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What is an arthropod?
• A typical arthropod is a segmented,
coelomate invertebrate animal with
bilateral symmetry, an exoskeleton, and
jointed structures called appendages.
• An appendage is
any structure, such
as a leg or an
antenna, that grows
out of the body of
an animal.
What is an arthropod?
• Arthropods are the earliest known
invertebrates to exhibit jointed appendages.
• Joints are advantageous because they allow
more flexibility in animals that have hard,
rigid exoskeletons.
Arthropod exoskeletons provide
protection
• The exoskeleton is a hard, thick, outer
covering made of protein and chitin (KI tun).
Arthropod exoskeletons provide
protection
• The exoskeleton protects and supports
internal tissues and provides places for
attachment of muscles.
• In many aquatic species, the exoskeletons
are reinforced with calcium carbonate.
Why arthropods must molt
• A second and more important disadvantage
is that exoskeletons cannot grow, so they
must be shed periodically. Shedding the old
exoskeleton is called molting.
Why arthropods must molt
• Most arthropods molt four to seven times in
their lives before they become adults.
• When the new exoskeleton is soft,
arthropods cannot protect themselves from
danger because they move by bracing
muscles against the rigid exoskeleton.
Segmentation in arthropods
• In most groups of arthropods, segments have
become fused into three body sections—head,
thorax, and abdomen.
Segmentation in arthropods
• In other groups,
even these
segments may
be fused.
• Some arthropods
have a head and
a fused thorax
and abdomen.
Segmentation in arthropods
• In other groups, there is an abdomen and a
fused head and thorax called a
cephalothorax.
• Fusion of the body segments is related to
movement and protection.
Arthropods have efficient gas exchange
• Three types
of respiratory
structures
have evolved
in arthropods:
gills, tracheal
tubes, and
book lungs.
Arthropods have efficient gas exchange
• Most insects have tracheal tubes, branching
networks of hollow air passages that carry air
throughout the body.
Arthropods have efficient gas exchange
• Muscle activity helps pump the air through
the tracheal tubes.
• Air enters and leaves the tracheal tubes
through openings on the thorax and
abdomen called spiracles.
Arthropods have efficient gas exchange
• Most spiders and their relatives have book
lungs, air-filled chambers that contain leaflike
plates.
Arthropods have efficient gas exchange
• The stacked plates of a book lung are arranged
like pages of a book.
Arthropods have acute senses
• Antennae are also used for sound and odor
communication among animals.
Arthropods have acute senses
• Have you ever watched as
a group of ants carried
home a small piece of
food?
• The ants were able to work together as a
group because they were communicating
with each other by pheromones, chemical
odor signals given off by animals.
Arthropods have acute senses
• Antennae sense the odors of pheromones.
• Accurate vision is also important to the
active lives of arthropods.
• Most arthropods have one pair of large
compound eyes and three to eight simple
eyes.
• A simple eye is a visual structure with only
one lens that is used for detecting light.
Arthropods have acute senses
• A compound eye is a visual structure with
many lenses.
Arthropod nervous systems are well
developed.
• The nervous system consists of a double
ventral nerve cord, an anterior brain, and
several ganglia.
• Arthropods have ganglia that have become
fused. These ganglia act as control centers for
the body section in which they are located.
Arthropods have other complex body
systems
• Arthropod blood is pumped by a heart in an
open circulatory system with vessels that carry
blood away from the heart.
• The blood flows out of the vessels, bathes the
tissues of the body, and returns to the heart
through open body spaces.
Arthropods have other complex body
systems
• Arthropods have a complete digestive system
with a mouth, stomach, intestine, and anus,
together with various glands that produce
digestive enzymes.
• The mouthparts of most arthropod groups
include one pair of jaws called mandibles.
Arthropods have other complex body
systems
• Most terrestrial arthropods excrete wastes
through Malpighian tubules.
• In insects, the tubules are all located in the
abdomen rather than in each segment.
• Malpighian tubules are attached to and empty
into the intestine.
Arthropods reproduce sexually
• Most arthropod species have separate males
and females and reproduce sexually.
• Fertilization is usually internal in land
species but is often external in aquatic
species.
Arthropods reproduce sexually
• Some species, including bees, ants, aphids,
and wasps, exhibit parthenogenesis, a form
of asexual reproduction in which a new
individual develops from an unfertilized
egg.
• Reproductive diversity is one reason there
are more arthropod species than all other
animal species combined.
Objective 2: Subphylums for
Arthropods
• -Subphylum Trilobita:
Examples include trilobites.
All extinct forms; Cambrian
to Carboniferous; body
• divided by two longitudinal
furrows into three lobes;
distinct head, trunk, and
abdomen, biramous (twobranched) appendages.
• -Subphylum Chelicerata:
eurypterids, horseshoe
crabs, spiders, ticks. First
pair of appendages modified
to form chelicerae; pair of
pedipalps and four pairs of
legs; no antennae; no
mandibles; cephalothorax
and abdomen usually
unsegmented.
Objective 2: Subphylums for
Arthropods
• -Subphylum Crustacea:
crustaceans. Mostly aquatic,
with gills; cephalothorax
usually with dorsal
carapace; biramous
appendages, modified for
various functions; head
appendages consisting of
two pairs of antennae, one
pair of mandibles, and two
pairs of maxillae;
development primitively
with Nauplius stage
• -Subphylum Uniramia:
All appendages
currently thought of as
uniramous; head
appendages consisting
of one pair of antennae,
one pair of mandibles,
and one or two pairs or
maxillae. Examples
include: centipedes,
millipedes, and insects.
Objective 3: Classes of Chelicerata
• -Class Merostomata: aquatic chelicerates.
Cephalothorax and abdomen; compound
lateral eyes; appendages with gills; sharp
telson; subclass Eurypterida (all extinct)
and Xiphosurida, horseshoe crabs.
Examples include Limulus.
Objective 3: Classes of Chelicerata
• -Class Pycnogonida: Sea Spiders. Small
(3 to 4 mm). Some reach 500mm; body
chiefly cephalothorax; tiny abdomen;
usually four pairs on long walking legs
(some with five or six pairs); mouth on
long proboscis; four simple eyes; no
respiratory or excretory system. Example:
Pycnogonum.
Objective 3: Classes of Chelicerata
• -Class Arachnida: scorpions, spiders, mites, ticks,
harvestmen. Four pairs of legs; segmented or
unsegmented abdomen with or without appendages
and generally distinct from cephalothorax; respiration
by gills, tracheae, or book lungs; excretion by
Malpighian tubules and /or coxal glands; dorsal bilobed brain connected to ventral ganglionic mass with
nerves, simple eyes; chiefly oviparous; no true
metamorphosis. Examples Argiope, Centruroides.
Objective 4: Crustaceans
• Antennae: are also used for sound
and odor communication among
animals.
• Mandibles: The mandible of an
arthropod is a pair of mouthparts
used for either for biting, cutting
and holding food. Mandibles are
often simply referred to as jaws.
• Maxillae: Paired maxillae cut
food and manipulate it during
mastication. Maxillae can have
hairs and "teeth" along their inner
margins.
Objective 4: Crustaceans
•
•
•
•
•
•
Cephalothorax: an abdomen and a
fused head and thorax.
Gills: respiratory structures for
aquatic organisms which extract
oxygen from the water.
Rostrum: (beak) A snout like
projection on the head of an
arthropod.
Carapace: A shield like plate covering
the cephalothorax of certain
crustaceans.
Telson: A nonsegmented part of tail
fan of arthropods.
Uropod: The last abdominal segments
which make up the tail fan of
arthropods.