Arachnid
An arthropod in the class Arachnida, including spiders, scorpions, ticks, and mites. Most adults have four pairs of legs and no antennae.
Arthropod: An invertebrate animal with a segmented body, jointed appendages, and an external skeleton made of chitin. Arachnids are one of the major arthropod groups.
Book lung: A respiratory organ made of stacked air-filled plates, found in some arachnids. Spiders and scorpions use book lungs to exchange gases through the body wall.
Spider: An arachnid in the order Araneae, typically producing silk and bearing venom-delivering fangs. Spiders are the most familiar arachnids and include web builders and active hunters.
Insect: An arthropod in the class Insecta, typically with six legs, antennae, and three main body regions. Adult insects generally have six legs and antennae, unlike the usual arachnid pattern.
Chelicerata: An arthropod subphylum that includes arachnids, horseshoe crabs, and sea spiders. Arachnids belong to this broader group, whose members lack antennae.
Tracheal system: A network of air-filled tubes that delivers oxygen directly to tissues in many arthropods. Several arachnid lineages breathe through tracheae rather than book lungs.
Scorpion: An arachnid in the order Scorpiones, with grasping pedipalps and a venomous tail. Scorpions show a distinctive arachnid body plan adapted for seizing and stinging prey.
Crustacean: An arthropod belonging to a diverse group that includes crabs, shrimp, and woodlice. Many crustaceans have antennae and gills, traits absent from the typical arachnid body plan.
Prosoma: The front body region of a chelicerate, bearing the eyes, mouthparts, and walking legs. This fused front region carries the legs and feeding appendages of an arachnid.
Haller's organ: A specialized sensory organ on the forelegs of hard ticks that detects chemical and environmental cues. It helps ticks locate hosts despite their lack of antennae.