What Are Arachnids?

Traits of the Class Arachnida

Spider.
Spiders belong to the class Arachnida. Getty Images/Biosphoto/Michel Gunther

The class Arachnida includes a diverse group of arthropods: spiders, scorpions, ticks, mites, harvestmen, and their cousins. Scientists describe over 100,000 species of arachnids. In North America alone, there are about 8,000 arachnid species. The name Arachnida derives from the Greek aráchnē, which means spider. The vast majority of arachnids are spiders.

Most arachnids are carnivorous, typically preying on insects, and terrestrial, living on land.

Their mouthparts often have narrow openings, which restricts them to eating liquified prey. Arachnids provide an important service, keeping insect populations under control. 

Although technically the word arachnophobia refers to a fear of arachnids, this term is widely used to describe a fear of spiders.

Arachnid Characteristics

To be classified in the class Arachnida, an arthropod must have the following characteristics.

  1. Arachnid bodies are usually divided into two distinct regions, the cephalothorax (anterior) and the abdomen (posterior).
  2. Adult arachnids have four pairs of legs, which attach to the cephalothorax. In immature stages, the arachnid may not have four pairs of legs (such as in mites).
  3. Arachnids lack both wings and antennae.
  4. Arachnids have simple eyes, called ocelli. Most arachnids can detect light or its absence, but don't see detailed images.

Arachnids belong to the subphylum Chelicerata.

Chelicerates, including all arachnids, share the following characteristics.

  1. They lack antennae.
  2. Chelicerates typically have 6 pairs of appendages.

The first pair of appendages are the chelicerae, also known as the fangs. The chelicerae are found in front of the mouthparts and look like modified pincers.

The second pair are the pedipalps, which function as sensory organs in spiders and as pincers in scorpions. The remaining four pairs are the walking legs.

Although we tend to think of arachnids as being closely related to insects, their nearest relatives are actually horseshoe crabs and sea spiders. Like arachnids, these marine arthropods possess chelicerae and belong to the subphylum Chelicerata.

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:

  • Order Amblypygi - tailless whipscorpions
  • Order Araneae - spiders
  • Order Uropygi - whipscorpions
  • Order Opiliones - harvestmen
  • Order Pseudoscorpiones - pseudoscorpions
  • Order Schizmoda - short-tailed whipscorpions
  • Order Scorpiones - scorpions
  • Order Solifugae - windscorpions
  • Order Acari - ticks and mites

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.

Sources:

  • Borror and DeLong's Introduction to the Study of Insects, 7th Edition, by Charles A. Triplehorn and Norman F. Johnson

  • "Class Arachnida - Arachnids," Bugguide.net website. Accessed online November 9, 2016.