Arachnids are a class (Arachnida) of chelicerate arthropods studied across zoology, ecology, and evolutionary biology as a diverse group that includes spiders, scorpions, mites, ticks, harvestmen, and related lineages. Research on arachnids focuses on their morphological specializations (e.g., chelicerae, pedipalps, spinnerets in spiders, pectines in scorpions), respiratory structures (book lungs, tracheae), and reproductive and developmental strategies. They serve as key models in venom biochemistry, neurobiology, sensory ecology, and biomechanics, and several groups (notably ticks and mites) are of major importance in medical and agricultural entomology as vectors, parasites, and bioindicators within terrestrial and, to a lesser extent, freshwater ecosystems.