Phylum Arthropoda

Phylum Arthropoda: The Arthropods

• Most successful animal group ever to live
• Consists of over 1 million named species, 2 of every 3 animals are arthropods. ~ 10¹⁸ individuals.
• Inhabit all ecosystems

General characteristics

• All have a hard exoskeleton
• Composed of protein and chitin
• Allows for great variation in function - biological "swiss army knives"
• Must molt as they grow
• Limits their size
• Have well developed body segments and appendages
• Have well developed sensory organs including true eyes and antennae
• Have open circulatory systems & special gas exchange organs

3 Subphylums
Subphylum Chilicerata (arachnids, horseshoe crabs & sea spiders)

• Divided into 3 classes; Arachnida, Merostomata & Pycnogonida
• Body divided into 2 regions
• Abdomen
• Cephalothorax (fused head & thorax)
• Lack jaws
• Have 6 appendages & no antennae
• First appendages form chilicerae (frequently fangs)

Class Arachnida (Spiders, Scorpions, Ticks & Mites)

• Very diverse class
• Most species parasitic or predatory
• Many possess book lungs for gas exchange
• Spiders are able to produce a strong polymer - silk
• Chilicera in form of fangs

Class Merostomata - Horseshoe crabs

• Ancient group of species
• Changed little over 350 million years
• Aquatic, mostly found on Atlantic & gulf coasts of United States.

Subphylum Uniramia: 3 classes

• Class Insecta (insects)
• Class Chilopoda (Centipedes)
• Class Diplopoda (Millipedes)

Classes Chiopoda & Diplopoda

• Millipedes (diplopoda) are segmented worm-like animals
• Have 2 pairs of legs per segment
• Primarily herbivores & decomposers

• Centipedes (Chilopoda)
• Usually terrestrial carnivores
• Have 1 pair of antennae
• Are often poisonous, using modified front claws to immobilize prey

Class Insecta (The Insects)

• Far & away the most diverse of animal groups
• More types of insects alone than all other animal groups combined
• Inhabit all terrestrial & freshwater ecosystems.
• Success largely attributed to coevolution with flowering plants.

Body Plan:

• Insects have 6 legs
• 3 body parts
• Head
• Thorax
• Abdomen
• Most insects have wings, however in many species these are vestigal
• Have advanced excretory system composed of malphygian tubules
• Exchange gasses through a complex tracheal system

•  
• Have complex compound eye which is usually extremely sensitive to motion and allows 360⁰ vision

• Have specialized jaws/mouthparts suited to their ecological niche

•  
• Most insects undergo a process of metamorphosis - 2 types
• Incomplete metamorphosis
• Larva similar to adult, with differing body proportions
• Undergoes a series of molts resulting in adult phenotype

• Complete metamorphosis
• Larva is very unlike adult phenotype
• Envelopes self in a coccoon or chrysalis where body breaks-down and reforms into adult form.

Subphylum Crustacea

• Includes crabs, lobsters, crayfish, shrimp, krill & barnacles
• Over 40,000 species
• Appendages are often highly specialized
• Gas exchange is usually through gills
• Many species taste delicious in butter