Trilobites,
Chelicerates, and
Myriapods
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19-2
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Japanese Spider Crap
4 M in length
Follicle Mite
0.1 mm in length
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Characteristics
Relationships among arthropod subgroups

Divided into subphyla
 Centipedes,
millipedes are placed into
subphylum Myriapoda
 Insects are placed in subphylum
Hexapoda
 Spiders, and ticks form subphylum
Chelicerata
 Lobsters, crabs, barnacles, and others
form subphylum Crustacea
19-4
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2 Important Parts
in Arthropods:
Appendages and
Mouthparts
Uniramous: single branched
Biramous: 2 branches
Chelicerae: pointed
appendages to grasp food
Mandible: chewing appendage
19-6
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Arthropod Characteristics
WHY HAVE ARTHROPODS SURVIVED…(Top 5
Reasons) - 600 million years old
 1. Versatile Exoskeleton
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Cuticle is protective and jointed, providing mobility
Consists of inner thick molting procuticle (made of
exocuticle and endocuticle) and outer thin
epicuticle
Ecdysis, or molting
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Arthropods typically molt four to seven times
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19-7
Process of shedding outer covering and growing a new,
larger one
Controlled by hormones and environmental cues
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19-8
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Survival of Arthropods
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2. Segmentation and Appendages
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3. Metamorphosis
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19-9
Paired Appendages specialized for
division of labor (sensing, food handling,
walking, or swimming)
Larvae and adults feed on different
organisms and occupy different habitats
 Avoid competition
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Survival of Arthropods
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4. Air Piped Directly to Cells
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Terrestrial arthropods
 Use tracheal system for oxygen transport
directly to tissues
Aquatic arthropods
 Respire via gills
5. Highly Developed Sensory Organs
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19-10
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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Early Arthropod fossils: Trilobites and Eurypterid
Subphylum Trilobita - extinct for over 200 million years
19-11
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Subphylum Chelicerata
Chelicerata Characteristics
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Chelicerates have six pairs of
cephalothoracic appendages including
chelicerae (eating), pedipalps (mating) and
four pair of legs
Lack mandibles and antennae
Most suck liquid food from prey
Example: spiders, horseshoe crabs, sea
spiders, ticks, scorpions
19-12
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Spiders
Sea
Spiders
Horseshoe Crabs
19-13
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Horseshoe Crab: Illustrating 6 pairs of appendages
19-14
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Sea spiders: Illustrating
6 sets of appendages
Oviger: In males only,
carry developing eggs
19-15
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Subphylum Chelicerata
Class Arachnida
Structures
 Divided into two body parts: a cephalothorax
and an abdomen
 Cephalothorax bears ALL appendages
 Example: Spiders, Ticks, and Scorpions
19-16
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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
Pedipalps of males are modified, sometimes
elaborately, for sperm transfer
19-17
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Subphylum Chelicerata
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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
19-18
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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
Anterior appendages are a pair of chelicerae
with terminal fangs
Pair of pedipalps are used by males to
transfer sperm and handle food
Four pairs of walking legs terminate in claws
All are predaceous, mostly on insects
19-19
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External Anatomy of Jumping Spider
19-20
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Subphylum Chelicerata
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Injected venom liquefies and digests the tissues which
is sucked into spider’s stomach
RESPIRATORY SYSTEM (Book lungs or Trachea)
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Book lungs unique to spiders
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Parallel air pockets extend into blood-filled chamber
Air enters chamber through a slit in body wall
Trachea system
 Transports air directly to tissues/blood through an opening
called a spiracle
Excretory System

19-21
In spiders and insects, Malpighian tubules serve as excretory
structures
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Internal Anatomy of a Spider
19-22
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Subphylum Chelicerata

Sensory Systems
 Most spiders have eight simple eyes, each
with a lens, optic rods, and a retina
 Detect movement and may form images
(overall poor vision)
 Sensory hairs detect air currents, web
vibrations, and other stimuli
19-23
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Subphylum Chelicerata

Web-Spinning Habits
Spinning silk critical ability for spiders
 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

19-24
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19-25
Grasshopper snared in Golden Garden Spider silk.
If the spider is not hungry, it will save the live prey for later.
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Fisher Spider: Catches prey, paralyzes them,
pumps in digestive enzymes,
then sucks out predigested contents.
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Subphylum Chelicerata

Reproduction
Before mating, male stores sperm in
pedipalps
 Mating involves inserting pedipalps into
the female genital openings
 Use a courtship ritual
 Eggs may develop in a cocoon in the web
or carried by female
 Young hatch in about two weeks and may
molt before leaving the egg cocoon

19-27
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Wolf spider with egg sack
Black Widow with egg sack
19-28
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Subphylum Chelicerata
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Are spiders really dangerous?
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19-29
American tarantulas rarely bite, and bite is not
dangerous
Species of black widow spiders are dangerous
 Venom is a neurotoxin
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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Tarantula: Not Dangerous. Bite similar to Bee Sting
Bite when threatened or defending eggs/young.
19-30
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A: Black Widow displaying Red Hourglass
B: Brown Recluse Spider displaying violin marking on cephalothorax
19-31
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Subphylum Chelicerata
Order Scorpionida: Scorpions
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Approximately 1,400 species worldwide
Nocturnal and feed largely on insects and
spiders
Sand-dwellers locate prey by detecting
surface waves with leg hairs
Appendages attached to cephalothorax
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Pair of medial eyes and lateral eyes
Preabdomen has 7 segments
Postabdomen has long, slender tail of five
segments that ends in a stinging apparatus
19-32
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Scorpion epicuticle is fluorescent when exposed to UV light.
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Subphylum Chelicerata
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Stinger on last segment has venom that
varies from mildly painful to dangerous
Perform complex mating dances
Ovoviviparous or viviparous and produce from 1 to
100 young

19-34
Gestation is from several months to a year
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Emperor Scorpion with young.
Young stay with mother until their first molt.
(Gestation is up to a year, and 1 month on back)
Life span of a scorpion is about 15 years
19-35
Harvestmen: Order Opiliones
AKA- Daddy Longlegs
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Subphylum Chelicerata
Order Opiliones: Harvestmen
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Harvestmen or daddy longlegs
Approximately 5,000 species worldwide
Unlike spiders, abdomen and cephalothorax join
broadly
Can lose most of their eight long legs without ill
effect
Chelicerae are pincerlike - can’t bite a human
Are NOT venomous to humans
Mostly scavengers
Have 2 eyes, not 8
19-36
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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
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
19-37
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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
Adult mites and ticks possess 4 pairs of legs
Transfer sperm directly or by
spermatophores (packets of sperm)
19-38
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Wood Tick
All stages are ectoparasites
19-39
Red Velvet Mite: Trombicula
Similar to chiggers, only Larvae
is ectoparasite. Nymphs and Adults
are free-living.
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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 stage feed on dermal tissues and cause
skin irritation
Hair follicle mite Demodex

19-40
Harmless but may cause mild dermatitis
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Human Follicle Mite
19-41
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Subphylum Chelicerata

Human itch mite
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Causes intense itching
Popular in WWII - due to crowded conditions in
which people lived (Scabies)
Human Itch Mite
19-42
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Subphylum Chelicerata
Ticks:
2nd premier disease vector (after mosquito).
Some carry Lyme disease (bacteria)
Bacterial infection dicovered in 1970, in the
city of Lyme, Conneticut.
Transmit Rocky Mountain spotted fever (bacteria)
Transmits Texas cattle fever (Protozoan)
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Tick: Carries Texas Cattle Fever
19-44
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Subphylum Myriapoda
Characteristics
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Myriapods (“Many Footed”) include:
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Chilopoda (centipedes)
Diplopoda (millipedes)
Use trachea to transport respiratory gases to
all body tissue
Excretion usually by Malpighian tubules
19-45
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19-46
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Subphylum Myriapoda
Class Chilopoda
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Centipedes
Found under logs, bark and stones
Carnivorous, eating earthworms,
cockroaches, and other insects (use poison
claw) and then chew prey with mandible
Centipedes have one pair of legs on each
segment (except 1st and last 2)
Most harmless to humans


Few large, tropical centipedes are dangerous
Approximately 3,000 species worldwide
19-47
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Centipede from Amazon, Peru.
19-48
Head of a Centipede
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Subphylum Myriapoda
Characteristics
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Appendages of first body segment form
poison claws - comparable to wasp sting
Head has one pair of antennae
Eyes on either side of the head consist of
ocellus
19-49
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Subphylum Myriapoda
Reproduction
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Sexes separate
Most lay eggs and guard them (oviparous),
few are viviparous
Young resemble adults and do not undergo
metamorphosis
Legs and segments are added with each molt
19-50
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Subphylum Myriapoda
Class Diplopoda
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Millipedes (double footed) -10,000 species
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 - use mandible
Most are slow moving and roll into a coil for
defense
Some secrete toxic or repellant fluids from
special glands on side of body (hydrogen
cyanide)
19-51
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Tropical millipede.
19-52
Head of a Millipede
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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
Each abdominal segment has 2 sets of
legs
Head has 2 clusters of simple eyes and
a pair of antennae and mandibles
19-53
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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, as they molt more legs and body
segments are acquired
19-54
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Classification
Classification- Phylum Arthropoda
 Subphylum Trilobita (extinct)
 Subphylum Chelicerata
 Class Arachnida
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Order Araneae
Order Scorpiones
Order Opiliones (Daddy Longlegs)
Order Acari (Ticks and Mites)
Subphylum Myriapoda
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Class Diplopoda
Class Chilopoda
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Subphylum Crustacea
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Subphylum Hexapoda
19-55