Insect Cuticle
• Dynamic, not inert
• Functions as skin AND
skeleton
• Strong but transmits
information and
substances
• Gives shape, color,
pattern
SHAPE - the cuticle is
not a flat sheet
Outline
• Shape –macro and microstructure
• General cuticle structure: chitin, protein
• Factors changing cuticle properties:
sclerotization, water content
• Resilin - ‘cuticle’ without chitin
• Outer surface and coatings for special
properties: waxes, color
• Cuticle replacement by molting
scaffold for strength,
muscle attachments
• pleural (suture) (example)
• critical for flight
• major features
• microfeatures
Sculpture at many levels
• multiple cells
• one structure per
cell - cells
unspecialized
• specialized
hair and
socket cells
• multiple
structures per cell
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Functions of the two sections
Two major sections:
1. epicuticle
2. procuticle
epidermal cells
• The epicuticle and coatings made by
the cells give surface properties such as
waterproofing
• The procuticle, with its composite
structure, provides mechanical
properties such as stiffness and
elasticity
cuticulin
epicuticle
• Epicuticle –
made up of
cuticulin and
inner
epicuticle
epicuticle
formed by release
of material from
vesicles that
assemble under
envelope
cuticulin is produced at
plasma membrane
surface
Mostly highly
polymerized lipid
cuticulin
epidermis and
associated cells
• epidermal cells
• glandular cells
• oenocytes
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Cellular layer
• epidermal cells make new cuticle
• associated cells. For example,
oenocytes produce hydrocarbons,
lipids, and wax (icing on cuticle)
Procuticle
• What is it made of?
• How is it put together?
• How do the components vary to give
such a wide range of properties?
Cuticle is a composite material
Basic unit of chitin is
n-acetylglucosamine
• CHITIN fibers
• PROTEINS matrix
β - linkage
Chitin makes up as much as
half of the exoskeleton
forms long chains
3
Chitin fibrils form layers
Helical pattern
of layers
Means that
strength is same
in all directions
ƒchains interact with each other
ƒhydrogen bonds
ƒform microfibrils
Chitin
Composite
Materials
• n-acetylglucosamine units
• form chains
• form microfibrils
• form layers
• versatile, light, different properties
based on different combinations
• Growing field of materials
engineering and design that is
“bioderived and bioinspired”
ƒfibers stacked layers
ƒmatrix of proteins and other component
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Sclerotization
• hardening of the cuticle by chemical
interactions among components
Degree of sclerotization varies
in different body parts, stages,
species … etc
Proteins are key to diverse
mechanical properties
• interactions of protein with chitin
• interactions of protein with protein
• water content and pH change how proteins
interact
Across insect cuticle,
sclerotization varies
• Exocuticle =
hardened region
• Endocuticle=
not hardened
Exocuticle
Endocuticle
Regions of unsclerotized cuticle
give points/lines that can bend
a cuticle protein
• “cleft” full of aromatic
residues, which form “flat”
surfaces of aromatic
rings, for protein–chitin
interactions
• outer surface (lower side)
important for protein–
protein interactions in
cuticle.
5
How can proteins contribute to
different cuticle properties
(hard or soft)?
Making hard cuticle
n-acetyldopamine quinone
is common in sclerotized
(hard) cuticle
A protein in hard cuticle
• Histidines (blue)
are in right position
to participate in
sclerotization
• Or to be involved in
water binding
capacity of cuticle
A protein in soft cuticle
• lacks histidines
for sclerotization
additional hardening with
metal
• e.g., zinc in mandibles and ovipositor of
a wasp
Water
• Hard, stiff cuticles contain 15-35% chitin
and only 12 % water
• Soft cuticle contains equal parts chitin
and protein AND 40-75% water
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Effect of water content on
shear stiffness
• very small %
increase in water
makes huge
difference in
stiffness
insect cuticle shows a huge range of
stiffness across a very narrow range
of density
Some important factors are:
•Quinones
•Proteins and protein structure
•Metal
•Water content
Plasticization
• Rhodnius
• cuticle only 10% chitin
• increases water
content from 26 to
31% and increases its
extensibility from
about 10% to 100%
Young’s
modulus
=
stiffness
In some cases, properties can
change reversibly
Plasticization
• controlled by
hormones
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flea
Resilin
•
•
•
•
• Resilin in flea leg and
internal supports is a
key element in
building up energy
for a jump
contains NO chitin
rubber-like protein
stores energy
small bits are important in many insects
body parts
Dermaptera
• very important in
wing flexibility and
resilience
• blue areas
contain resilin
engineers at work
Resilin cloned
• Resilin gene cloned
into E. coli
• Product isolated
• Cross linked
photochemically
• Resilience is better
than man-made high
resilience rubber.
• Great potential in
biomedical
applications
Functions of the two sections
• The epicuticle and coatings,
made by the cells, give
surface properties
• The procuticle, with its composite structure,
provides mechanical properties such as
stiffness and elasticity
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epidermal cells have extensions that
reach up through the epicuticle
Pore Canals
Color
wax decorations
•
•
•
•
water barrier
reflection
camouflage
?other
Some Pigments
• Pterins - yellow, red, white
• Pigments
• Structural colors
• Ommochromes -yellow, red, brown
• Quinones - Homoptera only
Structural colors
• Entomologists don’t do optics,
physicists don’t do biology
• Entomological vocabulary has about 30
terms to distinguish shades of brown,
but only one for iridescence
3 main classes of iridescence
(color changes with angle)
• multilayer reflectors
• diffraction gratings
• photonic crystals (opalescent)
Seago et al. 2009. Gold bugs and beyond: a review of iridescence and
structural colour mechanisms in beetles. J. R. Soc. Interface 6, S165-S184.
9
Multilayer reflectors
Tiger beetles
Seago A E et al. J. R. Soc. Interface 2009;6:S165-S184
©2009 by The Royal Society
• unlayered
epicuticle
of a black
beetle
• layer spacing
has peak
green
reflectance
• layered
epicuticle of an
iridescent red
beetle
“Additive” coloration
• pointilistic disruption of even color
• another way to reduce iridescence
Seago A E et al. J. R. Soc. Interface 2009;6:S165-S184
©2009 by The Royal Society
10
Circularly polarized multilayer reflectors
Broadband multilayer reflectors
rare, only
in scarabs
one rotation=wavelength of light
analogous to cholesteric liquid crystal
Seago A E et al. J. R. Soc. Interface 2009;6:S165-S184
©2009 by The Royal Society
Seago A E et al. J. R. Soc. Interface 2009;6:S165-S184
©2009 by The Royal Society
Multilayer reflectors
• the broader the
range of
thicknesses, the
closer to pure
silver or gold
Physical color by diffraction
•
•
•
•
simple layered reflectors
additive color mixing (pointilistic)
circular polarizing reflectors
broad band reflectors
Butterfly scales
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The iridescent scales of the Morpho sulkowskyi butterfly
give a different optical response to different individual
vapours. This optical response dramatically outperforms
that of existing nano-engineered photonic sensors.
And every molt
they make a new one!
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What happens if these cells
make new cuticle?
new cuticle will form on top of
this larger epidermis
• It will be the
same size as the
one before
• FIRST, cell
division!
APOLYSISseparation of old cuticle
from epidermis, formation of space
• Molting
fluid
• New
cuticulin
and
epicuticle
Enzymes activated
Inner epicuticle produced
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Endocuticle
digested
Fluid
reabsorbed
Procuticle laid
down
Procuticle deposition a
2 stage process
pharate pupal stage inside
larval cuticle
• chitin and specific proteins that coat it
• then other proteins
ecdysis
Expansion, sclerotization
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