuticle

Structure Of Insect Cuticle
@Bhubanananda
Adhikari

@Bhubanananda
Adhikari
 Basal part of the body wall formed
from the degenerated epidermal
cells
 0.5μ in thickness and consists of
fibrous protein, collagen and
glycoproteins etc.
 Forms a continuous sheet beneath
the epidermis, where muscles are
attached.
a) Basement Membrane
Structure Of Insect Cuticle …

Structure Of Insect Cuticle…
@Bhubanananda
Adhikari
b) Epidermis
 Unicellular continuous layer formed from
polygonal cells (modifies in to cuboidal or
columnar during the process of moulting).
 Adjacent epidermal cells are held together by
desmosomes.
 All the epidermal cells are glandular and
secrete cuticle and the enzymes involved in
production and digestion of old cuticle .
 Absorb the digestion products of the old
cuticle and repair the wound.

@Bhubanananda
Adhikari
b) Epidermis
 The epidermal cells get differentiated in some
regions of the integument and constitute
various types of mechano- and chemoreceptor
organs.
i. Dermal glands : (Produce cement layer, secretion of
defensive substances, silk wax etc.)
ii. Trichogen cell: (produce hair like seta or trichome)
iii. Moulting glands : (Secrete moulting fluid)
iv. Peristigmatic glands : (around the spiracles )

@Bhubanananda
Adhikari
c) Cuticle
 Outermost thick layer of
integument.
 secretory product of epidermis.
 Determines surface pattern and
physico-chemical properties of
integument.
Function of the cuticle:
 Provide support and shape of insects.
 Muscles are attached to cuticle
 Invaginated to provide internal support
 Epicuticular wax layer helps in conservation
of water.
 Cuticular sense organs.
 Cuticular intima and peritrophic membrane
protects the epithelial membrane of
alimentary canal.
 Certain pigments of cuticle impart colour to
insects.
Cuticle is divided in to two regions
A) Upper epicuticle
B) Inner procuticle
(Endocuticle + Exocuticle)

@Bhubanananda
Adhikari
c.1. Cuticular components
 Two major components i.e, Chitin and Protein
Chitin: (named by Odier in 1834)
 Nitrogenous polysaccharide. (C8H13O5N)x.
 Consists of polymer of N-acetyl-D-glucosamine.
 The adjacent chitin chains are held together by β-glycosidic linkages.
 Chitin is insoluble in water, alcohol, organic solvents, but soluble only
in concentrated mineral acids and sodium hypochlorite.
 Chitin is synthesized by the insect epidermis. Its specific gravity is 1.4
(wt/vol)

@Bhubanananda
Adhikari
c.1. Cuticular components
 Two major components i.e, Chitin and Protein
 Arthropodin :  Soft water soluble protein present in
endocuticle.
 The conversion of arthopodin in to sclerotin is
known as sclerotization or tanning.
 Sclerotin : Tanned protein which is amber coloured and
present only in exocuticle.
 Resilin : Rubber like elastic protein which is colourless
and present in joints such as wing hinge
ligaments, leg joints.

@Bhubanananda
Adhikari
c.2. Epicuticle
Cement layer:
 The cement layer is very thin layer outside most of wax
layer and consists of mucopolysaccharide. It protects
underlying wax and not produced by all insects. It is
secreted by dermal glands and protects the body from
external damage.
Wax layer:
 The wax layer contains many different compounds but
wax comprises over 90%. It is consisting of long chain
hydrocarbons, esters of fatty acids and alcohols. It
serves as water proof layer preventing water loss from
the body.

@Bhubanananda
Adhikari
c.2. Epicuticle
Polyphenol layer:
 It is a non-static layer containing various types
of phenols which are mainly used in the
formation of the proteins. It is resistant to acids
and organic solvents.
Cuticulin layer:
 It is an amber coloured thin layer over the
surface of the epidermis which is strengthened
by outer polyphenol layer. It serves as
permeability and also acts as growth barrier. It
is lipoprotein in nature

@Bhubanananda
Adhikari
c.4. Endocuticle
 It is a thick layer which consists of mainly chitin
and proteins, the later being tanned by phenolic
substances to produce a hard, brown material
called sclerotin, which gives rigidity to cuticle.
 Many larvae have a soft thin cuticle over most of
their body . This cuticle has little or no exocuticle.
c.3. Exocuticle
 It is the innermost and thickest layer of cuticle,
which also contains chitin and protein. Since
the proteins are not tanned, this part is soft and
flexiable
Pore canals
 Vertical channels traversing both
exo and endocuticle measuring 0.15
to1μ in diameter.
 Run perpendicularly from epicuticle
through out the length of the
cuticle.
 Transportation of cuticular material
and enzymes from the epidermal
cells onto the upper surface of the
procuticle

@Bhubanananda
Adhikari
Endoskeleton of insect Body
Apodeme:
Hollow invagination of
body wall (ridge like)
Apophysis:
Solid invagination of
body wall (spine like)

@Bhubanananda
Adhikari
Cuticular Appendages
1. Non-cellular:
 No epidermal association
but rigidly attached
 Hairs and throns
1. Cellular:
 Epidermal association
 Unicellular or multicellular

@Bhubanananda
Adhikari
Unicellular Appendages
1. Clothing hairs, plumose hairs. e.g. Honey bee.
2. Bristles. e.g.flies.
3. Scales - flattened out growth of body wall e.g. Moths
and butterflies
4. Glandular seta. e.g. caterpillar
5. Sensory setae - associated with sensory neuron or
neurons
6. Seta
 hair like out growth from epidermis. Epidermal cell
generating seta is known as Trichogen, while the socket
forming cell housing trichogen is known as Tormogen.
Study of arrangement of seta known as Chaetotaxy.
Cuticular Appendages

@Bhubanananda
Adhikari
Multicellular Appendages
 Spur: Movavble
 Spine: Immovable

@Bhubanananda
Adhikari
Moulting
 The mechanism of periodical sheding of old cuticle
and formation of new cuticle is known as moulting
or ecdysis.
 The old cuticle which is shed off in the process of
moulting is known as exuviae.
 The interval between two subsequent moulting is
known as stadium. The form assumed by the insect
in any stadium is called instar.
 Moulting occurs many times during immatured
stages before attaining the adult-hood.

@Bhubanananda
Adhikari
Moulting
1. Behavioural Changes
2. Changes in epidermis
3. Apolysis (Detachment of cuticle from epidermis)
4. Formation of subcuticular space
5. Secretion of moulting gel
6. New epicuticle formation
7. Procuticle formation
8. Activation of moulting gel
9. Wax layer formation
10. Cement layer formation
11. Rupturing and removal of old cuticle
12. Formation of Exocuticle
13. Formation of Endocuticle
Apolysis
Ecdysis
Sclerotization and Melanization

@Bhubanananda
Adhikari
1. Behavioural Changes
 Larvae stops feeding and become inactive
 Neurosecretory cells of brain produce
prothoracico-tropic hormone
 Prothoracicotropic hormone stimulate
prothoracic gland to produce moulting
hormone
Moulting…
2. Changes in Epidermis
 Cell size, activity, protein content and
enzyme level increase in epidermal cells.
 Mitotic division starts ,increases tension
 Loosening of cells of cuticle
Apolysis

@Bhubanananda
Adhikari
3. Detachment of cuticle from epidermis
 Due to loosening and creation of pressure, cuticle
is detached from epidermis
Moulting…
4. Formation of subcuticular space
 Space between detached cuticle and epidermis is
called as sub cuticular space
5. Secretion of Moulting gel
 Secretion of moulting gel in the sub cuticular
space which is rich in chitinase and protease
Apolysis

@Bhubanananda
Adhikari
6. New Epicuticle formation
 A new epicuticle layer (cuticulin) formed and laid over epidermis.
Moulting…
7. Procuticle formation
 The process continues with formation of new procuticle layer
below the newly formed epicuticle (cuticulin).
8. Activation of Moulting gel
 Moulting gel is converted into moulting fluid which favours the
digestion process as well as absorption of old endocuticle. Only
a thin layer of the old endocuticle is left out.
 This layer is called as Ecdysial membrane. These lines are
located in the median lines of head and thorax.
Apolysis

@Bhubanananda
Adhikari
9. Wax layer formation
 Wax threads of pore canals secrete
wax layer
 Wax layer is deposited in new cuticle
Moulting…
10. Cement layer formation
 Dermal glands secretes cement layer
 Cement layer is otherwise known as
Tectocuticle
 Cement layer gives the strength of new
cuticle.
Apolysis

@Bhubanananda
Adhikari
11. Rupturing and removal of old cuticle
 Insect increases its body volume
through intake of air or water which
enhances the blood flow to head and
thorax.
 There by the old cuticle ruptures along
predetermined line of weakness known
as ecdysal line.
 Peristaltic movement of body and
lubricant action of moulting fluid helps
in the removal of old cuticle.
Moulting…
Ecdysis

@Bhubanananda
Adhikari
12. Formation of Exocuticle
 Upper layer of procuticle developed as new
exocuticle
 Addition of protein and tanning by phenolic
sdubstance
Moulting…
13. Formation of Endocuticle
Sclerotization and Melanization
 Lower layer of procuticle developed as new
endocuticle
 Addition of chitin and proteins, increase its
thickness

@Bhubanananda
Adhikari
Hormonal Control of Moulting
 Moulting is controlled by neurosecretory cells in the brain which in turn stimulate
Corpora allata (small glands behind the brain) which produce juvenile hormone
(JH)
 Prothoracic glands which produce moulting hormones (ecdysteroids). Ecdysteroids
stimulate the epithelial cells in the cuticle to begin the moulting process.
 The outcome of a moult is determined by the level of juvenile hormone. Juvenile
hormone suppresses adult characters
 Large amounts of juvenile hormone = larva -> larva
 Small amounts of juvenile hormone = larva -> pupa
 No juvenile hormone = pupa -> adult
Moulting…

@Bhubanananda
Adhikari
Hormonal Control
of Moulting
Moulting…
 Large amounts of
juvenile hormone =
larva -> larva
 Small amounts of
juvenile hormone =
larva -> pupa
 No juvenile hormone =
pupa -> adult

@Bhubanananda
Adhikari
Diagrammatic presentation Moulting

Insect Cuticle and Moulting

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