Arthropods!

Ch 28 Presentation

Common Characteristics of all Arthropods

• Typical arthropod:– Segmented– Coelomate with Bilateral symmetry– Invertebrate with exoskeleton– Jointed structures called appendages

• Appendage—any structure (e.g. leg, antenna) that grows out of the body of an animal

• In arthropods, appendages are adapted for walking, sensing, feeding, mating, etc.

• Arthropods are earliest known invertebrates to exhibit jointed appendages– Joints allow for flexibility of rigid exoskeleton (think knight in suit of armor)– Joints enable many uses for various appendages (e.g. mating in spiders,

stinging in scorpions)

Exoskeletons Support and Protect

• Exoskeleton—hard, thick outer covering of protein and chitin

• In some species, exoskeleton is continuous over entire body, in others it is separate plates held together by hinges

• Exoskeleton supports internal tissues, provides places for muscle attachment

• In many terrestrial species, exoskeleton covered with waxy layer to minimize water loss

• In many aquatic species, exoskeleton reinforced with calcium carbonate

Oh no, it’s MOLTING!

• Why molt? – Exoskeletons are heavy and thick (the larger the muscles, the

heavier/thicker must the exoskeleton be), so weight is limited for arthropods• Terrestrial and flying arthropods have thinner, lighter (but more fragile)

exoskeletons but are less protected

• Molting—shedding the old exoskeleton (new soft skeleton formed underneath first)

• Animal takes in air/water and contracts muscles, which swell and split old skeleton

• Most arthropods molt 4-7x during lifetime before becoming adults• Mid-molt, animals are vulnerable to predators, so many hide or

remain motionless until their new skeleton hardens• http://youtu.be/2A1i10ZIB-w http://youtu.be/ffo37DlH5DM http://youtu.be/4QIgW639Oog

Segmentation

• Most arthropods are segmented but do not have as many segments as worms

• In most groups, segments fused into three zones:– Head, thorax, abdomen

• Sometimes head and thorax fused together into

cephalothorax

Breathing• Arthropods have efficient gas exchange—generally quick, active animals with high

oxygen requirements• Three types of respiratory structures:

– Gills– Tracheal tubes– Book lungs

• In some arthropods, gas exchange occurs across exoskeleton (so it must be thin and permeable)

• Aquatic arthropods use gills• Land arthropods use either tracheal tubes or book lungs• Most insects use tracheal tubes (branching networks of hollow air passages that

carry air throughout body)• Air enters and leaves body through openings on thorax and abdomen called

spiracles• Spiders and their relatives use book lungs, air-filled chambers with leaf-like plates

(maximize surface area for gas exchange)

• Tracheal Tubes and Spiracles

• • Book Lungs

Book Lungs

Acute Sensory Perception• Why can arthropods respond so quickly to their

environment?– Strong muscular contraction– Detect movement, chemicals and sound with

antennae• Antennae have odor and sound receptors that are used in

communication with other members of species (spiders lack these)

• Pheromones—chemical odor signals given off by animals (signal animals to engage in variety of behaviors)

– Accurate vision: EYES• Most arthropods have one pair of large compound eyes and

3-8 simple eyes• Simple eyes- single lensed-structures for detecting light• Compound eyes—visual structures with many lenses

– Each lens registers a tiny portion of the field of view, so the total image is made of thousands of parts (panoramic view)

Eyes

What structures allow this damselfly to sense its environment?

Well-developed Nervous Systems

• Arthropods process information from sense organs with well-developed nervous systems

• Consists of double ventral nerve cord (pearl necklace), an anterior brain, and several ganglia http://youtu.be/0uTdTRXNdEY

Arthropod Circulation

• Open circulatory system—blood is pumped by a heart, through vessels that bathe the tissues of the body, returns to heart through open spaces

Complex Digestive Systems

• Mouth, stomach, intestine, anus, glands that produce digestive enzymes

Diversity of Mouthparts

• Mouthparts are grouped and adapted for a vast variety of foods eaten by arthropods– Mandibles for chewing, pinching, holding– Pairs of appendages for moving food around while

chewing http://youtu.be/yBHqJrKtlso http://youtu.be/ZJyeFSq3STM

• Some insects have blade- or needle-like mouthparts for piercing for drawing blood, etc., while others have sponging tongues to lap up food, rolled-up proboscises for sucking up food

Insect Mouthparts• Most mouthparts of arthropods

include one pair of mandibles (green) http://youtu.be/QOIJPG25gr0

– These are adapted for holding, chewing, or biting various foods

• Labrum: upper-most mouthpart (red)– fused plate to hold food in place

during chewing• Maxillae (yellow): hold and

manipulate food during mastication– Hairs and “teeth” along inner ridge– Palps used for sensing the

characteristics of potential foods– Modified into proboscis or straw-

like tube for sucking up food http://youtu.be/MYWPWTme_YI

• Labium: “floor” of the mouth, assists during chewing (blue)

• Hypopharynx: base of labium, assists in swallowing

Crustacean Mouthparts

• Three pairs of appendages modified into mouthparts that manipulate and bring food into mouth (mandibles followed by two pairs of maxillae) http://youtu.be/25F7xMVNt-w

– Paired appendages offer opposable surfaces for grinding and biting food

– Often followed by setae used to collect food

Arachnid Mouthparts• Spiders and scorpions have

chelicerae—pointed appendages used to grasp food– In spiders, they are hollow and

contain venom glands

Other well-developed systems

• Excretory: Malpighian tubules that remove waste from abdomen; attached to and empty into the intestine

• Muscular: muscle is attached to inner surface of exoskeleton on both sides of joints

Sexual Reproduction

• Most arthropods have separate males and females and reproduce sexually– Fertilization usually internal, sometimes external in aquatic

species• A few species hermaphroditic– Barnacles

• Parthenogenesis—asexual reproduction in which new individual develops from unfertilized egg (ex. Bees, wasps, ants, aphids; males with only one X chromosome)

• Reproductive diversity one reason why more arthropod species than ALL the other animal species combined

Arthropod Diversity• Arachnidae: Spiders, scorpions, mites, and ticks

• Spiders have two body sections: cephalothorax, abdomen• Six pairs of jointed appendages• Chelicerae: first pair of appendages located near the mouth

• Modified into pincers or fangs, spiders have no mandibles for chewing

• Pedipalps: second pair, adapted for handling food and sensing food

• Arachnids have no antennae• Spiders can make elaborate webs– though all spiders spin silk, not

all make webs• Silk secreted in glands in abdomen, then spun together into

thread by spinnerets located at rear of spider• Ticks, mites, scorpions: only one body section (head, thorax, and

abdomen completely fused• Mites feed on fungi, plants, animals

• Small but can expand body size up to 3x original size• Scorpions have long tail with venomous stinger at tip,

enlarged pincers

Arthropod Diversity• Crustacea: body sections, two pairs of antennae for

sensing, two compound eyes, mandibles for crushing food (ex. Shrimp, lobsters, crabs, barnacles, crawdads– Crustacean mandibles open and close side to side– Many have five pairs of walking legs

• Used for walking, seizing prey, cleaning appendages• First pair of walking legs often modified into strong claws for

defense

Arthropod Diversity• Chilopoda: centipedes, Diplopoda: millipedes– Have Malpighian tubules for excreting wastes– Tracheal tubules for gas exchange– Centipedes are carnivorous with painful bites

• May have 15-181 body segments, each with one pair of legs

– Millipedes don’t bite, eat mostly plants and dead material on forest floors• Spray foul-smelling fluid from stink glands as defense• May have 20 to 100+ segments, with two spiracles and legs each

Arthropod Diversity• Merostomata: Horseshoe crabs

—”living fossils”– Three living genera, one (Limulus)

lives along North American East Coast, two live in Asian tropics

– Limulus fossils unchanged since Triassic Period 220 mya

– Heavily protected by extensive exoskeleton; live in deep coastal waters; forage for algae, annelids, mollusks

– Migrate to shallow water in spring, mate at night during high tide

Arthropod Diversity• Insecta: the most successful class of

arthropods• 3 body segments, six legs, more

species of insects than all other animals combined

• Reproduction: usually once during lifetime, internal fertilization– Parthenogenesis: in aphids all female

offspring; in bees/ants/wasps all male– Most lay large # of eggs, increases

chances of offspring survival to reproductive maturity

Metamorphosis• Metamorphosis—change in body shape from egg to

adult, controlled by series of chemical changes in animal– EggLarvaPupaAdult– If insect goes through all four stages: complete– If insect goes through some stages: incomplete (nymph

stage)– Complete metamorphosis advantageous for arthropods

because larvae don’t compete with adults for the same food

Origins of Arthropods

• Successfully covered the whole surface of Earth more than any other group of animals because of their ability to survive in almost every habitat– Exoskeletons, life cycles, high reproductive output, small

size, jointed appendages• Most likely evolved from annelid ancestor– Evolved fused body segments that adapted for

movement, sensing environment– More complex than annelids (eyes, nervous tissue)– Trilobites were major group of ancient arthropods, but

have been extinct 248 million years