Many online searches for “ropod” are misspellings of the term arthropod. Arthropods represent the most abundant and diverse phylum in the animal kingdom, making up about 84 percent of all known animal species. This group includes familiar invertebrate animals such as insects, spiders, crabs, and centipedes. They are defined by three primary characteristics: a hard outer covering called an exoskeleton, a body divided into distinct segments, and appendages with flexible joints.
This unique body plan has allowed them to adapt to nearly every habitat on Earth, from the deepest oceans to the highest mountains.
Defining Characteristics of Arthropods
An arthropod’s body is supported and protected by its exoskeleton, a nonliving external skeleton. This structure is primarily composed of a complex sugar called chitin, bound to various proteins. In many aquatic arthropods like crabs and lobsters, this covering is hardened with calcium carbonate, providing additional strength. The exoskeleton serves as a barrier against water loss, offers structural support, and provides a solid anchor for muscle attachment.
This external skeleton is formed in segments, which allows for movement despite its rigid nature. An arthropod’s body is typically divided into three main regions, or tagmata: the head, thorax, and abdomen. In some groups, such as spiders and crabs, the head and thorax are fused into a single structure known as a cephalothorax.
The name “arthropod” means “jointed foot,” which points to another of their defining features: paired, jointed appendages. These limbs have flexible joints, enabling a wide range of motion for walking, swimming, feeding, and sensing the environment. Over evolutionary time, these appendages have become specialized into structures as diverse as antennae for sensing, claws for grasping, and complex mouthparts for feeding.
Because the exoskeleton is rigid, it cannot grow with the animal. To increase in size, an arthropod must periodically shed its exoskeleton through a process called molting, or ecdysis. During this vulnerable time, the arthropod secretes a new, soft cuticle underneath the old one. It then sheds the old covering and expands its body before the new exoskeleton hardens.
Major Arthropod Groups
The phylum Arthropoda is organized into several major subphyla, distinguished by unique body structures. The four largest extant groups are Chelicerata, Myriapoda, Crustacea, and Hexapoda.
Chelicerata
This group includes spiders, scorpions, mites, and ticks. Their defining feature is a pair of specialized appendages near the mouth called chelicerae, often modified into fangs used to grab food. They lack antennae and their bodies are divided into two main parts: a cephalothorax and an abdomen.
Myriapoda
Myriapoda, meaning “many-footed,” encompasses centipedes and millipedes. These terrestrial arthropods are recognized by their long, segmented bodies, with each segment bearing legs. Centipedes possess one pair per segment and are carnivorous, while millipedes have two pairs per segment and feed on decaying plant matter.
Crustacea
The subphylum Crustacea is composed of mostly aquatic animals like crabs, lobsters, shrimp, and barnacles. They are distinguished by having two pairs of antennae and appendages that are often biramous, meaning they are split into two branches. Many crustaceans have a hard, calcified exoskeleton that forms a protective carapace over the cephalothorax.
Hexapoda
Hexapoda is the most diverse group, consisting primarily of insects. They are defined by a body plan of a head, thorax, and abdomen, with three pairs of legs attached to the thorax. This group includes beetles, butterflies, ants, and flies, and their adaptations for flight have allowed them to become the most widespread arthropod group.
Global Presence and Ecological Roles
Many arthropods perform functions that are beneficial to their ecosystems and human activity. Insects like bees, butterflies, and even some flies are pollinators for a vast number of plants, including many agricultural crops. In soil and leaf litter, decomposers such as millipedes and various insect larvae break down dead organic material, recycling nutrients back into the environment. Arthropods also serve as a food source for countless other animals, including birds, fish, reptiles, and mammals.
Conversely, some arthropod species have roles that are detrimental to human interests. Many insects are considered agricultural pests, causing significant damage to crops and stored food products. Others are vectors for disease; mosquitoes are well-known carriers of pathogens that cause malaria and dengue fever, while ticks can transmit Lyme disease.