Chapter 28
Arthropods
Section 1
Characteristics of Arthropods
What is an Arthropod?
• A segmented, coelomate invertebrate
animal with bilateral symmetry, an
exoskeleton, and jointed structures
called appendages
What is an Appendage?
• An appendage is any structure that
grows out of the body of an animal (such
as a leg or an antenna)
– Arthropods are the earliest known
invertebrates to exhibit jointed appendages
• These appendages are adapted for many
purposes including: sensing, walking, feeding,
and mating
Why are jointed appendages
advantageous?
• Allow more flexibility in animals that have
hard, rigid exoskeletons
• Allow powerful movements of appendages
• Allow an appendage to be used in many
different ways
What is an exoskeleton?
• A hard, thick outer covering
• Made of protein and chitin
– Often reinforced with calcium carbonate in
aquatic species
• Can be continuous over the body or can
be made of separate plates held
together by hinges
What does an exoskeleton do?
• Protects and supports inner tissues
• Provides places for muscle attachment
• Provides protection against water loss
when covered by a waxy layer
What are the disadvantages of
an exoskeleton?
• Heavy…weight of the exoskeleton limits
the size arthropods
• Cannot grow…so they must be
periodically shed through the process of
molting
What happens during molting?
1. Before molting occurs, a new soft exoskeleton is
formed underneath the current one
2. When the new exoskeleton is ready the animal
contracts muscles and takes in water or air causing
the animal’s body to swell until the old exoskeleton
splits open
3. The old exoskeleton is shed
4. The animal’s body gets bigger due to increased blood
circulation (and sometimes also due to the animal
taking in air)
5. The new exoskeleton hardens in a larger size leaving
room for the animal to grow
What happens during molting?
• During molting arthropods are vulnerable
to predators because they are soft and
cannot use the exoskeleton for the bracing of
muscles (can’t move)
• So, during molting, arthropods have to hide
or remain motionless for a few hours or
days until their new exoskeleton hardens
How often does molting occur?
• Most arthropods molt 4 to 7 times
before they become adults
How are arthropods
segmented?
• In most arthropods, segments are fused into
three body sections – head, thorax, and
abdomen
– Some arthropods have a head and a fused thorax
and abdomen
– Some arthropods have an abdomen and a fused
head and thorax (called a cephalothorax)
– Some arthropods have all three body
sections fused together
Why are arthropod segments
fused?
• Fusion of segments protect the
arthropod better
– However, fusion of segments also limits the
movement of the arthropod
What type of respiratory
structures do arthropods have?
• Arthropods need to have efficient
respiratory structures that ensure rapid
oxygen delivery to cells because active
lifestyle of an arthropod requires a high
metabolism and a large oxygen
demand
What type of respiratory
structures do arthropods have?
• Three types of respiratory structures
have evolved in arthropods:
– Gills
– Tracheal tubes
– Book lungs
• (*Some arthropods also diffuse air directly across
the exoskeleton and body wall but this requires a
very thin and permeable exoskeleton)
What type of respiratory
structures do arthropods have?
• Tracheal tubes
– Found in most insects
– Consist of branching networks of
hollow air passages that carry air
throughout the body
– Details:
• Muscles pump air through the tracheal
tubes
• Air enters and leaves the tracheal tubes
through openings on the thorax and
abdomen called spiracles
What type of respiratory
structures do arthropods have?
• Book lungs
– Found in most spiders and spider relatives
– Consist of air-filled chambers that contain
leaf-like plates that are stacked or arranged
like the pages in a book
What type of respiratory
structures do arthropods have?
• Gills
– Found in aquatic arthropods
– Extract oxygen from the water and release
carbon dioxide into the water
What type of senses do
arthropods have?
• Arthropods have antennae
– Stalk-like structures
– Detect changes in the environment (such
as changes in chemicals, movement, or
sound)
– Used for sound and odor communication
• Antennae sense the odors of pheromones
(chemical odor signals) which signal the
arthropod to behave in a certain manner
What type of senses do
arthropods have?
• 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
– A compound eye is a visual structure with many
lenses
• Each lens contributes a tiny portion to the total
image allowing the arthropod to easily
detect movements and color
What type of nervous systems
do arthropods have?
• Have well developed nervous systems
that process information from sense
organs
– The nervous system consists of a double
ventral nerve cord, an anterior brain, and
several fused ganglia that act as control
centers for the body section in which they are
located
What other body systems do
arthropods have?
• Arthropods have an open circulatory
system
– A heart pumps blood into vessels that carry
the blood away from the heart
– Blood then leaves the vessels and goes
directly into the tissues
– Blood returns to the heart through open
body spaces
What other body systems do
arthropods have?
• Arthropods have a complete digestive
system with a mouth, stomach,
intestine, anus, and various digestive
enzyme producing glands
– The mouthparts of most arthropods include
one pair of jaws called mandibles that are
adapted for holding, chewing, sucking, or
biting various types of food
What other body systems do
arthropods have?
• Most terrestrial arthropods secrete
wastes through Malpighian tubules that
are located in the abdomen and empty into
the intestine
What other body systems do
arthropods have?
• Arthropods have well developed
muscular systems
– Muscles are attached to the inner surface of
the exoskeleton on both sides of joints
How do arthropods reproduce?
• Most arthropods have separate males and females and
reproduce sexually
– Fertilization is usually internal in terrestrial species but is often
external in aquatic species
• A few species (like barnacles) are hermaphrodites
• Some species (such as bees, ants, aphids, and wasps)
can reproduce through parthenogenesis which is a type 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