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What Are Arachnids?

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What Are Arachnids?

Spider.

Photo: © Debbie Hadley, WILD Jersey

The class Arachnida includes a diverse group of arthropods: spiders, scorpions, ticks, mites, harvestmen, and their cousins. Scientists describe over 75,000 species of arachnids, the majority of them spiders. Most arachnids are carnivorous, typically preying on insects, and terrestrial, living on land. Arachnids provide an important service, keeping insect populations under control.

Arachnid Characteristics:

To be classified in the class Arachnida, an arthropod must have certain characteristics. Arachnid bodies are divided into two distinct regions, the cephalothorax and the abdomen. Four pairs of legs attach to the cephalothorax. Arachnids lack wings and antennae.

Arachnid Classification:

Arachnids, like insects, are arthropods. All animals in the phylum Arthropoda have exoskeletons, segmented bodies, and at least three pairs of legs. Other groups belonging to the phylum Arthropoda include: Insecta (insects), Crustacea (crabs), Chilopoda (centipedes) and Diplopoda (millipedes).

The class Arachnida is divided into orders and subclasses, organized by common characteristics. These include:

Here is an example of how an arachnid, the cross spider, is classified:

  • Kingdom Animalia: the animal kingdom
  • Phylum Arthropoda: arthropods
  • Class Arachnida: arachnids
  • Order Araneae: spiders
  • Family Araneidae: orb weavers
  • Genus: Araneus
  • Species: diadematus

The genus and species names are always italicized, and are used together to give the scientific name of the individual species. An arachnid species may occur in many regions, and may have different common names in other languages. The scientific name is a standard name that is used by scientists around the world. This system of using two names (genus and species) is called binomial nomenclature.

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