Chapter 28 Notes
Arthropods
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Includes insects, centipedes, millipedes, spiders, ticks, scorpions, mites, lobsters, crab, crayfish
They range in size from 2mm (hairy beetle) to 4m (Japanese spider crab)
The phylum is arthropoda
There is 1 million known species of arthropods
2 out of every 3 animals living on earth are arthropods
What is an arthropod
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Despite enormous diversity, all share some common characteristics
They are all invertebrates
They all have bilateral symmetry
They all have coelom
They have an exoskeleton
They have jointed structure called appendages
An appendage is any structure such as a leg or an antennae that grows out of the body of the
animal
A. Sensing
B. Walking
C. Feeding
D. Mating
Advantages of jointed appendages
1. Arthropods were the 1st invertebrates to evolve jointed appendages
2. Joints allow for more powerful movement during locomotion
3. An appendage can be used in many different ways
A. The 2nd pair of appendage in spiders is used for sensing and mating
B. In scorpions, this pair is used for seizing prey
Arthropods exoskeleton give protection
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The exoskeleton is a hard, thick outer covering made of protein and chitin(fungal cell walls)
The exoskeleton protects and support internal tissues
It provides places for attachment of muscles
Some species(live on land) the exoskeleton is covered by a waxy layer that provides protection
against water loss
Why arthropods must molt
1. Disadvantages of exoskeletons
A. Heavy structures
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Exoskeletons cannot grow
Must shed
Molting is shedding of the old exoskeleton
Most arthropods molt from 4 to 7 times in their lives
Are vulnerable when they molt- must hide
Some arthropods have thinner and lighter exoskeleton have less protection but more
freedom to fly & jump
Segmentation in arthropods
1. In arthropods bodies, segment have become fused into one to three body sections
a. Head
b. Thorax
c. Abdomen
2. In some segments are fused- ex- the head & thorax is fused to form the “cephalothorax”
Arthropods have efficient gas exchange
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They are usually quick, active animals
They crawl, run, climb, dig, swim, fly, etc.
Some flies beat their wings 1000/second
Arthropods have evolved three types of respiratory structure for taking in oxygen
a. Gills
b. Trachea tubes
c. Book lung
5. Aquatic arthropods exchange gases through gills- they extract O2 from the water and release
CO2 into the water
6. Land arthropods have trachea tubes- branching network of hollow air passages- the opening on
the thorax & abdomen are called “spiracles” when air enters and leaves
7. Most spiders have “book lung”- they are like stacked plate or pages of a book and they serve for
gas exchange
Arthropods have acute sensors
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Movement, sound, or chemical can be detected with great sensitivity by “antenna”
Antenna are also used for communication between animals
“pheromones” are chemical odor signals given off by animals
Accurate vision is also important for the active lives of arthropods
Most have one pair of large compound eyes & from 3 to 8 simple eyes
A simple eye is a visual structure with only one lens- detects light
A compound eye has many lenses
However the image formed by the human eye
Arthropod nervous systems are developed by the human eye
1. Their nervous system consists of a double ventral nerve cord, anterior brain, and several ganglia
Arthropods have evolved other complex body systems
1. Blood is pumped by one or more hearts
2. Open circulatory system with vessels that carry blood away from the heart
3. Blood flows out of the vessel, bathes the tissues of the body, and returns to the heart through
open body spaces
4. Have complex digestive systems
A. Mouths, stomach, intestines, and anus
B. Various glands produce digestive enzymes
C. Mouthparts include a variety of jaws called mandible
D. Mouthparts are adapted for holding, chewing, sucking, and biting,
E. Most terrestrial arthropods excrete wastes through malphigian tubules
F. Another well developed system is the muscular system
1. An arthropod muscle is attached to the exoskeleton on both sides of the joint
Arthropods reproduce sexually
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Have separate males and females
Internal fertilization in land species- external in aquatic species
A few are hermaphrodites
Some exhibit parthenogenesis
A. They develop from an unfertilized egg
Ecology of arthropods
1. Arthropods are found in so many habitats because the enormous variety of adaptations that
have evolved for obtaining and digesting different foods
Arthropods and humans
1. Beneficial to humans in a variety of ways
A. Pollinate flowers
B. Provide food- honey, sea food, shellac, wax, silk, etc
C. Provide chemical control of other insects
D. Research advances in field of genetics, evolution, biochemistry, etc.- the fruit fly
E. Shells, artificial skin
Insects cause problems
1. They eat important crops
2. Spread disease- to plant and animals, humans- malaria- yellow fever
Origins of arthropods
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They most likely evolve from the annelids
Body segments fused
The head of the arthropod is more complex
The development of eyes, large brain
The exoskeleton
The arachnids
1. It includes spiders, scorpions, mites and ticks belong to the class arachnida
2. Spiders are the largest only about 1 dozen are dangerous to humans- in the U.S. the black
widow and the brown recluse
3. They have only 2 body regions
4. They have 6 pairs of jointed appendages
5. The first pair of appendages is called chelicerae
A. Located near the mouth and modified as pinchers to hold food or are the fangs- insect
poison
6. They have no mandibles for chewing
7. The 2nd pair of appendages are the pedipelps- they handle food and are sense organs and also
carry sperm during reproduction
8. The 4 remaining appendages in arachnids are modified legs for locomotion
9. No antennae
10. Most spiders spin webs from silk from silk glands
11. Silk is secreted by silk glands spun into threads by structures called spinnerets
Close relatives of spiders
1. Ticks and mites- have a single body section- head- thorax- abdomen are fused
2. Scorpions- many abdominal body segment and enlarged pinchers
3. Horseshoe crab
Crustaceans
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Most crustaceans are aquatic and exchange gases as water flows over feathery gills
All have mandibles for crushing food
2 pair of antennae for sensing
2 compound eyes
Their mandibles open and close from side to side
5 pairs of walking legs
Crab, lobster, shrimp, crayfish, barnacles, water fleas, and pill bugs are members of the class
crustacean
8. Some have 3 body sections and some 2
9. Pill bugs- are the only land crustaceans
Centipedes and millipedes
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Centipedes belong to the class chilopoda
Millipedes are members of the class diplopoda
Centipedes are carnivores eat soil, arthropods, snails, slugs, and worms
Their bite is painful to humans
Centipedes have:
A. Malphigian tubules for metabolic wastes
B. Tracheal tubes for gas exchange
6. Millipedes eat mostly plants
A. Do not bite
B. Have stink glands- spray
Insects
1. Flies, grasshoppers, lice, butterflies, bees, and beetles are just a few members of the class
insecta
2. By far the largest group of arthropods
Insect reproduction
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Insects mate once, or at most, only a few times during their lifetime
Eggs are fertilized internally and shells form around them
Most have an appendage mofified to dig and lay eggs in the ground
Females lay a large number of eggs
Increases the chance some will survive
Metamorphasis
1. Complete metamorphosis
A. Most insects go through 4 stages
1. Egg
2. Larva
3. Pupa
4. Adult
2. The larva in a caterpillar
3. Pupa- the quiet stage
4. Hatch into the beautiful butterfly
5. These 4 stages are known as complete metamorphosis
6. Is an advantage- larva do not compete with adults for food
Incomplete metamorphosis
1. Has only 3 stages
A. Egg
B. Nymph
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C. Adult
A nymph hatches from an egg- has the same general appearance as the adult but is smaller
Nymphs lack wings and cannot reproduce
Nymphs grow and become an adult
Grasshoppers and cockroaches have incomplete metamorphosis