Chapter 30

Chapter 30Chapter 30

ArthropodsArthropods

• A typical arthropod is a segmentedsegmented, coelomatecoelomate invertebrate animal with bilateral symmetry, an exoskeletonexoskeleton, 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.

I. Features of ArthropodsI. Features of Arthropods

• 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

What is an arthropod?What is an arthropod?

Origins of ArthropodsOrigins of Arthropods

Origins of ArthropodsOrigins 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: trilobitestrilobites• -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: scorpionsscorpions

The total number of arthropodsThe 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.)

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.

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.

Arthropod Body Plan

Segmentation in arthropods

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)

• In other groups, even these segments may be fused.

Segmentation in arthropodsSegmentation in arthropods

• Some arthropods have a head and a fused thorax and abdomen.

• In other groups, there is an abdomen and a fused head and thorax called a cephalothorax.

Segmentation in arthropodsSegmentation in arthropods

• Fusion of the body segments is related to movement and protection.

• A compound eye is a visual structure with many lenses.

Arthropods have acute sensesArthropods have acute senses

• See motion much more quickly than humans.

• 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.

• The exoskeleton is a hard, thick, outer covering made of protein and chitin (KI tun).

• Brittle and can break easily

Arthropod exoskeletons provide protectionArthropod exoskeletons provide protection

• 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.

• In some species, the exoskeleton is a continuous covering over most of the body.


• In many aquatic species, the exoskeletons are reinforced with calcium carbonate.

• The exoskeleton protects and supports internal tissues and provides places for attachment of muscles.


• First, they are relatively heavy structures. The larger an arthropod is, the thicker and heavier its exoskeleton must be to support its larger muscles.

• Exoskeletons have their disadvantages.

Why arthropods must moltWhy arthropods must molt

Why arthropods must moltWhy 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.

• 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.


• Thus, the new exoskeleton hardens in a larger size, allowing some room for the animal to continue to grow.

• Before the new exoskeleton hardens, the animal puffs up as a result of increased blood circulation to all parts of its body.


• When the new exoskeleton is soft, arthropods cannot protect themselves from danger because they move by bracing muscles against the rigid exoskeleton.

• Most arthropods molt four to seven times in their lives before they become adults.


• This large oxygen demand is needed to sustain the high levels of metabolism required for rapid movements.

Respiration: Arthropods have efficient gas exchangeRespiration: Arthropods have efficient gas exchange

• Arthropods have efficient respiratory structures that ensure rapid oxygen delivery to cells.

• Three types of respiratory structures have evolved in arthropods: gills, tracheal tubes, and book lungs.

Arthropods have efficient gas exchangeArthropods have efficient gas exchange

• Aquatic arthropods exchange gases through gills, which extract oxygen from water and release carbon dioxide into the water.


• Land arthropods have either a system of tracheal tubes or book lungs.


• Most insects have tracheal tubes, branching networks of hollow air passages that carry air throughout the body.


• Air enters and leaves the tracheal tubes through openings on the thorax and abdomen called spiracles.

• Muscle activity helps pump the air through the tracheal tubes.


• Most spiders and their relatives have book lungs, air-filled chambers that contain leaflike plates.


• The stacked plates of a book lung are arranged like pages of a book.


• 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.

Excretion:Excretion:

II. Spiders and other Arachnids II. Spiders and other Arachnids

• Spiders, scorpions, mites, and ticks belong to the class Arachnida (uh RAK nud uh).

• Spiders are the largest group of arachnids.

What is an arachnid?What is an arachnid?

• 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.

• The first pair of appendages, called chelicerae, is located near the mouth.

chelicerae


• Chelicerae are often modified into pincers or fangs.

• Pincers are used to hold food, and fangs inject prey with poison.


• 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.


• 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.

• Although all spiders spin silk, not all make webs.

• Spider silk is secreted by silk glands in the abdomen.


• As silk is secreted, it is spun into thread by structures called spinnerets, located at the rear of the spider.


A SpiderA Spider

• Spiders are predatory animals, feeding almost exclusively on other arthropods.(=carnivores)

Simple eyes

Legs

Cocoon

Silk glandsBook Lungs

Chelicerae

Pedipalps

• Ticks and mites differ from spiders in that they have only one body section.

tick

Ticks, mites, and scorpions: Spider relativesTicks, mites, and scorpions: Spider relatives

• The head, thorax, and abdomen are completely fused.

• Plant mites-Plant mites- while feeding may pass viral and fungal infections to plants.

• Dust mites-Dust mites- live in carpet, bedding, clothing. (cause allergies)

• ChiggersChiggers-known for their irritating bite

• TicksTicks feed on blood from reptiles, birds, and mammals.


• 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


• 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.

• Flies, grasshoppers, lice, butterflies, bees, and beetles are just a few members of the class Insecta.

InsectsInsects

• Insects have three body segments (head thorax and abdomen) and six legs.

• Head- Head- mandibles (chewing mouth part) pair of antennae, compound eyes

• ThoraxThorax - - (composed of 3 fused segments.)

• 3 pair of jointed walking legs and some have wings that are attached to the thorax.

• AbdomenAbdomen- is composed of 9-11 segments

Arthropods have other complex body systemsArthropods 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.

A GrasshopperA Grasshopper

Wings

Legs Eyes

Antennae

Nervous SystemTympanumMalpighian tubules

Spiracles

• Insects usually mate once during their lifetime.

Insect reproduction/Life CycleInsect reproduction/Life Cycle

• The eggs usually are fertilized internally.

• Some insects exhibit parthenogenesis, reproducing from unfertilized eggs.

• Most insects lay a large number of eggs, which increase the chances that some offspring will survive long enough to reproduce.

Insect reproductionInsect reproduction

• 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.

Metamorphosis: Change in body shape and formMetamorphosis: Change in body shape and form


• These insects go through successive molts until the adult size is reached.

EggsNymph

Molt

Nymph

Molt

Adult

• In some cases, the adult insect bears little resemblance to its juvenile stage.

AdultEgg

LarvaPupa


• Insects that undergo metamorphosis usually go through four stages on their way to adulthood: egg, larva, pupa, and adult.

• This series of changes, controlled by chemical-substances in the animal, is called metamorphosis.


• The larva is the free-living, wormlike stage of an insect, often called a caterpillar.


• 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.


• 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.


Metamorphosis: Change in body shape and form

Metamorphosis: Change in body shape and form

pupa

larvae

adult

fertilized egg

• The complete metamorphosis of a butterfly is illustrated.

• Other insects that undergo complete metamorphosis include ants, beetles, flies, and wasps.


Incomplete metamorphosis has three stagesIncomplete 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.


• A nymph, which hatches from an egg, has the same general appearance as the adult but is smaller and wingless.

EggsNymph

Molt

Nymph

Molt

Adult

Incomplete metamorphosis has three stagesIncomplete 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.


• Grasshoppers and cockroaches are insects that undergo incomplete metamorphosis.

Incomplete metamorphosis of a harlequin bug

• Two orders of insects, Hymenoptera (bees) and Isoptera (termites) have elaborate social systems.

Social InsectsSocial Insects

• 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.caste.

• Its Caste is determined by a combination of heredity, diet, hormones and pheromones

• Like spiders, millipedes and centipedes have Malpighian tubules for excreting wastes.

Insect relativesInsect relatives

Centipedes and Millipedes

Insect relativesInsect relatives

Centipedes and Millipedes

• In contrast to spiders, centipedes and millipedes have tracheal tubes rather than book lungs for gas exchange.

• Centipedes are carnivorous and eat soil arthropods, snails, slugs, and worms.

• Centipedes have one pair of legs /segment

Centipedes and MillipedesCentipedes and Millipedes

• The bites of some centipedes are painful to humans.

• A millipede eats mostly plants and dead material on damp forest floors. (herbivores)

Centipedes and MillipedesCentipedes and Millipedes

• Millipedes do not bite, but they can spray foul-smelling fluids from their defensive stink glands.

• Millipedes have 2 pair of legs /segment

• Members of the class Crustacea include crabs, lobsters, shrimps, crayfishes, water fleas, pill bugs, and barnacles.

• Barnacles are sessile

CrustaceansCrustaceans

• 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)

• 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.

• Sow bugs and pill bugs, two of the few terrestrial crustaceans, must live where there is moisture, which aids in gas exchange.


• 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

Decapods Decapods

• Swimmerets- are appendages attached to the underside for swimming and reproduction

• The first pair of walking legs are often modified into strong claws for defense. (Cheliped)

claw legs


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Chapter 30