Introduction to Phylum Arthropoda

• The word Arthropoda is derived from Greek: Arthro = Jointed, Poda = Legs.
• It is the largest phylum in the Animal Kingdom, comprising over 85% of all known animal species.
• Arthropods exhibit bilateral symmetry, segmented bodies, and jointed appendages.
• They have a chitinous exoskeleton, which is periodically shed in a process called molting (ecdysis).
• Includes insects, crustaceans, arachnids, myriapods, and extinct trilobites.

General Characteristics of Phylum Arthropoda

• Body Segmentation: Body is divided into head, thorax, and abdomen (in some, head and thorax fuse to form a cephalothorax).
• Exoskeleton: Hard chitinous cuticle, providing protection and support.
• Appendages: Jointed appendages adapted for locomotion, feeding, and sensing.
• Respiration: Through trachea, gills, book lungs, or body surface.
• Circulatory System: Open type with hemocoel.
• Excretion: Malpighian tubules (terrestrial forms), Green glands (Crustaceans).
• Nervous System: Well-developed with a ventral nerve cord and a brain.
• Reproduction: Mostly sexual, with internal or external fertilization.

Classification of Phylum Arthropoda (Up to Classes)

Phylum Arthropoda is the largest phylum in the Animal Kingdom, characterized by jointed appendages, a segmented body, and an exoskeleton made of chitin. It includes insects, crustaceans, arachnids, and myriapods.

Kingdom: Animalia

Phylum: Arthropoda

Subphylum 1: Trilobitomorpha (Extinct)

Class: Trilobita (Extinct trilobites) Ancient marine arthropods, now extinct. Body divided into three lobes (hence the name “trilobite”).

Subphylum 2: Chelicerata (Chelicerates – Spiders, Scorpions, Horseshoe Crabs, etc.)

🔹 Characteristics:

• Body divided into cephalothorax and abdomen.
• No antennae.
• First pair of appendages are chelicerae (mouthparts) instead of mandibles.

Classes under Chelicerata:

• Class: Merostomata (Horseshoe Crabs) Example: Limulus (horseshoe crab). Marine, have book gills for respiration.
• Class: Arachnida (Spiders, Scorpions, Ticks, Mites) Example: Spiders (Araneae), Scorpions (Scorpiones), Mites (Acari). Mostly terrestrial, breathe via book lungs or tracheae.
• Class: Pycnogonida (Sea Spiders) Marine arthropods with long, slender legs.

Subphylum 3: Mandibulata (Includes Crustaceans, Myriapods, and Insects) Characteristics: Possess mandibles (jaws) for feeding. Most have antennae.

Superclass 1: Crustacea (Crabs, Lobsters, Shrimps, etc.) Characteristics: Mostly aquatic, breathe via gills. Two pairs of antennae.

Classes under Crustacea:

• Class: Branchiopoda (Fairy Shrimps, Water Fleas) Example: Daphnia, Artemia. Found in freshwater.
• Class: Ostracoda (Seed Shrimps) Small, enclosed in a bivalve-like shell.
• Class: Copepoda (Planktonic Crustaceans) Example: Cyclops. Important part of aquatic food chains.
• Class: Cirripedia (Barnacles) Sessile marine crustaceans with hard shells.
• Class: Malacostraca (Larger Crustaceans: Crabs, Lobsters, Prawns) Example: Penaeus (prawn), Cancer (crab), Palinurus (lobster). Most economically important class of crustaceans.

Superclass 2: Myriapoda (Millipedes & Centipedes)

🔹 Characteristics:

• Long, segmented bodies.
• One or two pairs of legs per segment.
• Terrestrial, breathe via tracheae.

Classes under Myriapoda:

• Class: Chilopoda (Centipedes) Example: Scolopendra (giant centipede). One pair of legs per body segment. Carnivorous, have venomous fangs.
• Class: Diplopoda (Millipedes) Example: Julus. Two pairs of legs per body segment. Herbivorous or detritivorous.

Superclass 3: Hexapoda (Insects and Relatives)

Characteristics:

• Body divided into head, thorax, and abdomen.
• Three pairs of legs, one pair of antennae.
• Most can fly (insects are the only flying arthropods).

Classes under Hexapoda:

• Class: Entognatha (Wingless, primitive hexapods) Example: Springtails (Collembola). Small, soil-dwelling arthropods.
• Class: Insecta (True Insects) Example: Butterflies (Lepidoptera), Beetles (Coleoptera), Flies (Diptera). Largest group of arthropods. Possess one or two pairs of wings (except in some wingless species).

Summary Table of Arthropoda Classification