arthropods

Arthropods are a diverse and ubiquitous animal phylum (Arthropoda) that includes insects, arachnids, myriapods, and crustaceans. They are characterized by a segmented body, an external exoskeleton of chitin, and jointed appendages. Their bodies are typically organized into tagmata, such as a head, thorax, and abdomen in insects, or prosoma and opisthosoma in many arachnids.

Key features common to arthropods include an open circulatory system with hemolymph, a ventral nerve cord and brain, and highly developed sensory organs in many groups. The exoskeleton provides protection and muscle attachment but requires periodic molting as the animal grows. Metabolic and respiratory adaptations are diverse: insects mostly use a tracheal system for gas exchange, crustaceans frequently have gills, and many arachnids possess book lungs or tracheae.

Arthropods exhibit a wide range of life cycles. Insects show incomplete metamorphosis (hemimetabolism) with nymphs resembling adults, or complete metamorphosis (holometabolism) with larval stages that look very different from adults. Molting hormonally regulated by ecdysone and juvenile hormone is common across groups.

Ecologically, arthropods occupy nearly every habitat and play essential roles as pollinators, decomposers, herbivores, and predators. They are also a major source of food for other animals and have significant economic impacts as pests and beneficial species. The arthropod lineage has a rich fossil record and demonstrates extensive diversification linked to tagmosis and specialized appendages.