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Arthropods
Chapter 46
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Jointed Appendages and Exoskeleton
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Arthropods all have jointed
appendages.
Rigid external skeleton
(exoskeleton).
 Protects animal and
provides sites for
muscle attachment.
- Brittle, thus there is a
limit to arthropod
body size due to
exoskeleton
thickness.
 Estimates of a quintillion
insects alive at any one
time.
- 1,000,000 species.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Classification of the Arthropods
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Traditional Classification
 Three subphyla based on
morphology.
- Trilobites (extinct)
- Chelicerates - (pointed
mouthparts,sometimes
connected to venom
glands)
- Mandibulates (chewing mouthparts,
sometimes modified)
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Classification of the Arthropods
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Doubts About Traditional Classification
 Key morphological traits are not as
powerful as previously assumed.
- Revised taxonomy places insects and
crustaceans as sister groups.
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General Characteristics of Arthropods
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Exoskeleton
 Molting (Ecdysis)
Compound Eye-insects
 Composed of many
ommatidia.
- Each covered with a
lens and linked to a
complex of eight retinal
cells and a light
sensitive core
rhabdom.
 Apposition Eyes - Daylight
 Superposition Eyes Night
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General Characteristics of Arthropods
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Circulatory System
 Greatly reduced coelom.
 Open circulatory system.
Nervous System
 Double chain of
segmented ganglia
running along the
animal’s ventral surface.
- Brain appears to be
inhibitor rather than
stimulator.
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
General Characteristics of Arthropods
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Respiratory System
 No single major respiratory
organ.
 Small branched air ducts tracheae.
- Air passes into trachea
through spiracles.
 Spiders contain book
lungs (series of leaflike
plates within a chamber).
Excretory System
 Malpighian Tubes
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Class Arachnida
Major Orders
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Have 2 body sections
One pair of chelicerae
& one pair of pedipalps
Four pairs of walking
legs.
 Most are
carnivorous, except
for mites.
Order Araneae: Spiders
Order Opiliones: Daddy Longlegs
Order Scorpiones: Scorpions
Order Acari: Mites & ticks
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Class Arachnida
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Order Opiliones: Daddy Longlegs
Acari: Mites and Ticks
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Order Scorpiones: Scorpions
Order Araneae: Spiders
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Class Merostomata
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Horseshoe Crabs
 Ancient Group.
 Live in deep water, but migrate to shallow
water in the spring.
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Class Pycnogonida
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Sea Spiders
 Very small (1-3 cm).
 Body almost entirely cephalothorax.
 Completely lack excretory and respiratory
systems.
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Class Crustacea
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Most have two pairs of antennae, three types
of chewing appendages, and various
numbers of leg pairs.
 All pass through nauplius larval stage.
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Crustaceans
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Decapods
 “Ten footed”
- Exoskeleton usually
reinforced with calcium
carbonate.
- Most body segments are
fused into cephalothorax
covered by carapace.
 Lobsters and crayfish
have swimmers and
uropods to aid in
swimming, and may
have a telson (tail
spine).
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The Myriapods
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Diplopoda-millipedes
Head region followed by
many identical body
segments.
Two pairs of walking legs
per segment, tubular body
Not venomous
Scavengers, herbivores
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Class Chilopoda-centipedes
Head and many identical
body segments
One pair of walking legs per
segment, body flattened
Venomous
Carnivores
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Class Insecta
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Largest group of organisms on earth.
 More than half of all named species on
earth are insects.
- Hectare of lowland tropical rainforest is
estimated to be inhabited by as many as
41,000 insect species.
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Class Insecta
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External Features
 Three body segments
- Head, Thorax, and Abdomen
 Three pairs of legs.
 One pair of antennae.
 Modified mouthparts
 Solid wings
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Class Insecta
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Internal Organization
 Tubular digestive system.
- Dilute digestive enzymes.
 Trachea extends throughout body.
 Fat Body for food storage.
Sense Receptors
 Sensory Hairs - Linked to nerve cells.
 Tympanum - Found with tracheal air sacs.
 Pheromones - Communication signals.
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Insect Life Histories
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Most hatch from fertilized
eggs laid outside mother’s
body.
Undergo edysis multiple
times.
 Instars
Metamorphosis
 Simple
- Nymphs
 Complete
- Pupa (chrysalis)
- Larvae
Controlled by hormones.
 Molting hormone
(ecdysone).
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies