Arthropods are the largest and most diverse phylum in the animal kingdom, encompassing millions of species—from insects and spiders to crabs and centipedes. Found in nearly every environment on Earth, arthropods play vital roles in ecosystems, agriculture, and even human health. But what exactly makes an animal an arthropod?

Here are the defining characteristics that set them apart:

Exoskeleton Made of Chitin

Arthropods have a tough, external skeleton called an exoskeleton, made primarily of chitin. This protective armor shields them from predators, prevents water loss, and supports muscle attachment. However, since it doesn’t grow with the animal, arthropods must periodically molt (shed) their exoskeleton in a process called ecdysis.

Segmented Body

Their bodies are typically divided into segments, often organized into three main regions:

• Head – where sensory organs and mouthparts are located

• Thorax – the center of locomotion, bearing legs and wings (in insects)

• Abdomen – where digestion, reproduction, and other internal processes occur

Some groups, like spiders, have different segment arrangements, but segmentation remains a consistent feature across the phylum.

Jointed Appendages

As their name implies (arthro = joint, pod = foot), arthropods have jointed limbs. These flexible appendages allow for complex movement and are adapted for various functions—walking, swimming, flying, sensing, feeding, and defense.

Bilateral Symmetry

Arthropods have bilateral symmetry, meaning their body can be divided into two mirror-image halves. This symmetry aids in balance, directional movement, and coordination.

Open Circulatory System

Instead of a closed circulatory system like humans, arthropods pump hemolymph (a fluid analogous to blood) into body cavities, where it bathes organs directly. This open circulatory system is less efficient but well-suited to their relatively small body size and energy needs.

Ventral Nerve Cord and Dorsal Brain

Arthropods possess a centralized nervous system consisting of a brain and a ventral nerve cord. Ganglia (clusters of neurons) control local body segments, allowing fast and efficient responses to stimuli.

Diverse Sensory Organs

Arthropods are equipped with a range of sophisticated sensory organs:

• Compound eyes for detecting movement and color

• Antennae for touch and smell

• Setae (bristles) on their body for detecting vibrations and chemicals

Incredible Diversity and Adaptability

Arthropods have adapted to virtually every environment—deep oceans, mountaintops, deserts, rainforests, and even human homes. Their diversity includes:

• Insects (e.g., bees, ants, butterflies)

• Arachnids (e.g., spiders, scorpions, ticks)

• Crustaceans (e.g., crabs, shrimp, lobsters)

• Myriapods (e.g., centipedes, millipedes)