PHYLUM ARTHROPODA

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PHYLUM ARTHROPODA

• between 6-9 MILLION SPECIES
• 80 of described animal species
• are Arthropoda
• live in ALL
• environments and are
• tolerant of extremes

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ARTHROPODA BODY PLAN I
TAGMATIZATION - Tagma are fused
segments that form functional units - Tagma in
Arthropods are external segments not
reflecting internal metamerism
e.g head, thorax and abdomen in insects are tagma
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ARTHROPODA BODY PLAN II
EXOSKELETON
The exoskeleton of Arthropods is a unique
characteristic and a key feature in their
success Exoskeleton - protects against
predators - prevents dessication - provides
structural support - layered (wax layer, outer
epicuticle, inner epicuticle,
procuticle) - stiff and non-living thus needs
to be molted regrown - prevents water loss,
but also prevents exchange of gases.
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ARTHROPODA BODY PLAN II
EXOSKELETON
Wax layer and Outer epicuticle composed of many
layers of waterproofing materials (in
terrestrial forms) Inner epicuticle consists
mostly of tanned lipoproteins Procuticle It
contains microfibers of chitin surrounded by a
matrix of protein
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ARTHROPODA BODY PLAN II
EXOSKELETON

• PROCUTICLE
• After molting, procuticle is SOFT.
• Procuticle differentiates into hard
• exocuticle and softer endocuticle.
• Exocuticle hardened by a process called
• sclerotization or by addition of calcium
• carbonate (crustaceans, millipedes)
• Sclerotization protein molecules are
• cross-linked by quinone compounds.
• This solidifies protein matrix, forms sclerites
• Some areas in arthropods do NOT form
• exocuticle.

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ARTHROPODA BODY PLAN II
EXOSKELETON ANOTHER
VIEW
wax layer
epicuticle
procuticle
epidermis
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ARTHROPODA BODY PLAN II
EXOSKELETON
ANOTHER VIEW II
hairs (pile in insects) embedded in exoskeleton
muscles attach to inner surface of exoskeleton
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ARTHROPOD EXOSKELETON III Mechanics

• Flexible joints form lever system
• Exoskeleton provides high stiffness per weight
• (but theres the problem of buckling
  think aluminum can)

joint
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ARTHROPOD EXOSKELETON III Mechanics
Imagine we are looking at a very simplified
walking leg joint. One type of possible
joint but the weight of the insect would cause
the joint to buckle.
stiff
flexible
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ARTHROPOD EXOSKELETON III Mechanics
This is a more realistic look at the outside of
a walking leg. Weight can be placed on
foot without compressing leg but joint is still
flexible.
stiff
procuticle flexible
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ARTHROPOD EXOSKELETON III Mechanics/Molting
Segueway
How does animal grow in an exoskeleton? - the
only solution is to get rid of old
exoskeleton and then make bigger one. - this
solution leaves animal vunerable while new
exoskeleton is forming. - it also limits size
since if animal is large, it cannot support
itself during molt. - terrestrial arthropods
more size limited than those living in water.
Why?
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ARTHROPOD EXOSKELETON III Molting I
Molting also called ecdysis.
Tissue growth is dependent on amount of room
allowed in exoskeleton. Tissues can continue
to expand in exoskeleton only up to a point
then next molt must happen.
body length
tissue weight
time
molt 3
molt 2
molt 1
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ARTHROPOD EXOSKELETON III Molting II

• MOLTING PROCESS 5 steps
• premolt new cuticle is secreted underneath
  (inside)
• old cuticle

epicuticle
initial state
procuticle
epithelium
epicuticle
early premolt separation of epithelia from
epi-,procuticle
procuticle
epithelium
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ARTHROPOD EXOSKELETON III Molting II
MOLTING PROCESS 5 steps 1 cont. premolt new
cuticle is secreted underneath (inside) old
cuticle
epicuticle
early premolt separation of epithelia from
epi-,procuticle
procuticle
epithelium
old epicuticle
late premolt digestion of old matl. formation
of new.
old procuticle
new epicuticle
new procuticle
epithelium
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ARTHROPOD EXOSKELETON III Molting II

• MOLT old cuticle shed along
• perforations.

Some areas of exoskeleton not tanned (besides
joints). These are spots where cuticle splits.
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ARTHROPOD EXOSKELETON III Molting II Remaining
Steps

• 3. INFLATION rapid increase in size
• by inhaling water or air pushing out the
• new cuticle.
• Postmolt hardening of the skeleton
• by tanning, sclerotization.
• Intermolt tissue grows to fill in new
• cuticle.

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MOLTING - EXAMPLES
two molting nymphs shield-bugs
molting bush-cricket
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ARTHROPOD EXOSKELETON III Molting II Remaining
Steps
CUTICLE ALSO LINES THE GUT OF INSECTS SO
MOLTING ALSO INCLUDES BOTH EXTERNAL AND INTERNAL
BODY ELEMENTS.
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ARTHROPOD EXOSKELETON III Molting II Remaining
Steps
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ARTHROPOD EXOSKELETON III Molting III Hormonal
Control
How do arthropods control the timing of molting
process? Ecdysis is under hormonal
control Hormones chemical messengers produced
by gland cells, neuronal cells and carried in
blood. Interact only with certain cell types.
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ARTHROPOD EXOSKELETON III Molting III Hormonal
Control
corpus allatum
neuro-secretory cells
Hormone system best studied in insects that
eat blood meals. Blood meals trigger ecdysis.
HOW?
prothoracic gland
corpus candiacum
head thorax
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ARTHROPOD EXOSKELETON III Molting III Hormonal
Control
corpus allatum
neuro-secretory cells
Blood meal triggers neurosectory cells to
activate corpus cardiacum. Corpus cardiacum
simulates prothoracic gland to secrete ecdysone.
Ecdysone (molting hormone) released into blood.
Ecdysone stimulates epithelial cells to make
new cuticle.
prothoracic gland
corpus cardiacum
head thorax
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ARTHROPOD EXOSKELETON III Molting III Hormonal
Control
Trigger in non-blood sucking insects? May
be size? Photoperiod? In some insects,
photoperiod triggers other hormones which
eventually trigger ecdysone. Photoperiod timing
differs between species.
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ARTHROPOD EXOSKELETON III Molting III Hormonal
Control

• CRUSTACEAN HORMONE CONTROL
• Y-gland (in head) secretes molting hormone
• X-organ (neurosecretory cells in eyestalk)
  secretes
• hormone into blood sinus that inhibits Y-gland
• MOLTING
• stimulus shuts off X-organ
• Y-gland not inhibited
• Y-gland produces molting hormone
• epithelial cells produce new cuticle.
• molting hormone in crustaceans similar to
  ecdysone in insects.
• How could we know?

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ARTHROPOD EXOSKELETON III Molting IV Change in
Form
SILVERFISH no change in body form
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ARTHROPOD EXOSKELETON III Molting IV Change in
Form
BUG wings form late in adult stage nymphjuvenil
e, adult form winged.
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ARTHROPOD EXOSKELETON III Molting IV Change in
Form
Butterfly dramatic change in adult
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ARTHROPOD EXOSKELETON III Molting IV Change in
Form
Larval forms of insects can be quite different
from (or similar to) the adult form. WHAT
CONTROLS WHAT AN ARTHROPOD BECOMES IN NEXT MOLT?
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ARTHROPOD EXOSKELETON III Molting IV Change in
Form
corpus allatum
neuro-secretory cells
Corpus allatum secretes the juvenile hormone
(JH). IF JH present ? epidermal cells
secrete cuticle that it is juvenile in form
(juvenile hormone explicitly mediates
ecdysone) IF JH absent ? epidermis secretes
adult cuticle. What controls JH?
prothoracic gland
corpus candiacum
head thorax
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ARTHROPOD EXOSKELETON III Molting IV Change in
Form
young catepillar JH high so epithelial cells
make catepillar cuticle. old catepillar JH
lower. Epithelial cells make pupa. pupa JH
absent. Epithelial cells make adult cuticle
(and thus adult body form).
JH low
JH absent
JH high
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Gnathobases are used to help crush food as it is
collected