The arthropods are bilaterally symmetrical invertebrates with jointed exoskeletons covering body and appendages. They must be the most successful group of animals on the planet, accounting for over 80% of all known living animals. They are incredibly diverse inhabiting all environments on Earth.

The name arthropod comes from the greek arthron = joint and pous = foot, due to their distinct jointed appendages. They also have segmented bodies. The exoskeleton consists of a stiff cuticle made of chitin, which is often further stiffened with calcium.

They range in size from the microscopic to quite large with both the smallest, Stygotantulus stocki at 94 μm, and the largest, the Japanese spider crab, Macrocheira kaempferi, with an extended arm span of up to 3.8m, being crustaceans.

The arthropods are divided into four subphyla, Chelicerata, Crustacea, Hexapoda, and Myriapoda. There is some disagreement regarding the classification and what should be included in this phylum.

Subphylum Chelicerata — Chelicerates

1. Class: Arachnida — Arachnids (spiders, mites, scorpions, and ticks)
2. Class Merostomata — Horseshoe Crabs
3. Class Pycnogonida — Pycnogonids (sea spiders)

Subphylum Crustacea — Crustaceans

1. Superclass Multicrustacea — Typical Crustaceans
2. Superclass Oligostraca — Oligostracans
3. Class Branchiopoda — Branchiopods
4. Class Cephalocarida — Horseshoe Shrimps
5. Class Remipedia — Remipedes

Subphylum Hexapoda — Hexapods

1. Class Entognatha — Entognathans
2. Class Insecta — Insects

Insect quick-links:
Butterflies & moths, dragonflies & damselflies, beetles, bugs

Subphylum Myriapoda — Myriapods

1. Class Chilopoda — Centipedes
2. Class Diplopoda — Millipedes
3. Class Pauropoda — Pauropodans
4. Class Symphyla — Symphylans

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