What Are Arachnids?

Learn the Characteristics of Spiders, Scorpions, Ticks, and More

Spider on a white background
Most spiders hunt insects.

Getty Images / Biosphoto / Michel Gunther

The class Arachnida includes a diverse group of arthropods: spiders, scorpions, ticks, mites, harvestmen, and their cousins. Scientists describe more than 100,000 species of arachnids. In North America alone, there are about 8,000 arachnid species. The name Arachnida is derived from the Greek aráchnē with ties to a myth. In Greek mythology, Aráchnē was a woman who was turned into a spider by the goddess Athena, and so Arachnida became a fitting name for spiders and the vast majority of arachnids.

Most arachnids are carnivorous, typically preying on insects, and they're terrestrial (living on land). Their mouthparts often have narrow openings, which restricts them to eating liquefied prey. They provide an important service by 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 (e.g., 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 six 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 is 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 (e.g., crabs), Chilopoda (centipedes), and Diplopoda (millipedes).

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

  • Order Amblypygi – tailless whip scorpions
  • Order Araneae – spiders
  • Order Uropygi – whip scorpions
  • Order Opiliones – harvestmen
  • Order Pseudoscorpiones – pseudoscorpions
  • Order Schizmoda – short-tailed whip scorpions
  • Order Scorpiones – scorpions
  • Order Solifugae – wind scorpions
  • 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:

"Class Arachnida - Arachnids," Bugguide.net. Accessed 9 Nov. 2016.

Triplehorn, Charles and Norman F. Johnson. Borror’s Introduction to the Study of Insects, 7th ed., Cengage Learning, 2004.