ArthropodsChapter 28

Arthropod characteristics

• Segmented bodies

• Tough exoskeleton made of chitin– Multiple shapes and textures

• Jointed appendages: legs and antennae

Arthropod evolution

• Modern arthropods have fewer body segments and more specialized appendages.

Fossil of a Trilobite, extinct class of Arthropod

Fossil of Leanchoilia, Phylum Arthopoda

www.burgess-shale.bc.ca/ gallery/gallery.htm

Feeding• Herbivores, carnivores, omnivores,

parasites, blood suckers, filter feeders, and detritovores

• Variety of mouthparts: pincers, fangs, sickle-shaped jaws, feeding tubes

stag beetle pincers

Respiration

• Most arthropods– Tracheal tubes: branching network of tubes

that deliver and expel air through spiracles

• Some arthropods (including spiders)– Book lungs: layers of respiratory tissue

stacked like pages of a book

Circulation

• Open circulatory system: heart pumps blood to tissues, sinuses, and cavities but does not return to the heart directly

Excretion

• Malpighian tubules: saclike organs that extract wastes from blood and then add them to feces to move through the gut

Chrysina quetzalcoatli is freed in Cusuco

National Park, leaving a fecal trail

National Geographic

Response

• Sensitive sensory organs (sight, smell, touch)

• Well-developed nervous system – Brain– Ventral nerve cord– Ganglia: groups of nerve cells

Movement

• Flexor muscles

• Extensor muscles

• Muscle pulling against exoskeleton enables movement

Reproduction

• Terrestrial arthropods: internal fertilization– Male places sperm in female– Sperm sac that female picks up

• Aquatic arthropods: internal or external fertilization

Growth and development

• Molting: period of shedding exoskeleton when outgrown– Glands secrete

digestive enzymes to break down exoskeleton.

– Glands secrete new exoskeleton.

– Vulnerable stage

Insect molting case left on tree bark

Arthropod subphylums

• Crustaceans• Spiders and their relatives• Insects and their relatives

Crustacean characteristics

• Two pairs of antennae• Two or three body sections• Mandibles: chewing mouthparts• Body plan: celphalothorax, abdomen, carapace• Barnacles have “lost” appendages.• Decapods have chelipeds and swimmerets.

– Ex: shrimp, lobster, crayfish

Section 28-2

AbdomenCephalothorax

Walking legs

Tail

Swimmerets

Carapace

Cheliped

MandibleSecond antenna

First antenna

The Anatomy of a Crayfish

Subphylum Crustacea – hermit crab

Subphylum Crustacea –crab

Subphylum Crustacea – fire shrimp

Chelicerate characteristics

• Four pairs of walking legs• Chelicerae mouthparts: fangs that sting

and pedipalps that grab prey• Two body sections: cephalothorax and

abdomen• Classes: Merostomata (horseshoe crabs)

and Arachnida (spiders, scorpions)• Spiders have spinnerets, silk glands.

Section 28-2

AbdomenCephalothorax

Book Lung

Eyes

Brain

Pumpingstomach

Heart

Intestine

Ovary

Malpighiantubules

Anus

SpinneretsSilk glands

Spiracle

Bases ofwalking legs

Fanglikechelicera

Pedipalp

Poisongland

Airflow

The Anatomy of a Spider

Figure 28–9 

Subphylum Arachnida – black widow spider

Scorpion

Gum shoe spider web Theridae family

National Geographic

Photo by Darlyne A. Murawski

Uniramia characteristics

• Jaws

• One pair of antennae

• Unbranched appendages

• Groups: centipedes, millipedes, insects

Millipede

Centipede

Why insects are evolutionarily successful

• Flight

• Variety of stimuli responses

• Differential feeding methods between young and adult

• Different morphology between young and adult

Insect characteristics

• Compound eyes

• Some with wings

• Three part body plan

• Three pairs of legs

• Sensitive taste and smell receptors• Three appendages for mouth,

including pair of mandibles

Section 28-1

Ventral View

Antennae

Compound eyeBrain Digestive

tract Malpighian tubules

Heart

Reproductive organs

Anus

SpiraclesTrachealtubes

Nervecord

Ganglia

Salivaryglands

Mouth

Tracheal tubes

Spiracles

Legs

The Anatomy of a Grasshopper

Figure 28–4 

Insect life cycle

• Metamorphosis: process of changing shape and form from juvenile to adult stage– Incomplete: juvenile stage (nymphs) appear

like adults, just smaller– Complete: juvenile stage (larvae) becomes

pupa, final stage before becoming an adult

Section 28-3

IncompleteMetamorphosis

CompleteMetamorphosis

Eggs

Larva

Pupa

Adult

Nymph

Nymph

Nymph

Adult

Eggs

Larva

Adult

Adult

Immature

Metamorphosis

Figure 28–18 

• Although they can be a nuisance or pest, insects are significant to agriculture.

• They facilitate flower pollination.

Insects and humans

Insect communication and societies

• Chemical cues: pheromones

• Visual cues: light, flight patterns

• Some insects form societies where castes, groups of individuals, perform specific tasks.

Polyphemus moth Pheromone chemical: (E,Z)-6,11-Hexadecadienyl acetate

National Geographic

Image  by Joseph Scheer

Subphylum Insecta – lady bug

Subphylum Insecta – bee moth