1. Chapter 30
Arthropods

2. Section 28.1 Summary – pages 741 - 746
I. Features of Arthropods
A typical arthropod is a segmented, coelomate invertebrate animal with bilateral symmetry, an exoskeleton, and jointed structures called appendages.
A. Jointed appendages:
“joint foot”=arthropoda
An appendage is any structure, such as a leg , an antenna, or mouthpart that grows out of the body of an animal.
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What is an arthropod?
Joints also allow powerful movements of appendages, and enable an appendage to be used in many different ways.
Mouthparts have sucking, ripping or chewing parts
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4. Section 28.2 Summary – pages 747 - 755
Origins of Arthropods
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Origins of Arthropods
Arthropods most likely evolved from an ancestor of the annelids.
Oldest, best –preserved
multicellular animal fossils
600 million years old
Most numerous early arthropod: trilobites
-became extinct about 250 million years ago
Lived in the sea
Segmented bodies
Jointed appendages
First animals with eyes capable of forming images.
First terrestrial arthropods: scorpions
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6. Section 28.2 Summary – pages 747 - 755
The total number of arthropods
Exceeds that of all other kinds of animals
combined .
5,000,000 species
More species of beetles than vertebrates.
Size varies from 80 micrometers (parasitic mite)- to 3.6 m (giant crab found in the sea near Japan.)
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7. Two main groups Arthropods with jaws 1.-Uniramia – (subphylum) insects
chilopoda and diplopoda
2. Crustacea- (subphylum) shrimp, crab lobster
Arthropods with fangs and pincers
1. Chelicerata (subphylum) – scorpions, mites,
spiders
***Each subphyla represents a distinct evolutionary line.

8. Section 28.1 Summary – pages 741 - 746
Arthropod Body Plan
Segmentation in arthropods
In most groups of arthropods, segments have become fused into three body sections—head, thorax (mid body region), and abdomen.
Individual body segments often exist only in larval stages. (ex: catepillar)
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Segmentation in arthropods
In other groups, even these segments may be fused.
Some arthropods have a head and a fused thorax and abdomen.
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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.
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Arthropods have acute senses
A compound eye is a visual structure with many lenses.
See motion much more quickly than humans.
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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.
In dragonflies and locusts, these simple eyes function as horizon detectors. –helps them stabilize their position in flight.
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Arthropod exoskeletons provide protection
The exoskeleton is a hard, thick, outer covering made of protein and chitin (KI tun).
Brittle and can break easily
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Arthropod exoskeletons provide protection
In some species, the exoskeleton is a continuous covering over most of the body.
In other species, the exoskeleton is made of separate plates held together by hinges.
Crustaceans : thick relatively inflexible exoskeleton.
Insects and arachnids: soft and flexible exoskeleton.
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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.
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Why arthropods must molt
Exoskeletons have their disadvantages.
First, they are relatively heavy structures. The larger an arthropod is, the thicker and heavier its exoskeleton must be to support its larger muscles.
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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.
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Why arthropods must molt
When the new exoskeleton is ready, the animal contracts muscles and takes in air or water.
This causes the animal’s body to swell until the old exoskeleton splits open, usually along the back.
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Why arthropods must molt
Before the new exoskeleton hardens, the animal puffs up as a result of increased blood circulation to all parts of its body.
Thus, the new exoskeleton hardens in a larger size, allowing some room for the animal to continue to grow.
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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.
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Respiration: Arthropods have efficient gas exchange
Arthropods have efficient respiratory structures that ensure rapid oxygen delivery to cells.
This large oxygen demand is needed to sustain the high levels of metabolism required for rapid movements.
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Arthropods have efficient gas exchange
Three types of respiratory structures have evolved in arthropods: gills, tracheal tubes, and book lungs.
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Arthropods have efficient gas exchange
Aquatic arthropods exchange gases through gills, which extract oxygen from water and release carbon dioxide into the water.
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Arthropods have efficient gas exchange
Land arthropods have either a system of tracheal tubes or book lungs.
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Arthropods have efficient gas exchange
Most insects have tracheal tubes, branching networks of hollow air passages that carry air throughout the body.
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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.
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Arthropods have efficient gas exchange
Most spiders and their relatives have book lungs, air-filled chambers that contain leaflike plates.
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Arthropods have efficient gas exchange
The stacked plates of a book lung are arranged like pages of a book.
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Excretion:
Most terrestrial arthropods excrete wastes through Malpighian tubules.
Slender, fingerlike extensions from the arthropods gut that are bathed in blood.
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.
Metabolic wastes remain in the gut and leave the body through the anus.
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31. Section 28.2 Summary – pages 747 - 755
II. Spiders and other Arachnids
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What is an arachnid?
Spiders, scorpions, mites, and ticks belong to the class Arachnida (uh RAK nud uh).
Spiders are the largest group of arachnids.
Spiders and other arachnids have only two body regions—the cephalothorax and the abdomen.
They have no antennae
Arachnids have six pairs of jointed appendages.
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What is an arachnid?
The first pair of appendages, called chelicerae, is located near the mouth.
chelicerae
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What is an arachnid?
Chelicerae are often modified into pincers or fangs.
Pincers are used to hold food, and fangs inject prey with poison.
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What is an arachnid?
Spiders have no mandibles for chewing.
Using a process of extracellular digestion, digestive enzymes from the spider’s mouth liquefy the internal organs of the captured prey. The spider then sucks up the liquefied food.
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What is an arachnid?
The second pair of appendages, called the pedipalps, are adapted for handling food and for sensing.
pedipalps
Sometimes the pedipelps are for reproduction.
Following the pedipalps, are 4 pairs of appendages called walking legs.
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What is an arachnid?
Although all spiders spin silk, not all make webs.
Spider silk is secreted by silk glands in the abdomen.
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What is an arachnid?
As silk is secreted, it is spun into thread by structures called spinnerets, located at the rear of the spider.
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A Spider
Legs
Simple eyes
Cocoon
Silk glands
Book Lungs
Pedipalps
Chelicerae
Spiders are predatory animals, feeding almost exclusively on other arthropods.(=carnivores)
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Ticks, mites, and scorpions: Spider relatives
Ticks and mites differ from spiders in that they have only one body section.
tick
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Ticks, mites, and scorpions: Spider relatives
The head, thorax, and abdomen are completely fused.
Plant mites- while feeding may pass viral and fungal infections to plants.
Dust mites- live in carpet, bedding, clothing. (cause allergies)
Chiggers-known for their irritating bite
Ticks feed on blood from reptiles, birds, and mammals.
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Ticks, mites, and scorpions: Spider relatives
Mites feed on fungi, plants, and animals.
They are so small that they often are not visible to the unaided human eye.
Like ticks, mites can transmit diseases.
Lyme disease is spread by deer ticks
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Ticks, mites, and scorpions: Spider relatives
Scorpions are easily recognized by their many abdominal body segments and enlarged pincers.(=pedipalps)
They have a long tail with a venomous stinger at the tip. Which is used to stun their prey.
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Insects
Flies, grasshoppers, lice, butterflies, bees, and beetles are just a few members of the class Insecta.
Insects have three body segments (head thorax and abdomen) and six legs.
Head- mandibles (chewing mouth part) pair of antennae, compound eyes
Thorax - (composed of 3 fused segments.)
3 pair of jointed walking legs and some have wings that are attached to the thorax.
Abdomen- is composed of 9-11 segments
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Arthropods have other complex body systems
The mandibles, together with other mouthparts are adapted for holding, chewing, sucking, or biting the various foods eaten by arthropods.
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47. Section 28.2 Summary – pages 747 - 755
A Grasshopper
Antennae
Legs
Eyes
Wings
Spiracles
Malpighian tubules
Tympanum
Nervous System
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Insect reproduction/Life Cycle
Insects usually mate once during their lifetime.
The eggs usually are fertilized internally.
Some insects exhibit parthenogenesis, reproducing from unfertilized eggs.
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Insect reproduction
Most insects lay a large number of eggs, which increase the chances that some offspring will survive long enough to reproduce.
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Metamorphosis: Change in body shape and form
After eggs are laid, the insect embryo develops and the eggs hatch.
In some wingless insects development is direct; the eggs hatch into miniature forms that look just like tiny adults.
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Metamorphosis: Change in body shape and form
These insects go through successive molts until the adult size is reached.
Molt
Nymph
Eggs
Nymph
Molt
Adult
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Metamorphosis: Change in body shape and form
In some cases, the adult insect bears little resemblance to its juvenile stage.
Adult
Egg
Larva
Pupa
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Metamorphosis: Change in body shape and form
This series of changes, controlled by chemical-substances in the animal, is called metamorphosis.
Insects that undergo metamorphosis usually go through four stages on their way to adulthood: egg, larva, pupa, and adult.
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Metamorphosis: Change in body shape and form
The larva is the free-living, wormlike stage of an insect, often called a caterpillar.
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Metamorphosis: Change in body shape and form
The pupa stage of insects is a period of reorganization in which the tissues and organs of the larva are broken down and replaced by adult tissues.
Usually the insect does not move or feed during the pupa stage. After a period of time, a fully formed adult emerges from the pupa.
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Metamorphosis: Change in body shape and form
The series of changes that occur as an insect goes through the egg, larva, pupa, and adult stages is known as complete metamorphosis.
Complete metamorphosis is an advantage for arthropods because larvae do not compete with adults for the same food.
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Metamorphosis: Change in body shape and form
larvae
fertilized egg
The complete metamorphosis of a butterfly is illustrated.
adult
pupa
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Metamorphosis: Change in body shape and form
Other insects that undergo complete metamorphosis include ants, beetles, flies, and wasps.
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Incomplete metamorphosis has three stages
Many insect species, as well as other arthropods, undergo a gradual or incomplete metamorphosis, in which the insect goes through only three stages of development.
These three stages are egg, nymph, and adult.
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Incomplete metamorphosis has three stages
A nymph, which hatches from an egg, has the same general appearance as the adult but is smaller and wingless.
Molt
Nymph
Eggs
Nymph
Molt
Adult
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Incomplete metamorphosis has three stages
Nymphs cannot reproduce.
As the nymph eats and grows, it molts several times. With each molt, it begins to resemble the adult more.
Gradually, the nymph becomes an adult.
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Incomplete metamorphosis has three stages
Grasshoppers and cockroaches are insects that undergo incomplete metamorphosis.
Incomplete metamorphosis of a harlequin bug
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Social Insects
Two orders of insects, Hymenoptera (bees) and Isoptera (termites) have elaborate social systems.
They have division of labor. Each having their own job. (worker, drone, soldier, queen ,king)
The role played by the individual in a colony is called its caste.
Its Caste is determined by a combination of heredity, diet, hormones and pheromones
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Insect relatives
Centipedes and Millipedes
Like spiders, millipedes and centipedes have Malpighian tubules for excreting wastes.
In contrast to spiders, centipedes and millipedes have tracheal tubes rather than book lungs for gas exchange.
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Centipedes and Millipedes
Centipedes are carnivorous and eat soil arthropods, snails, slugs, and worms.
Centipedes have one pair of legs /segment
The bites of some centipedes are painful to humans.
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Centipedes and Millipedes
A millipede eats mostly plants and dead material on damp forest floors. (herbivores)
Millipedes do not bite, but they can spray foul-smelling fluids from their defensive stink glands.
Millipedes have 2 pair of legs /segment
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Crustaceans
Members of the class Crustacea include crabs, lobsters, shrimps, crayfishes, water fleas, pill bugs, and barnacles.
Barnacles are sessile
Many have a distinctive larval form called a nauplius .
The nauplius has 3 pair of branched appendages and undergoes a series of molts.(pg 680)
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Crustaceans
Sow bugs and pill bugs, two of the few terrestrial crustaceans, must live where there is moisture, which aids in gas exchange.
Most crustaceans are aquatic and exchange gases as water flows over feathery gills.
Shrimp, water fleas, ostracods ,copepods
Copepods are among the most abundant multi-cellular organism on earth and along with krill they are a major food source in the oceans.
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Decapods
Crustaceans (krus TAY shuns) this type have two pairs of antennae for sensing.
5 pairs of legs
Anterior pair of legs are modified into large pincers called chelipeds.
Cephalathorax (called a carapace)
Uropods-flattened, paddle like appendages
Telson- tail spine
Swimmerets- are appendages attached to the underside for swimming and reproduction
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Crustaceans
claw
legs
The first pair of walking legs are often modified into strong claws for defense. (Cheliped)
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