Arachnids are a diverse group of invertebrates within the phylum Arthropoda. They inhabit nearly every terrestrial environment globally, with some species in freshwater and marine habitats. Comprising over 110,000 named species, arachnids include many familiar animals.
Defining Characteristics of Arachnids
Arachnids possess distinct anatomical features that set them apart within the animal kingdom. Most adult arachnids have two main body segments: the prosoma (cephalothorax), a fused head and thorax, and the opisthosoma (abdomen). While this two-part body plan is common, some groups like mites and harvestmen exhibit a more fused body appearance where these segments are not as distinctly separated. A defining feature is the presence of eight legs, attached to their cephalothorax. This characteristic generally holds true for adult forms, although some larval mites may have only six legs, gaining additional pairs as they mature. They have two other pairs of specialized appendages: the chelicerae, which are mouthparts used for feeding and defense, and pedipalps, which can serve various functions like sensing, manipulating prey, or reproduction. Respiration in arachnids occurs through structures such as book lungs or a system of tracheal tubes.
Common Arachnid Groups
The class Arachnida encompasses a wide array of creatures, each with unique adaptations. Spiders, belonging to the order Araneae, constitute the largest group of arachnids, with over 51,000 known species. They are distinguished by their ability to produce silk from spinnerets, typically located at the tip of their abdomen, and most species possess venom used to subdue prey.
Scorpions, members of the order Scorpiones, are readily identifiable by their prominent grasping pincers, which are modified pedipalps, and a segmented tail that typically curves over their back, ending in a venomous stinger. They are found on all continents except Antarctica. Mites and ticks, classified under the order Acari, are often microscopic. Their body segments are frequently fused, and numerous species are known for their parasitic lifestyles, feeding on plants or animals.
Harvestmen, or “daddy longlegs,” from the order Opiliones, often have a more globular body where the cephalothorax and abdomen are broadly joined, appearing almost as a single segment. They are characterized by their exceptionally long, slender legs, with the front pair often used for sensing their environment. Unlike spiders, harvestmen are not venomous and do not produce silk or build webs.
Arachnids vs. Insects: Key Differences
While both arachnids and insects belong to the phylum Arthropoda, they differ in body plans and characteristics. Insects typically have three distinct body segments: a head, a thorax, and an abdomen. In contrast, arachnids generally have only two primary body sections: the cephalothorax and the abdomen.
Another distinction lies in the number of legs. Adult insects have six legs, attached to the thorax. Adult arachnids have eight legs. Furthermore, insects are equipped with antennae, which they use for sensing their surroundings, and many species also possess wings for flight. Arachnids lack both antennae and wings, relying on their specialized pedipalps and sensory hairs for environmental perception.