Spiders are invertebrates that belong to the Phylum Arthropoda, a massive group of animals known for their jointed limbs and hard external skeletons. These eight-legged predators are often mistakenly grouped with insects, which belong to a different class entirely. Understanding a spider’s classification reveals its true place in the natural world.
Spiders Belong to the Class Arachnida
Spiders are members of the Class Arachnida, a classification that places them within the larger Phylum Arthropoda alongside crustaceans, millipedes, and insects. Within this phylum, spiders belong to the subphylum Chelicerata, defined by a unique pair of feeding appendages called chelicerae. These specialized mouthparts, which often take the form of fangs in spiders, are used to grasp, crush, or inject venom into prey.
Distinguishing Spiders from Insects
The most striking difference between a spider and an insect lies in their fundamental body structure, separating them into the classes Arachnida and Insecta. Spiders have their bodies divided into two main sections: the cephalothorax and the abdomen. In contrast, insects possess three distinct body parts: the head, the thorax, and the abdomen.
Another distinction is the number of walking legs. Spiders have four pairs of legs, totaling eight, all attached to the cephalothorax. Insects have three pairs of legs, totaling six, attached to the thorax. Spiders also lack antennae, the sensory appendages that insects use to navigate. Furthermore, adult spiders do not possess wings, unlike many insect species.
Immediate Relatives of Spiders
The closest living relatives of spiders are found within the Class Arachnida, sharing the defining features of eight legs and a two-part body structure.
Scorpions (Order Scorpiones) are ancient relatives that share chelicerae and pedipalps. Their pedipalps have evolved into large, prominent claws used for defense and prey capture. The scorpion’s abdomen is segmented and ends in a telson, which is modified into a stinger.
Harvestmen (Order Opiliones), commonly known as daddy long-legs, are also arachnids, but they are not true spiders. They lack venom glands or silk-producing spinnerets. Their bodies appear more fused, with the cephalothorax and abdomen broadly joined into a single, compact unit.
Mites and Ticks (Subclass Acari) represent another diverse group of relatives. Adults possess eight legs, though their two body segments are often completely fused into a single, sac-like body.