CHAPTER 19
Trilobites,
Chelicerates, and
Myriapods
19-1
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19-2
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Characteristics
Anthropodization
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Soft cuticle of the arthropod ancestors was
stiffened by deposition of protein and chitin
Joints had to provide flexibility
Sequence of molts was necessary to allow
for growth
Molting required hormonal control
Hydrostatic skeleton function was lost
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19-3
Coelom regressed and was replaced by open
sinuses
Motile cilia were lost
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Characteristics
Phylum Arthropoda
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19-4
Contains over 3/4 of all known species
Approximately 1,100,000 species of
arthropods have been recorded
Rich fossil history dating to late Precambrian
Eucoelomate protostomes with welldeveloped organ systems
Segmented
Molecular analyses indicate annelids and
arthropods evolved from different ancestors
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Characteristics
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19-5
Sizes range from the Japanese crab (four
meters in leg span) to the 0.1 mm long
follicle mite
Abundance and wide ecological distribution
makes them the most diverse animal group
Some are agents of disease and compete
with humans for food
Others are beneficial
All modes of feeding occur among
arthropods but most are herbivorous
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Characteristics
Relationships among arthropod subgroups
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Divided into subphyla based on relationships
between subgroups
Groupings among subphyla based on
molecular data
Centipedes, millipedes, pauropods, and
symphylans are placed in subphylum
Myriapoda
Insects are placed in subphylum Hexapoda
Spiders, ticks, horseshoe crabs and their
relatives form subphylum Chelicerata
Lobsters, crabs, barnacles, and others form
subphylum Crustacea
19-6
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19-7
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19-8
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Characteristics
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Also included in Crustacea are tongue worms
Extinct trilobites are placed in subphylum
Trilobita
Relationships are controversial
“Mandibulate hypothesis”
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19-9
Myriapods, hexapods, and crustaceans more
closely related due to a shared mouthpart, the
mandible
Molecular evidence of a close relationship
between hexapods and crustaceans unites
subphylum Crustacea with subphylum
Hexapoda in clade Pancrustacea
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Great Diversity and Abundance of Arthropods
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Versatile Exoskeleton
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19-10
Cuticle is highly protective and jointed, providing
mobility
Consists of inner thick procuticle and outer thin
epicuticle
Procuticle has an exocuticle secreted before a molt
and an endocuticle secreted after molting
Both layers of procuticle contain chitin bound with
protein
Procuticle is lightweight, flexible, and protects
against dehydration
Chitin content varies from 40% of the procuticle in
insects to as much as 80% in crustaceans
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19-11
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Great Diversity and Abundance of Arthropods
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Calcium salts responsible for hardness of
procuticle of lobsters and crabs
Cuticle is laminated and further hardened by
tanning
Cuticle is thin between segments, allowing for
movement at the joints
Ecdysis, or molting
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19-12
Process of shedding outer covering and growing a new,
larger one
Arthropods typically molt four to seven times
 Weight is a limit to ultimate body size
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Great Diversity and Abundance of Arthropods
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Segmentation and Appendages for Efficient
Locomotion
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Primitive pattern
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19-13
Linear series of similar somites with jointed appendages
Many somites may be fused or combined into
specialized groups called tagmata
Appendages often highly specialized for division
of labor
Limb segments are hollow levers with internal
striated muscles
Appendages may function in sensing, food
handling, walking, or swimming
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Great Diversity and Abundance of Arthropods
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Air Piped Directly to Cells
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Terrestrial arthropods
 Use an efficient tracheal system for oxygen
transport
Aquatic arthropods
 Respire via various forms of gills
Highly Developed Sensory Organs
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19-14
Eyes vary from simple light sensitive ocelli to a
compound mosaic eye
Other sensory structures for touch, smell,
hearing, balancing, and chemical reception
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Great Diversity and Abundance of Arthropods
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Complex Behavior Patterns
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Arthropods surpass most other invertebrates in
complex and organized activities
Most behavior is innate but some is learned
Use of Diverse Resources through
Metamorphosis
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19-15
Many arthropods undergo metamorphic changes
leading to different larval and adult stages
Larvae and adults feed on different organisms
and occupy different habitats
 Avoid competition
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Subphylum Trilobita
History of an Ancient Group
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Trilobites arose before the Cambrian,
flourished, and then became extinct 200
million years ago
Trilobed body shape due to a pair of
longitudinal grooves
Bottom dwellers and probably were
scavengers
Ranging from 2 to 67 centimeters long
Could roll up like pill bugs
Exoskeleton contained chitin strengthened by
calcium carbonate
19-16
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19-17
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Subphylum Trilobita
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Body was divided into a cephalon, trunk, and
pygidium
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Cephalon was a fusion of segments
Trunk varied in number of somites
Pygidium was fused into a plate
Cephalon bore antennae, compound eyes, a
mouth, and jointed appendages
Each body somite except the last had a pair of
biramous appendages
One of the branches of biramous appendage
was fringed
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19-18
May have been a gill
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Subphylum Chelicerata
Characteristics
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Chelicerates have six pairs of
cephalothoracic appendages including
chelicerae, pedipalps and four pair of legs
Lack mandibles and antennae
Most suck liquid food from prey
19-19
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19-20
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Subphylum Chelicerata
Class Merostomata: Subclass Eurypterida
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Eurypterids (giant water scorpions) were the
largest of all fossil arthropods
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3 m in length
Fossils date from Ordovician to Permian periods
Resemble both marine horseshoe crabs and
terrestrial scorpions
Head: 6 fused segments, 6 pairs of appendages,
simple and compound eyes, chelicerae,
pedipalps, and 4 pairs of walking legs
Abdomen: 12 segments and spike-like telson
19-21
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19-22
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Subphylum Chelicerata
Class Merostomata: Subclass Xiphosurida,
Horseshoe Crabs
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Modern horseshoe crab nearly unchanged from
ancestors in the Triassic period
5 species in 3 genera survive
Most live in shallow water
Structures
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19-23
Unsegmented carapace covers body in front of a
broad abdomen and a telson
Cephalothorax has 5 pairs of walking legs and a pair
of chelicerae
Abdomen bears six pairs of broad, thin, appendages
fused in the median line
Book gills exposed on some abdominal appendages
Carapace has 2 compound and 2 simple eyes
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Subphylum Chelicerata
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Walk with walking legs and swim with
abdominal plates
Feed at night on worms and small molluscs
During mating season, come to shore at high
tide to mate
Females burrow into sand to lay eggs
Males follow to add sperm before eggs are
covered
Young larvae hatch and return to sea at next
high tide
Larvae are segmented and resemble trilobites
19-24
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Subphylum Chelicerata
Class Pycnogonida: Sea Spiders
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All have small, thin bodies
Approximately 1,000 species
Some species duplicate somites, and may
have five or six pairs of legs
Males may have a pair of legs (ovigers) to
carry developing eggs
Many have chelicerae and palps
Mouth, at the tip of a proboscis, sucks juices
from cnidarians and soft-bodied animals
19-25
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19-26
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Subphylum Chelicerata
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Most have four simple eyes
Simple dorsal heart
No excretory and respiratory systems
Digestive system sends branches to the legs
Most gonads are in the legs
Occupy all oceans but most common in polar
waters
Some suggest that pycnogonids belonged to
an early-diverging arthropod lineage
19-27
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Subphylum Chelicerata
Class Arachnida
Great diversity
 Most are free living and more common in
warm, dry regions
Structures
 Divided into two tagmata: a cephalothorax
and an abdomen
 Cephalothorax bears a pair of chelicerae, a
pair of pedipalps, and 4 pairs of walking legs
 No antenna and mandibles
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19-28
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Subphylum Chelicerata
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Most are predaceous and have claws, fangs,
poison glands, or stingers
Sucking mouthparts ingest fluids and soft
tissues from bodies of their prey
Spiders have spinning glands
A few spiders may have a segmented
abdomen, a primitive character
Pedipalps of males are modified, sometimes
elaborately, for sperm transfer
19-29
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Subphylum Chelicerata
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Over 80,000 species have been described
Scorpions appeared on land in the Silurian,
mites and spiders by the end of the
Paleozoic Era
Most harmless to humans and provide
essential control of injurious insects
Some spiders are venomous and can cause
pain or death in humans
Ticks may carry human diseases
Mites can be crop pests
19-30
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Subphylum Chelicerata
Order Araneae: Spiders
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Approximately 40,000 species
Body consists of an unsegmented
cephalothorax and abdomen joined by a
slender pedicel
Anterior appendages are a pair of chelicerae
with terminal fangs
Pair of pedipalps have sensory functions and
are used by males to transfer sperm
Basal parts of pedipalps used to handle food
Four pairs of walking legs terminate in claws
All are predaceous, mostly on insects
19-31
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19-32
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Subphylum Chelicerata
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Injected venom liquefies and digests the
tissues which is sucked into spider’s stomach
Breathe by book lungs and/or tracheae
Book lungs unique to spiders
Parallel air pockets extend into blood-filled
chamber
Air enters chamber through a slit in body wall
Tracheae system is less extensive than in
insects
 Transports air directly to tissues
Tracheal systems of arthropods represent a
case of evolutionary convergence
19-33
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19-34
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Subphylum Chelicerata
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In spiders and insects, Malpighian tubules
serve as excretory structures
Potassium, other solutes, and waste are
secreted into tubules
Rectal glands reabsorb the potassium and
water, leaving wastes and uric acid for
excretion
Conserves water and allows the organisms
to live in dry environments
Many spiders have coxal glands, modified
nephridia, at the base of legs
19-35
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Subphylum Chelicerata
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Sensory Systems
 Most spiders have eight simple eyes, each
with a lens, optic rods, and a retina
 Detect movement and may form images
 Sensory setae detect air currents, web
vibrations, and other stimuli
 Spider’s vision usually poor
 Awareness of environment depends
largely on cuticular mechanoreceptors
such as sensory setae
19-36
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Subphylum Chelicerata
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Web-Spinning Habits
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19-37
Spinning silk critical ability for spiders and some
other arachnids
Two or three pairs of spinnerets contain
microscopic tubes that run to silk glands
Liquid scleroprotein secretion hardens as it is
extruded from spinnerets
Silk threads are very strong and will stretch
considerably before breaking
Silk is used for orb webs, lining burrows, forming
egg sacs, and wrapping prey
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19-38
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Subphylum Chelicerata
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Reproduction
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19-39
Before mating, male stores sperm in pedipalps
Mating involves inserting pedipalps into the
female genital openings
A courtship ritual is often required before the
female will allow mating
Eggs may develop in a cocoon in the web or may
be carried by female
Young hatch in about two weeks and may molt
before leaving the egg cocoon
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19-40
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Subphylum Chelicerata
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Are spiders really dangerous?
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19-41
Most people fear spiders without good reason
Spiders are allies of humans in our battle with
insects
American tarantulas rarely bite, and bite is not
dangerous
Species of black widow spiders are dangerous
 Venom is neurotoxic
Brown recluse spider
 Hemolytic venom that destroys tissue around
the bite
Some Australian and South American spiders are
the most dangerous and aggressive
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19-42
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19-43
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Subphylum Chelicerata
Order Scorpionida: Scorpions
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More common in tropical and subtropical
zones but occur in temperate areas
Approximately 1,400 species worldwide
Nocturnal and feed largely on insects and
spiders
Sand-dwellers locate prey by detecting
surface waves with leg sensillae
Appendages attached to cephalothorax
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Pair of medial eyes and 2–5 lateral eyes
Preabdomen has 7 segments
Postabdomen has long, slender tail of five
segments that ends in a stinging apparatus
19-44
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19-45
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Subphylum Chelicerata
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Under the abdomen are comblike pectines
 Explore the ground and aid in sex
recognition
Stinger on last segment has venom that
varies from mildly painful to dangerous
Ovoviviparous or viviparous and produce
from 6 to 90 young
Perform complex mating dances
 In some species the male stings the female
on pedipalp or on edge of cephalothorax
19-46
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Subphylum Chelicerata
Order Opiliones: Harvestmen
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Harvestmen or daddy longlegs
Common, particularly in tropical regions
Approximately 5,000 species worldwide
Unlike spiders, abdomen and cephalothorax
join broadly without a narrow pedicel
Can lose most of their eight long legs
without ill effect
Chelicerae are pincerlike
Mostly scavengers
19-47
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Subphylum Chelicerata
Order Acari: Ticks and Mites
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Medically and economically the most
important arachnids
About 30,000 species have been described
Both aquatic and terrestrial
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Inhabit deserts, polar areas, and hot springs
Most mites are less than 1 millimeter long
Ticks may range up to 2 cm
Complete fusion of cephalothorax and
abdomen
No sign of external segmentation
19-48
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Subphylum Chelicerata
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Mouthparts on tip of the anterior capitulum
Chelicerae on each side help pierce, tear, or
grip food
Other mouthparts include pedipalps with a
fused base, hypostome, rostrum, and tectum
Adult mites and ticks possess 4 pairs of legs
Transfer sperm directly or by
spermatophores
Egg hatches, releasing a six-legged larva
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19-49
Eight-legged nymphal stages follow
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19-50
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Subphylum Chelicerata
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House dust mites
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Spider mites
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One of many important agricultural pest mites
that suck out plant nutrients
Chiggers
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Free-living and often cause allergies
Larval Trombicula mites
Feed on dermal tissues and cause skin irritation
Hair follicle mite Demodex
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19-51
Harmless but other species cause mange in
domestic animals
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19-52
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19-53
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Subphylum Chelicerata
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Human itch mite
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Tick species of Ixodes
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Carry Lyme disease
Tick species of Dermacentor
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Causes intense itching
Transmit Rocky Mountain spotted fever
Cattle tick
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19-54
Transmits Texas cattle fever
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19-55
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19-56
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Subphylum Myriapoda
Characteristics
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Myriopods include
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Chilopoda (centipedes)
Diplopoda (millipedes)
Pauropoda (pauropods)
Symphyla (symphylans)
Use trachea to transport
Excretion usually by Malpighian tubules
19-57
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19-58
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Subphylum Myriapoda
Class Chilopoda
Natural History
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Centipedes
Found under logs, bark and stones
Carnivorous, eating earthworms,
cockroaches, and other insects
House centipede has 15 pairs of long legs
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Most harmless to humans
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Common in bathrooms and damp cellars
Few large, tropical centipedes are dangerous
Approximately 3,000 species worldwide
19-59
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19-60
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Subphylum Myriapoda
Characteristics
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Terrestrial and have flattened bodies with up
to 177 segments
Each segment except the one behind the head
and last two
 Bears a pair of jointed legs, the last pair of
which serves a sensory function
Appendages of first body segment form
poison claws
Head has one pair of antennae, a pair of
mandibles, and one or two pairs of maxillae
Eyes on either side of the head consist of
groups of ocelli
19-61
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Subphylum Myriapoda
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Salivary glands empty into anterior end of
straight digestive tract
Two pairs of Malpighian tubules empty into
the hind intestine
Elongated heart has pair of arteries in each
somite
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Ostia provide return flow of hemolymph
Pair of spiracles in each somite allows air to
diffuse through branched air tubes of the
tracheae
Arthropod nervous system includes a portion
that serves as a visceral nervous system
19-62
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Subphylum Myriapoda
Reproduction
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Sexes separate with unpaired gonads and
paired ducts
Some lay eggs and others are viviparous
Young resemble adults and do not undergo
metamorphosis
19-63
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Subphylum Myriapoda
Class Diplopoda
Natural History
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Millipedes
Less active than centipedes
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Walk with graceful rather than wriggling motion
Most eat decayed plants but a few eat living
plant tissue
Most are slow moving and roll into a coil for
defense
Some secrete toxic or repellant fluids from
special repugnatorial glands on side of body
More than 10,000 species of worldwide
19-64
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19-65
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Subphylum Myriapoda
Characteristics
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Cylindrical bodies with 25 to more than 100
segments
Short thorax consists of 4 segments, each
bearing one pair of legs
Head has 2 clusters of simple eyes and a pair
each of antennae, mandibles, and maxillae
Each abdominal somite has 2 pairs of
spiracles opening into air chambers and
tracheal air tubes
Two genital apertures located toward anterior
end
19-66
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Subphylum Myriapoda
Reproduction
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Appendages of seventh segment specialized
as copulatory organs
After copulation, female lays eggs in a nest
and guards them
Larvae have only one pair of legs per
segment
19-67
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Subphylum Myriapoda
Class Pauropoda
Life History
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Live in moist soil, leaf litter, decaying
vegetation, or under bark and debris
Least well known of myriapods
Characteristics
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Soft-bodied, small (2 mm or less)
Approximately 500 species
Head lacks true eyes, has branched
antennae, and a pair of sense organs
19-68
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Subphylum Myriapoda
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12 trunks segments bear 9 pairs of legs but
none on the last 2 segments
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A tergal plate covers each of the two segments
Lack tracheae, spiracles, and circulatory
system
Probably most closely related to diplopods
19-69
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19-70
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Subphylum Myriapoda
Class Symphyla
Life History and Reproduction
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Live in humus, leaf mold, and debris
Male Scutigerella places a spermatophore at
end of a stalk
Female stores the sperm in special pouches
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Removes and smears eggs with sperm before
attaching them to moss or lichen
Young hatch with only 6 or 7 pairs of legs
19-71
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Subphylum Myriapoda
Characteristics
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Small (2–10 mm) with centipede-like bodies
Soft-bodied with 14 segments
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12 segments bear legs and one bears a pair of
spinnerets
Antennae are long and unbranched
About 160 species are known
Eyeless with sensory pits at base of
antennae
Tracheal system connects to a pair of
spiracles on the head and tracheal tubes to
the anterior only
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Phylogeny and Adaptive Diversification
Phylogeny
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Relationships between subphyla are debated
Taxon of Pancrustacea, which includes
hexapods and crustaceans, is well-supported
Phylogenies using molecular data rarely
support grouping Myriapoda with
Pancrustacea
There is support for placement of Myriapoda
as the sister taxon for Cheliceratae
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Phylogeny and Adaptive Diversification
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Biologists assume that the ancestral
arthropod had a segmented body with one
pair of legs per segment
Evolution caused adjacent segments to fuse
and to make body regions
Hox gene studies indicate that the first five
segments fused to form the head tagma in all
four extant subphyla
In spiders, Hox gene studies indicate that the
entire prosoma corresponds to the head of
other arthropods
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Phylogeny and Adaptive Diversification
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Sea spiders remain within subphylum
Chelicerata because Hox gene studies have
found that their head appendage arose from
the region of the head that corresponds to
the second segment
Genetic studies have been helpful in
understanding the evolution of uniramous
and biramous appendages
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Molecular evidence repeatedly places hexapods
with crustaceans even though hexapods have
uniramous appendages and crustaceans have
biramous appendages.
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Phylogeny and Adaptive Diversification
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Leads to the question: Did uniramous appendage
development evolve more than once?
Numbers of appendages per segment is another
variable character among arthropods that lends
itself to more testing
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Phylogeny and Adaptive Diversification
Adaptive Diversification
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In contrast to annelids, arthropods have
pronounced tagmatization by fusion of
somites
Those with primitive characters have
appendages on each somite
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Derived forms are specialized
Modification of exoskeleton and appendages
allowed variation in feeding and movement
Adaptations made possible by cuticular
exoskeleton and small size fostered high
diversity
19-77
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Phylogeny and Adaptive Diversification
Classification
 Subphylum Trilobita
 Subphylum Chelicerata
 Class Merostomata
 Class Pycnogonida
 Class Arachnida
 Subphylum Myriapoda
 Class Diplopoda
 Class Chilopoda
 Class Pauropoda
 Class Symphyla
 Subphylum Crustacea
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 Subphylum Hexapoda