Arthropods
Chapter 36
Phylum Arthropoda
Section 36.1
Arthropods
• Examples: lobsters, crabs, spiders,
millipedes, centipedes, insects
• Arthropod means “jointed foot”
– Appendages: body segments with jointed
extensions
• Used in feeding, moving, & mating
Characteristics:
• Exoskeleton
– Protection, support, prevents desiccation
– 3 layers excreted by epidermis
• Compound eye
– Optic nerves send impulse to the ventral nerve
cord
• Open circulatory system
Molting:
• Periodic shedding of the exoskeleton
to allow growth
• Few day process to develop hard
exoskeleton – extremely vulnerable times!
• Hormone, pressure, and enzyme induced
– enzymes digest old exoskeleton while
synthesizing new one
Evolution:
• 545 million years ago
– All from a common ancestor
• Tagmata: fused segments that perform a
specialized function of modern arthropods
• 5 subphlya
Classification: 5 Subphyla
1. Trilobita:
–
•
(trilobites)
Extinct!
2. Crustacea:
–
(shrimp, lobster, crayfish, pill bugs, barnacles)
3. Chelicerata:
–
(spiders, scorpions, ticks, horseshoe crab)
4. Myriapoda:
–
(centipedes, millipedes)
5. Hexapoda:
–
(insects, springtails)
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Subphylum Crustacea
Section 36.2
General Information:
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•
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“Crusta” means shell
Most are aquatic
Defining characteristic = 2 pairs of antennae
Exoskeleton:
– Made up of chitin (protein) or CaCO3
– Carapace has waxy coating
– 16 to 20 body segments that fuse to form tagmatas
General Information:
• Many have mandibles
– jawlike mouthparts
• Usually each segment has at least 1 pair of
appendages
• Open circulatory system
• Many respire with gills (aquatic only)
• Nauplius
– Free swimming larvae
Crustacean Diversity
• Aquatic:
– Plankton – collection of small animals that drift
near the surface of the water
• Copepods (crustaceans) are a part of plankton
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–
–
–
–
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Water flea
Barnacles
Shrimp
Lobster
Crabs
Crayfish
• Terrestrial: Isopods
– Pill and sow bugs
– Lose water quickly
Crayfish
• Decapods – “ten feet,” five pairs of legs
THIS IS WHAT WE
WILL BE DISSECTING
ON MONDAY – WE
CORRECT SHOES!
External Structure
• Two major sections:
– Cephalothorax: two tagmata
• Head – 5 segments
• Thorax – 8 segments posterior to
head
• Carapace – dorsal exoskeleton
– Abdomen:
• 7 segments
• Telson – 7th segment,
– flat paddle at posterior
Appendages:
• Antennules – touch, taste, equilibrium
• Antennae – touch, taste
• Mandible – chewing
• Maxilla – manipulate food, draw water
over gills
• Maxilliped – touch, taste, manipulate food
• Cheliped (claws) – capture food,
defense
• Walking legs – locomotion over solid
surfaces
• Swimmeret – create water currents,
transfer sperm (male), carry eggs and
young (female)
• Uropod – propulsion during tail flips
Digestion:
• Digestive System: mouth  esophagus
 stomach  enzymes secreted by
digestive gland  intestine & digestive
gland  anus
Respiration:
• Respiratory System:
– gills
– Base of each walking leg under carapace
– Diffusion of gases
Excretion:
• Excretory System: green glands
– Acts like a kidney
– Eliminate excess water due to hypotonic
environment
Circulation:
• Circulatory System:
– open system baths organs in hemolymph
– Pumping organ: heart (dorsal)
Neural Control:
– Brain = pair of ganglia above esophagus
– Ventral nerve cord
– Sensory hairs on exoskeleton
– 2,000 light sensitive units on each eye
– Statocyte at base of antennae for
balance
Reproduction:
Mate in fall:
– Male’s first 2 hollow legs transfer sperm
– Female holds sperm until eggs laid, then
fertilize; mated previous fall
– Eggs laid in spring (100)
– carried by female on swimmerets 6-8
weeks; “berried”
– regenerate missing parts
Male
Female
Subphyla Chelicerata & Mryiapoda
Section 36.3
Subphyla Chelicerata
• Examples: spiders, scorpions, mites,
horseshoe crab
• Typically 6 pair of appendages
– 1st pair = chelicerae (modified pincers or
fangs)
Video
Class Arachnida:
• Arachnids include spiders, scorpions, mites,
and ticks
• Cephalothorax has 6 pairs of jointed
appendages:
– one pair of chelicerae (pincers or fangs)
– one pair of pedipalps (aid in holding food &
chewing)
– Four pairs of walking legs
Anatomy of a spider:
• Chelicerae = modified fangs
• 8 simple eyes
• Spinnerets = produce silk
Respiratory System
• Book lungs = paired sacs with parallel
folds
• Tracheae = system of tubes carrying air
directly to tissues
– Air in through spiracles in exoskeleton
Some have one or the other
Some have both!
Excretory System
• Malpighian tubules = hollow projections
of digestive tract
– Collect body fluids & waste
– Waste leaves as feces
– Helps conserve water in spider
Feeding:
• Use webs to capture prey
• Inject venom to paralyze animal
• Two harmful spiders to humans = black
widow & brown recluse
Reproduction:
1. Transfer of sperm to tip of pedipalps
2. Sperm place into seminal receptacles on
female
3. Male flees to avoid being eaten
4. Eggs fertilized as passed out of female
5. Fertilized eggs into silken case
6. Female carries eggs or attaches to plant
7. Young spiders hatch in two weeks
Scorpions
• Large, pincerlike pedipalps in forward
position
• Large stinger on last segment of abdomen
curled over body
– Hunt insects & spiders at night
– Inject venom into prey
video
Mites
• Completely fused cephalothorax & abdomen
• Many are free living, some are parasitic
• Causes mange in dogs
Chiggers
•
•
•
•
Larvae of harvest mites
Break vertebrae skin, feed on blood
Causes swelling & itching
Can attack humans 
Ticks
• Many parasitic
• Pierce host’s skin, feed on blood
• Transmits bacteria & microorganisms
– Lyme disease
– Rocky Mountain spotted fever
Subphylum Myriapoda
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Myriapods = “many feet”
Examples: millipedes & centipedes
First animals on land
Segmented bodies
Live in damp environments
Class Diplopoda
• Examples: millipedes
• Round bodies
• Appendages:
– Two pair of legs on each segment
– Short antennae
– Two groups of simple eyes
• When threatened: coil up & secrete noxious
fluid with cyanide
• Food source: decaying plant material
Class Chilopoda
• Example: Centipedes
• Flat bodies
• Appendages:
–
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–
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Long, jointed legs
Poison claws
Long antennae
Two clusters of simple eyes
• Prey = earthworms, insects, NOT humans!
Video