arachnida

Arachnida is a diverse class of jointed invertebrates within the phylum Arthropoda. Members are characterized by eight legs and two main body segments, the prosoma (cephalothorax) and opisthosoma (abdomen). They lack antennae and true mandibles, instead possessing chelicerae for feeding and pedipalps for handling prey or reproduction. Most produce silk via spinnerets and many have book lungs or tracheae for respiration. Eyes, when present, are usually simple and variably arranged.

The class includes more than 110,000 described species, distributed among several orders such as Araneae (spiders), Scorpiones (scorpions), Acari (mites and ticks), Opiliones (harvestmen), Pseudoscorpiones, and Solifugae (camel or wind scorpions). Habitats range from deserts and forests to grasslands, caves, and freshwater margins. Silk use is widespread in spiders for webs, egg sacs, and shelter; other arachnids use silk less extensively or not at all.

Ecology and behavior vary, but arachnids are predominantly carnivorous predators, using venom and digestive enzymes to subdue prey. Parasitic mites and ticks have important roles as parasites or disease vectors. Spiders and some other groups contribute to natural pest control, while some species can impact human activities or health, such as ticks transmitting diseases or certain venomous spiders posing medical concerns.

Reproduction and development differ among lineages; many spiders lay eggs in silk cocoons and may exhibit parental care or elaborate courtship. Life cycles can range from simple direct development to multiple molts across several habitats, reflecting the ecological breadth of Arachnida.