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 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 cutaway view shows that the integument consists of a series of layers.
The integument is separated from the hemolymph by the basement membrane
the 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 to 30 chains lying parallel to each other and surrounded by protein.
The chitin-protein chains are deposited in the endocuticle as layers throughout the innermolt 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 the 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 epicutucle,
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.