Insect Cuticle or The Insect Integument.pptx

INSECT
INTEGUMENT
Dr. Mandeep Rathee
CCSHAU, Hisar

INTEGUMENTARY
SYSTEM
•Introduction
•Function of the integument
•Evolutionary significance to arthropods
•Structure of the cuticle
•Chemical composition
•Sclerotization
•Physical properties of cuticle
•Specializations in cuticle
•Coloration and melanization

The Integument
 The outer covering or cuticle of
an insect, plus the epidermal
cells that secrete the cuticle is
called as integument.
 Many of the characteristics that
make insects unique
are attributable to the
integument which is mainly
composed of:
• Cuticle
• Epidermis
• Basement membrane

Transmission Electron Micrograph of Cuticle – Adult female of
Heliothis zea
Epicuticle
Exocuticle
Endocutie
Epidermal
cell
Basement
membrane

Functions of
The Integument
• Protection of internal organs and tissues.
• Protective barrier against entry of pathogens, pesticides,
parasites, and predators.
• Preventive barrier against water loss.
• Provides for the insect the sensory “windows to the
outside world”
• Also lines the tracheae, tracheoles, salivary glands and
portions of reproductive tract. At the molt, all of this is
shed.
• Protective barrier for foregut and hindgut.
• Integument functions in locomotion, feeding, excretion,
protection from desiccation, breathing.

Immature insects are able to encapsulate certain parasites and get
rid of them at the molts. Here, a house fly larva is shown with two
encapsulated nematodes that are then moved to the area beneath
the cuticle. At the molt, these parasites are expelled with the old
exocuticle.
Parasitic expulsion

Evolutionary significance
of arthropod integument
POSITIVE EFFECTS
• Prevention of water loss because of wax layer and cuticular composition.
• Smaller the organism the greater the amount of surface area per unit
volume-greater tendency to lose water.
• Preventative barrier for pathogens, parasites and predators.
NEGATIVE EFFECTS
• As size increases, problems with cuticle being too heavy and also gaseous
exchange (oxygen uptake) adversely affected
• Evolutionary experiment with Meganeuron shows that cuticle is
responsible for smaller size of insects.

1. The Cuticle
• In all insects there is a thin layer of cuticle with special properties and is
comprised of following layers :
1. Epicuticle
2. Exocuticle
3. Endocuticle
NOTE It can be as thin as 1 µm in the hindgut and over gills (e.g.
Ephemeropteran larvae) and as thick as 200 + µm (elytra of large Coleoptera).
There is always an epicuticle layer and the epidermal cell layer, but the
exocuticle and endocuticle may be greatly reduced or absent in particular
parts of the cuticle of the same or different insects.
Often there are additional cuticle layers within one or more of
the three principal layers as evidenced by electron density in cross sections
viewed in the transmission electron microscope.

A. The Epicuticle
 Complex and the outermost layer and
probably the most important layer.
 Responsible for water proofing and
general impermeability of cuticle.
 Produced by epidermis and dermal
glands.
Insects needing a renewable layer
Soil dwelling insects due to erosion
of the cuticle by abrasion

Significance of each layer of epicuticle
1. Cement layer :
• Outermost layer. Closely associated with wax layer and may serve to
protect it.
• Not found in all insects.
2. Wax layer:
• Hydrocarbons constitute 90% of this layer. Important to insects for
water loss, thus waterproofing of cuticle.
• In some(e.g., Fulgoridae and scales), the insects produce a large bloom
of wax on outside.
• Bees have special glands, wax glands, on ventral abdominal segments 4-
7 that produce wax, which is then formed into flakes used by the bees
to make their cells.

3. Outer epicuticle-
• Cuticulin is a very thin layer of protein about0.0075 μm thick, which is the
typical thickness of an animal cell membrane. It is impregnated with lipids,
some of which may be covalently bound to proteins as lipoproteins.
• First layer formed following the molt and is the layer that protects the new
procuticle from digestion by molting enzymes (i.e., chitinases and
proteinases). Also called cuticulin layer.
4. Inner epicuticle-
• Function not that clear but it is a much thicker layer(1µm) than the outer
epicuticle.
5. Wax canal filament-
• A filament of wax that is produced by the hypodermal cells and extends to
the inner part of the epicuticle.

• wax channels about 0.006 μm–0.013 μm in diameter) that are 10–20 times
smaller than pore canals.
6. Pore canals-
● Tiny pores that run from the hypodermal cells to the inner part of the inner
epicuticular layer. Inside the canals are wax filaments that extend up to the
epicuticular layer. Probably serve as a transport passage for wax from
hypodermal cells up to wax layer.
● Pore canals are passageways from 0.1 μm to 0.15 μm diameter extending
from the epidermal cells through the endo and exocuticle.
The pore canals can be very numerous; for example,
-1.2 × 106/mm2 in some cockroaches, or
-as few as 15,000/mm2 in a flesh fly larva.

Electron micrograph of a transverse section of the cuticle from a larva of Galleria
showing the fibrous structure of the lamellae in the endocuticle
and pore canals each containing an axial filament.
pcf, pore canal filament ; pc, pore canal.
(Osmium tetroxide, Araldite). X 42,000.

Electron micrograph of a thick transverse section to show the main features.
Ep, epicuticlc; End, endocuticlc; Epith, epithelium.
(Osmium tctroxide, Araldite). X 3,200.

B. The Exocuticle
•The exocuticle contains chitin and protein.
• It lies just beneath the epicuticle.
•It is highly sclerotized and is therefore hard and rigid.
•Layers within the exocuticle may refract light in such a
way to produce structural colors in some insects. Many
of the iridescent greens and blues of insects are
structural colors due to refracted light rather than to
pigments.
•The thickness of the exocuticle is variable and species
specific. Adult insects generally have a thicker and more
sclerotized exocuticle than larval insects.

• Region in which cross-linking of occurs
to give cuticle hardness.
• Region not broken down by proteases
at molt and is what usually remains in
form of exuviae.
• Melanin and other pigments found in
this area. Sclerotization mainly occurs
here.
• Also arthropodin and sclerotin.
Cross-section of Oncopeltus
showing exocuticle --------»

C. The Endocuticle
•Is continually being synthesized
(in a dark/light way-24 hrs) and
often is laid down in layers, thus
can often be used to age-grade
some insects.
•Contains most of the chitin, which
is broken-down at the molt by
chitinase. Little cross-linking of
proteins, thus most is broken-down
at molting and reabsorbed.
•In soft-bodied insects and regions
of flexibility (in the intersegmental
membranes), this layer is well
developed and not the exocuticle.

2. The Epidermis
• A single layer of cells, underlying the
cuticle and is responsible for its
production.
• It the only living portion of the
integument that is ectodermal in origin.
• These cells can be modified to form
dermal glands, sensory receptors and
oenocytes.
• Epidermal cells are involved in wound
repair, and can move from an area of
undamaged cells into an area of
destroyed cells.

• The sex pheromone in most female Lepidoptera is secreted
by a small patch of tall, columnar epidermal cells located
beneath the cuticle of the ventral inter-segmental membrane
in the 8–9th segment of the abdomen.
• The Golgi complex is prominent in epidermal cells, and
probably serves
several functions including the following:
(i) processing of secretory substances necessary to
synthesize cuticle, (ii) production of material for the plasma
membrane of the cell,
(iii) processing and packaging of lysozymes needed for
autophagy.

• Oenocytes are large, prominent cells scattered among the
epidermal cells, and also clustered at spiracles, near origin of
larger tracheae, and scattered among fat body cells.
• These cells are usually large, polyploid, and always have
extensive tubular smooth endoplasmic reticulum and well
developed plasma membrane reticular system.
• Their function is not very clear, but their morphology suggests
lipid secretion and lipid metabolism.

Basement membrane or Basal lamina
• Muco-polysaccharide layer that is secreted by the hemocytes .
• It is penetrated by nerves and tracheae going to hypodermis, and is a
selective barrier between haemolymph and epidermal cells.
• Hormones and other nutrients can pass through this selectively
permeable layer to reach the hypodermal cells.
• Its important in the recognition of ‘self’; thus, the insect’s blood cells do
not recognize it as ‘foreign.’
• Molecules in this layer are charged and probably act like a molecular
sieve.
• Extremely important in self recognition and cuticular integrity.

In this micrograph we added cationized ferritin molecules
and one can see that they are attracted to the anionized
sites in the basal lamina. This was in Phormia regina
follicle cells (fc). He=hemolymph

3. Chemical composition of the cuticle
Lipids
• Waxes-
Wax blooms produced by several groups of insects
• Cuticular hydrocarbons-
used by systematists.
Carbohydrates
• Chitin-polysaccharide - with units of N-acetyglucosamine residues. Chitin
gives the cuticle its strength, not its hardness.
Proteins
• Arthropodin -Untanned protein that during sclerotization is crosslinked to
produce sclerotin and give the cuticle its hardness.

CHITIN
N-acetylglucosamine
polymer
CHITOSAN
CELLULOSE

• Sclerotin- Tanned protein that gives cuticle hardness. This is
brought about by a process called sclerotization and involves
eclosion hormone and bursicon, another rhormone.
• Chitinase- Enzyme involved in digestion of chitin at the molt.
It is released and produced by the epidermal cells.
• Proteinases- Enzymes other than chitinase that aid in the
digestion of the endocuticle at the molts.

Resilin- Colorless rubber-like protein.
Can be stretched and stores energy
due to tension.
Places where it is found:
Wing hinges;
Food pump of reduviid bugs;
Hind legs of jumpers;
Aids in inspiration in beetles,
which lack inspiration muscles.
resilin
pad.

Transverse section through the thoracic wall and wing base
of a grasshopper showing the position of the wing hinge
and the resilin pad.

Sclerotization of cuticle
• The process whereby untanned or unlinked proteins become cross-
linked to form strong linkages that give the cuticle its hardness. This
process occurs mainly in the exocuticle area.
• The greater the degree of sclerotization, the harder the cuticle. The
content of chitin does not control hardness of the cuticle, but
sclerotization does.
• Because of the sclerotization, little or none of the exocuticle is
digested by molting fluid, and it is shed, along with the epicuticle, at
molting.

5. Physical properties of the cuticle
• Hardness
• Strength
• Flexibility
• Plasticization
• Ability to heal or repair wounds
• Hydrophobicity

• Hardness-
Is due to the amount of sclerotization or tanning of the proteins that
takes place. This involves cross-linking of the proteins. A process
often called tanning or sclerotization. Mainly occurs in the exocuticle.
• Strength-
Is due to the presence of chitin in the endocuticle.
• Flexibility
It is due to less proteins being sclerotized and is usually due to more
endocuticle being present than exocuticle. Also certain proteins, such
as resilin provide flexibility.

Scanning electron micrograph of the inner surface of a
mandible. X-ray microanalysis of the same mandible
showing the presence of Zinc in the mandibular cusps,
which gives greater strength to the mandible.

Relationship between sclerotization and cuticular stiffness
in the abdominal tergite of the honeybee around time of
eclosion. Sclerotization is expressed as the inverse of the
% of protein that is extractable. At the same time, the
stiffness of the cuticle also increases.

Flexibility of the cuticle. Extrinsic articulations where
thes clerotized parts meet outside the membrane.
FLEXIBLE REGION

At the time of laying eggs, the
desert locust’s abdominal
cuticle becomes plasticized.
This permits the ovipositor to
extend further into the soil so
that the eggs can be laid as
deeply as possible.
Plasticization

Openings or specializations in the cuticle
1. Openings in the cuticle
a. Pore canals
b. ‘Sweat pores’ of Sonoran desert cicada
2. Specialized areas of cuticle
a. Anal organ of dipterous larvae
b. Sense organs
c. Pheromone or deterrent chemical site release

Pore canals-Tiny pores that extend from the top of the
hypodermal cells to the inner most part of the epicuticle.
Believed to be involved in the transport of lipids from the
hypodermal cells to the epicuticle.
Pore canals

Sonoran desert cicada.
Pores 7X size of pore canals
located on dorsal mesonotum.
These are connected to special
dermal glands via cuticular ducts
are involved in water transport to
the surface.
Cooling of 2-5oC below ambient
of42-45oC.

Anal organ of dipterous larvae
Organ is involved in osmoregulation
in the dipterous larva. Structure tells
you something about its function-
thinner cuticle, larger epidermal
cells, and separaation from rest of
cuticle.

Formation of sensilla
Trichogen cell- Creates the
shaft
of hair
Tormogen cell- Creates the
socket
of the hair or sensillum
Thecogen cell- Creates the
sheath that surrounds the
neurons and isolates them
and provides the neuron
with ions and nutrients.

6. Coloration and melanization
• Insect colors are due to either:
a. Pigments in the exocuticle such as
melanin and they are usually lacking in
endocuticle
b. Physical structure of the cuticle to form
defraction gradients that defract light in
various ways. This results in the iridescence of the
blue morpho butterflies and gold of beetles, gold
on monarch chrysalis, and color of tiger beetles,
etc.

REFRENCES:
• Chapman, R.F., 1998. The Insects: Structure and Function, 4th
ed., Cambridge University Press, Cambridge, UK.
• Chitra K. C., 2006. A Text Book of Insect Physiology, Xpress
Graphics, Delhi, India.
• Wigglesworth, V. B., 1965. The Principles of Insect Physiology,
English Language Book Society and Methven and Co. Ltd.
• Images and pdf on insect cuticle from www.google.com.

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