Have you ever gotten into a debate with a friend about what kind of creature a spider is? Despite their creepy-crawly behavior and abundance of legs (and regardless of what you may have heard), spiders aren’t actually insects. These critters belong to a scientific class called Arachnida, which makes them arachnids, along with other beings in the same class, like ticks, mites, and scorpions — but not insects like bees, beetles, or butterflies. Confused? Let’s take a look at how animals, particularly spiders, are classified. It might not make spiders any easier to handle if you’re an arachnophobe, but at least you’ll be able to yell at them with scientific accuracy.
Taxonomists Classify Living Things With the Linnaean System
Taxonomists are people who study taxonomy, the science of organizing living things according to their characteristics. They look at physical, genetic, behavioral, and other traits of a specific organism to figure out how it’s related to other organisms, then give it a name that indicates that relationship.
Swedish botanist Carl Linnaeus devised this system starting in the 1730s in his book Systema Naturae, which proposed grouping organisms into hierarchical categories (called a taxonomic rank) and giving them two-part Latin names, greatly simplifying the study of plants and animals. For example, the American house spider, one of the most common spiders in the U.S., has the two-part Latin name Parasteatoda tepidariorum.
All Animals (Including Spiders) Belong to the Kingdom Animalia
Scientists organize most non-plant organisms into seven main groups: kingdom, phylum, class, order, family, genus, and species. Kingdom is the most general, while species is the most specific. All animals fall into the kingdom Animalia by virtue of certain shared characteristics (with very few exceptions): They’re multicellular, they eat and digest organic material, they breathe air, they move, and they reproduce sexually. As you may have guessed, spiders are definitely animals based on these biological abilities.
Spiders and Insects Are Arthropods
Once they establish that something is an animal, taxonomists will continue to classify it into more specific categories of shared characteristics. Spiders belong to the phylum Arthropoda, the largest phylum of animals. This diverse group also includes insects, crabs, lobsters, and centipedes. These creatures all have exoskeletons, segmented bodies, and paired, jointed appendages (“arthropoda” actually means “jointed feet” in Latin).
Spiders and Insects Belong to Different Subphyla
Many taxonomists also place animals in a subphylum, a category that’s a little more specific than phylum, but less specific than class. Here’s where spiders and insects branch away from each other on the tree of life. Spiders belong to the subphylum Chelicerata, along with scorpions, mites, and horseshoe crabs. Organisms in Chelicerata have four pairs of walking legs and two pairs of mouthpart appendages (the pedipalps and the chelicerae).
Insects, on the other hand, belong to the subphylum Hexapoda — derived from Greek for “six feet.” Unlike spiders, the winged insects, springtails, and silverfish in this group have six legs and a pair of antennae, as well as jaws for crushing and chewing.
All Spiders Are Arachnids, But Not All Arachnids Are Spiders
With the class, order, and family categories, taxonomists further define which organisms spiders are closely related to and which ones are just distant relatives. Insects belong to the class Insecta, while spiders are in Arachnida. The term arachnid describes all the animals in Arachnida, not just spiders.
Arachnida contains 11 orders, the best known being Araneae (spiders), Scorpiones (scorpions), Acari (ticks and mites), and Opiliones (harvestmen, a.k.a. daddy long-legs). The organisms in each of the orders have physical and behavioral characteristics that differentiate them. Spiders in Araneae bear four pairs of legs and a combined head and thorax connected to the abdomen by a slender waist — in other words, your typical spider anatomy.
Within Their Order, Spiders Are Further Categorized Into Families
More than 100 families comprise the order Araneae. Included in this group are true spiders that have silk-producing spinnerets located on the underside of their abdomens. The families include Theraphosidae (tarantulas), Lycosidae (wolf spiders), Sicariidae (recluse spiders), Araneidae (orb-weaver spiders), and dozens of other types. Each family contains one or more genera (the plural of genus).
Genus and Species Are the End of the Taxonomic Line
The most specific classifications in the animal kingdom are genus and species. An organism’s genus is the first word in its Latin name, and its species is the second. (So for the common house spider Parasteatoda tepidariorum, “Parasteatoda” refers to the genus and “tepidariorum” to the species.) Animals in a genus are closely related from an evolutionary standpoint and have many physical characteristics in common. One genus may contain numerous species, each defined by a unique trait that only they possess. The Parasteatoda genus now has more than 40 species, for example. According to the World Spider Catalog, there are currently 49,482 spider species in 4,217 genera known to science.
Spiders’ Classifications Aren’t Set in Stone
Though naturalists have been organizing and naming spiders according to the Linnaean system for nearly 300 years, scientists are constantly learning new things about known species that refine their place on the tree of life. The common house spider, for example, has changed genus three times.
Some taxonomists now use phylogenetic nomenclature, a newer system that places animals into categories called clades according to their ancestry, which may better illustrate their non-linear evolutionary journey. This approach is controversial, though.
Taxonomists still examine an organism’s physical appearance to determine their taxonomic rank, but today, advancing genetic technology has allowed scientists to discover new species from their DNA. Some have been found in the wild, and others in museum collections. In 2019, the last year for which data is available, 652 new spider species were described in scientific journals.
But just remember: all of them are animals, and not one is an insect.
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