In most insects, the integument forms a rigid exoskeleton that surrounds the outer surface
of the animal.
The exoskeleton serves a variety of functions.
It gives the insect structure, prevents chemical and mechanical damage,
protects against invasion by parasites and infection by microorganisms,
inhibits water loss and serves as the attachment point of muscles
for locomotion.
It is also forms the trachea of the respiratory system, forms a lining for the foregut and hindgut
regions of the digestive system and forms the wings in adult insects.
This cut-away view shows that the integument consists of a series of layers.
The integument is separated from the hemolymph by the basement membrane – a connective
tissue layer comprised of glycoaminoglycans and proteins similar to collagen.
The epidermal cells are the living part of the integument.
Epidermal cells form a monolayer below the cuticle, and they secrete the overlying structural
layers, with the exception of the cement layer which is a product of the dermal glands.
Above the epidermal cells is the procuticle, a layer of protein intermixed with chitin.
Chitin is a complex polysaccharide comprised of mainly N-acetylglucosamine subunits mixed
with some glucosamine, and linked in chemically resistant beta-1,4 bonds similar to the inert
beta- glucose of cellulose.
Chitin gives the cuticle strength and stability and aids in water retention.
The insoluble chitin chains pack closely together to form microfibers of 15-30 chains lying
parallel to each other and surrounded by protein.
The chitin-protein chains are deposited in the endocuticle as layers throughout the intermolt
period.
Pore canals are minute tubular channels that extend from the epidermal cells through the
procuticle and end below the epicuticle.
The pore canals may be formed by cytoplasmic extensions of the epidermal cells as the procuticle
is formed following a molt.
Pore canals may provide an avenue for the transport of chemicals through the cuticle
and probably play a role in transporting the chemicals that comprise the structural parts
of the cuticle.
The chemicals may diffuse laterally from the canal to form the procuticle at the time of molting.
After the cuticle forms, the cytoplasmic extensions retract and the remaining channel becomes
the pore canal.
After the molt, the procuticle differentiates into the endocuticle and the exocuticle.
The thick, inner portion of the cuticle is termed the endocuticle.
It is usually the thickest layer of the cuticle and is soft and flexible.
Endocuticle is deposited throughout the time between molts.
Above the endocuticle is the exocuticle.
The exocuticle is the layer that gives the cuticle its hardness and rigidity.
Exocuticle becomes hard and rigid because it undergoes sclerotization or tanning.
Sclerotization is the cross-linking of proteins by quinones derived from polyphenols.
Sclerotization makes the exocuticle hard, strong and insoluble so it is resistant to
chemicals and mechanical damage and has low water permeability.
Sclerotization differentiates the original procuticle into the endo- and exocuticles.
Above the exocuticle is the epicuticle.
The epicuticle is thin and consists of four layers:
Cuticulin is the innermost epicuticle layer, and is composed of sclerotized proteins and lipids.
Some layers of the cuticle may be absent in regions of the body of some insect species,
but the cuticulin layer is always present.
A polyphenol layer is sometimes present above the cuticulin layer that may serve as a source
for the phenols used in tanning, A wax layer protects the insect from water
loss Pore canals may transport wax to the epicuticle,
and wax channels at the ends of the pore canals deposit the wax onto the inner epicuticle.
The wax consists of an inner monolayer of organized wax molecules and an outer "bloom" layer
of randomly mixed fatty acids and fatty alcohols.
Because insects are small animals, they have a large surface area relative to their volume
which means they have a potential for serious water loss through the cuticle, and the wax
serves to suppress cuticle transpiration.
The outermost cement layer is a product of the dermal glands and is comprised of lipids
and tanned proteins.
The cement layer is thought to protect the wax layer from abrasion, but it is variable
and may not always be present.
The insect exoskeleton is an effective integument, but, like a suit of armor, it restricts the
size that insects can attain, and its rigidity prevents growth except by replacing the existing
exoskeleton with a new, larger one by molting.
Let us see how an insect is able to molt to remove an exoskeleton that has become too
small, and replace it with a new one that allows for growth.
The molting process begins when cuticular epidermal cells are stimulated by exposure
to 20-hydroxyecdysone – the insect molting hormone.
The hormone enters the epidermal cells where it stimulates genes related to molting and
the formation of new cuticle.
The activated epidermal cells undergo mitosis or grow by cellular enlargement.
This is the period of growth to form a new, larger cuticle for the next instar.
The existing structural cuticle separates from the epidermal cells.
This is termed apolysis.
The ecdysial space between the endocuticle and the epidermal cells is filled with a gel
that contains inactive chitinase and protease enzymes.
A new outer epicuticle layer of cuticulin is secreted.
This new cuticulin layer protects the epidermal cells and newly forming cuticle from digestion
by the enzymes in the molting gel, which is then activated and becomes fluid.
The chitinase and protease enzymes of the molting fluid begin to digest the old endocuticle
As much as 90% of the chitin and protein breakdown products from the old endocuticle
are re-used by the epidermal cells to form a new procuticle.
Digestion of the endocuticle continues until it reaches the old exocuticle
The old exocuticle layer is resistant to enzyme action since it is sclerotized.
The remaining molting fluid is re-absorbed.
The wax layer and polyphenol layer of the new cuticle are deposited by the epidermal cells.
Just before the molt the cement layer is released by the dermal glands
Note that the section of old cuticle is smaller than the present region from which it came.
This is the result of the epidermal cell growth in the region from which the old cuticle was derived.
Molting, which is properly called ecdysis, occurs when the old exocuticle and epicuticle
are sloughed off.
The shed cuticle is called the exuvium.
A hormone called bursicon is released that stimulates the new procuticle layer that was
present at the time of the molt to undertake sclerotization by polyphenols and be converted to the
new exocuticle.
Once sclerotization is completed, no further sclerotization occurs during the remainder
of the instar.
During the time between the molts, new endocuticle is deposited continuously.
And the cycle starts over at the next molt.