Ch.14 The Arthropods
Arthropod Characteristics
• 70-85% of all named animals species are
arthropods! The majority of which are
insects (beetles, flies, wasps, butterflies)
• Segmented invertebrates
• Bilateral Symmetry
• Protostome development
• 3 main body segments:
– Thorax: middle body region
– Abdomen: posterior end
– Cephalothorax: head
Arthropod Characteristics
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Metamerism & Tagmatization
Chitinous exoskeleton-provides support & protection
Paired, jointed appendages
Grow by molting (ecdysis)
Ventral nervous system
Small coelom
Open circulatory system
– Blood released into hemocoel (tissue space)
• Complete digestive system
• Metamorphosis usually present
– Different body forms at different stages of development
(reduces competition between immature and adult stages)
Exoskeleton
Provides structural support, protection, impermeable surfaces for
the prevention of water loss, and a system of levers for muscle
attachment and movement.
• Two layers:
– Epicuticle – outermost layer
• Waxy lipoprotein
• Impermeable to water
• Barrier to microorganisms & many pesticides
– Procuticle – inner layer, thickest
• Made up of Chitin (tough substance-think leather)
• Hardens by sclerotization (hardening & tanning process)
Jointed Appendages
• Appendages: structures, such
as legs and antennae, that
grow and extend from an
animal’s body.
• Arthropods have paired
appendages.
• Appendages of arthropods are
adapted for a variety of
functions such as feeding,
mating, sensing, walking, and
swimming.
• Arthropod appendages are
jointed-enable flexible
movement.
Feeding & Digestion
• The mouthparts of most arthropods include a pair of
appendages called mandibles that can be adapted for
biting or chewing.
• Other arthropods have mouthparts modified like feathery
strainers, stabbing needles, cutting swords, or sucking
straws.
• Arthropods can be herbivores, carnivores, filter feeders,
omnivores, or parasites.
• Arthropods have a complete, one way digestive system
with a mouth, gut, and an anus, along with various
glands that produce digestive enzymes.
Respiration
1. Gills
– Most aquatic arthropods have gills that function in the same
way as they do for mollusks
2. Tracheal tubes
– Terrestrial arthropods, tubes branch into smaller and smaller
tubules that carry oxygen throughout the body.
3. Book lungs
– Saclike pockets with highly folded walls for respiration,
folded walls increase surface area and allow for efficient
exchange of gases.
• Both tracheae and book lungs open to the outside of the body
of the arthropod in openings called spiracles.
Excretion
• Cellular wastes are removed
from the blood through
Malpighian tubules.
• Malpighian tubules are attached
to and empty into the gut, which
contains the undigested food
wastes to be eliminated from
the body.
• Crustaceans and some other
arthropods do not have
Malpighian tubules, they have
modified nephridia, similar to
those in annelids.
Response to Stimuli
Vision:
– Arthropods have large, compound eyes.
– A compound eye has many facets, which are
hexagonal in shape.
– Each facet sees part of an image and the
brain combines the images into a mosaic.
– Compound eyes can detect the movements
of prey, mates, or predators, and also can
detect colors.
– Many arthropods have 3-8 simple eyes as
well as their compound eyes! (Only
distinguishes light and dark)
Response to Stimuli
• Chemicals:
– Pheromones: chemicals secreted by many animal
species that influence the behavior of other animals of
the same species.
– Arthropods give off a variety of pheromones that signal
behaviors such as mating or feeding.
• Hearing:
– In addition to eyes, many arthropods also have another
sense organ called a tympanum-a flat membrane used
for hearing.
– It vibrates in response to sound waves.
– Arthropod tympanums can be located on the forelegs as
in crickets, or on the abdomen as in some grasshoppers,
or on the thorax as in some moths!
Reproduction
• Most arthropods reproduce sexually and have a
variety of adaptations for reproduction.
• Most arthropods have separate sexes, but a few such
as barnacles, are hermaphrodites and undergo crossfertilization.
• Most crustaceans brood, or incubate, their eggs in
some way, but they do not care for their hatched
offspring.
• Some spiders and insects also incubate their eggs,
and some, such as bees, care for their young!
Ecdysis (molting)
• Four stages
– 1. enzymes digest old
procuticle
– 2. new procuticle and
epicuticle secreted
– 3. old exoskeleton splits along
predetermined lines when the
animal stretches by air or
water intake & then shedding
occurs
– 4. calcium carbonate deposits
& sclerotization harden new
exoskeleton
Cicada Molting
Metamorphism
• Reduces competition between adults and
immature stages
• Is a radical change in body form and physiology
as an immature stage, usually called a larva,
becomes an adult.
Taxonomy of Phylum Arthropoda:
• Page 217,
Table 14.1
Subphylum Trilobitomorpha
• Extinct, lived in
oceans 600
million years ago
• Oval body & body
could roll into a
ball to protect its
ventral surface!
• 3 tagmata:
– Head
– Thorax
– Pygidium
Subphylum Chelicerata
• Means “many claws”
• Tagmata:
– Prosoma (Cephalothorax)
• Anterior
• sensory & feeding
– Opisthosoma
• Posterior to prosoma
• Contains organs for reproduction, digestion, respiration, and
excretion
– Paired appendages – for locomotion
• Chelicerae – pincerlike, feeding
• Pedipalps – sensory, feeding
• Paired walking legs
Class Merostomata
1. Giant water scorpion
• Extinct
• six-legged
• about 5 feet long &3
feet wide
2. Horseshoe crabs
– Only 4 species of horseshoe
crabs live today, they were
in existence 600mya
– Can be found in the Gulf of
Mexico!
– 2 Chelicerae, 2 pedipalps, 6
chelate walking legs, 2
digging (or swimming)
appendages
– 5 pairs of book gills
– Dioecious,external
fertilization (congregate in
intertidal areas to mate)
– Oviparous – lays eggs
– Eat small invertebrates
Class
Merostomata
Class Arachnida
• Order Scorpionida
– Scorpions
• Order Araneae
– Spiders
• Order Opiliones
– Harvestmen (daddy longlegs)
• Order Acarina
– Ticks & mites
• Mostly nocturnal
• 2 small chelicerae near
mouth, 2 large pedipalps
with pincers, 8 walking legs
• Most sting like a wasp, only
a few have deadly venom
• Dioecious, courtship can
last hours, mating is internal
• Most Ovoviviparous –
internal but, large yolky
eggs
• Some Viviparous – mother
provides nutrients for
embryos
• Development about 1.5
years for 20 to 40 young
• After birth, young crawl onto
mother’s back (one month)
Scorpions
Spiders
• About 34,000 species (largest arachnid group)
• 2 poisonous chelicerae with fangs, 2 pedipalps for sperm
transfer, 8 walking legs
• 6 to 8 eyes
• 6 to 8 spinnerets eject silk
• Silk protein comes out as a liquid, then hardens
• Ballooning – using silk line to travel hundreds of kilometers
• Bite prey, paralyze them, wrap them in silk, inject enzymes, &
suck victim dry
• Females attract males to web with pheromones
• Internal fertilization & oviparous (egg outside body)
Black Widow
Lactrodectus mactans
*Recognized by its shiny black body
with a red hourglass pattern on its
ventral surface
*Neurotoxin: “nerve poison” can
cause paralyses, difficulty of breathing,
vomiting, fever, & sweating
Brown Recluse
Loxosceles reclusa
•Hemotoxin: destroys red blood cells,
disrupts blood clotting, and causes tissue
damage.
•Recognized by the dark brown, violinshaped mark on the dorsal surface.
Daddy Longlegs
or Harvestmen
• Long legs
• Most are
omnivores,
some
predators
• Not harmful
to humans
Mites & Ticks
• Mites 1mm or less
– Free-living are herbivores & scavengers
– Ectoparasites, most stay attached a few hours or
days
– Chigger (redbug) larvae burrow in skin, drop off to
molt
– Harmless follicle mite is permanent-most of us
actually have this mite!
– Itch mite causes Scabies (skin disease that causes
extreme itching)
• Ticks up to 3cm
– Ectoparasites all throughout their life
– Copulation on host, females drop to ground to lay
eggs
Chigger
Scabies
Tick
Class Pycnogonida
• Sea spiders
• All Marine and
most common
in cold water
• Predators
• Dioecious and
males carry
eggs until they
hatch!
Subphylum Crustacea
• Class Malacostraca
– Crabs, lobsters, crayfish, shrimp, krill,
isopods, amphipods
• Class Branchiopoda
– Fairy shrimp, brine shrimp, water fleas
• Class Maxillopoda
– Copepods, barnacles
Class Malacostraca
Greek “malakos”=soft + “ostreion”=shell
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Crabs, lobsters, crayfish, shrimp, krill, isopods, amphipods
Omnivores & scavengers
Have gills
Hemocoel present
Compound eyes on movable eye stalks
Dioecious (except for barnacle)
Abdomen is reduced and is held beneath the Cephalothorax
– Head & Thorax (using crayfish as example)
• 2 pairs of antennae, grinding mandibles, 2 pair maxillae, 3 pairs of
maxillipeds, 5 pairs of walking legs (1st is large and chelate), males
have an extra 2 pairs of claspers), 5 pairs of swimmerets, telson with
uropods (flipperlike)
Reproduction
• Dioecious
• Males flip
females over and
deposit sperm
• “Sticky” eggs
develop on tale of
female
• Larval stages can
lasts months
Class Branchiopoda
• Fairy shrimp & brine shrimp
– Mostly freshwater
– During droughts, embryos become dormant in
capsules
• Water fleas (Daphnia!)
– Look like fleas with large carapace
– In spring, females reproduce parthenogenetically
(without fertilization)
– In winter, sexual reproduction produces winter eggs
that hatch in spring
Class Maxillopoda
• Copepods
– Some or the most abundant crustaceans
– Marine and freshwater
– Few live on substrate and filter feed, some
are predatory, others are parasites
• Barnacles
– Sessile adults
– Marine only and ~1,00 species
– Filter-feeders
– Mostly monoecious
– Many will colonize ships and boats, rocks,
and even other animals, like whales!
Subphylum Myriapoda
• 4 Classes:
– Diplopoda (millipedes)
– Chilopoda (centipedes)
– Symphyla (symphylans)
– Pauropoda (pauropodans)
Class Diplopoda
Millipedes
• Have 11-100 trunk segments
• 2 pairs of appendages on each
trunk segment
• Worldwide in distribution and
are found in or under leaf litter
or decaying logs
• Feed on decaying plant matter
• Millipedes roll into ball when
disturbed
Class Chilopoda
Centipedes
• Mostly nocturnal
• Scurry about the surfaces of logs, rocks, or other forest-floor
debris
• Single pair of long legs on each segment
• 15 or more trunk segments
• Fast-moving predators
• Food is small arthropods, earthworms, and snails and
sometimes even frogs and rodents
• Poison claws kill or immobilize prey…most venom is
harmless to humans, although many have bites comparable
to a wasp and a few human deaths have been reported from
large, tropical species
• Legs and segments are added with each molt