Arthropods, the largest phylum in the animal kingdom, fundamentally do not possess backbones. This vast group, encompassing insects, spiders, and crustaceans, is defined by segmented bodies and specialized jointed limbs. Lacking an internal spinal column, the defining trait of animals like mammals and fish, arthropods are categorized as invertebrates. This structural difference accounts for the unique body plans and diversity that has allowed this group to colonize nearly every habitat on Earth.
The External Skeleton
Arthropods compensate for the lack of an internal spine with a complex, rigid outer casing known as the exoskeleton. This external structure is composed primarily of chitin, a tough, resilient material chemically bound to various proteins. The exoskeleton serves multiple purposes, providing structural support and a sturdy frame against which muscles pull for movement.
This hard shell forms a protective barrier for the soft internal tissues and organs. Terrestrial arthropods benefit from the epicuticle, a waxy outer layer that prevents rapid water loss and desiccation. Because the exoskeleton is a non-living, rigid structure, it cannot grow with the animal, necessitating ecdysis, or molting. During molting, the arthropod sheds its old casing to reveal a new, larger one, which remains soft until it hardens through processes like sclerotization or calcification.
Classification in the Animal Kingdom
The presence or absence of a spinal column forms the most basic and significant division within the Animal Kingdom. Animals that possess a spine or vertebral column, such as birds, reptiles, and mammals, are classified as vertebrates. In contrast, invertebrates are a collective grouping of all animals that lack this internal bony structure, a category that contains over 97% of all animal species.
Arthropods fall strictly into this invertebrate classification, representing a highly successful evolutionary alternative to the vertebrate model. The arthropod phylum alone contains an estimated 84% of all known animal species, demonstrating the immense success of the external skeletal system. This division between internal and external support systems highlights a fundamental divergence in life history, allowing for a massive radiation of small, segmented forms across diverse ecosystems.
Major Groups of Arthropods
The phylum Arthropoda is divided into four major subphyla of living organisms, all sharing the external skeleton and segmented body plan.
- Hexapoda: This is the largest and most diverse group, including all insects (e.g., beetles, ants, and butterflies). These animals are defined by three body segments and typically six legs.
- Chelicerata: This group includes spiders, scorpions, mites, and horseshoe crabs, characterized by specialized mouthparts called chelicerae.
- Crustacea: Comprised mostly of aquatic arthropods, such as crabs, lobsters, shrimp, and barnacles. Many species reinforce their exoskeletons with calcium carbonate.
- Myriapoda: This encompasses terrestrial arthropods like centipedes and millipedes, recognized for their elongated bodies and numerous pairs of legs.