Exoskeleton&molting of insects

EXOSKELETON

ByDarbaz AhmadShakar Ahmad

Insect Integument (Exoskeleton)

Outer epicuticle

Inner epicuticleEpicuticle

Exocuticle

Endocuticle

Schmidt’s layer

Epidermis

Basement membranePore canals

Procuticle

Insect Integument (Exoskeleton)

Outer epicuticle

Inner epicuticleEpicuticle

cement

waxoriented wax

cuticulin

Inner epicuticle

Exocuticle (procuticle)

pore canal

Insect Integument (Exoskeleton)

cement

waxoriented wax

WATER WATER

Wax Layers - Control of Water Movement

WATER IS RETAINED

Insect Integument (Exoskeleton)

NH

O = C - CH2

CH2OH

O

NH

O = C - CH2

O

N - acetylglucosamine polymer

OH H

H

OH

H

H

O

CH2OH

H

H

OH H

n

Insect Integument (Exoskeleton)

CO

NH CO

NH CO

NH CO

NH CO

NH CO

NH

CO

NH CO

NH

CO

NH CO

NH CO

NH CO

NH

hydrogen bonds

NH

O = C - CH2

CH2OH

O

NH

O = C - CH2

OH H

H

OH

H

H

O

CH2OH

H

H

OH H

Insect Integument (Exoskeleton)

N - acetylglucosamine polymer

microfibril

sheets of microfibrils

orientation of microfibrils changes

Insect Integument (Exoskeleton)

Protein Matrix

Insect Integument (Exoskeleton)

Cross linking of protein matrix

Insect Integument (Exoskeleton)

OH

Cross linking of protein matrix

OH

C

CH2

Protein Protein

OH

OH

CH2

CH2

O

O

CH2

CH2

OH

OH

CH2

CH2

Protein Protein

N-acetyl dopamine N-acetyl dopamine quinone

Insect Integument (Exoskeleton)

Tensile strength of sclerotized chitin (sclerotin)

kg/mm2

Molting

Molting

• Arthropods periodically shed exoskeleton to allow for growth and/or metamorphosis

• 7 steps during each molt cycle

1. Apolysis

• Retraction of epidermal cells from endocuticle

• Formation of subcuticular space

• Molting gel secreted

• New cuticle laid down

2. Epicuticle formation

• Epicuticle laid down

• It is extensively wrinkled

3. Procuticle deposition

• Formation of chitin microfibrils

• Endocuticular layers of old cuticle digested

• Enzymes in molting gel initially inactive

4. Ecdysis

• Old cuticle splits along middorsal suture

• Cast skin = epicuticle and exocuticle

• Endocuticle recovered and recycled into new procuticle

5. Expansion

• Insect swallows air

• Insect swells, removes wrinkles in epicuticle

6. Hardening and darkening

• New procuticle

stabilized

• Exocuticle formed

7. Endocuticle deposition

• Depositing chitin and protein takes time

• Some insects deposit one lamina of endocuticle every 24 hours

Note: Ecdysis under hormonal control