What is an Arachnid? - Definition, Characteristics & Anatomy

What is an Arachnid? - Definition, Characteristics & Anatomy
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  • 0:00 Definition
  • 0:45 Characteristics
  • 1:55 Physiology & Anatomy
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Lesson Transcript
Instructor: Meredith Mikell
Arachnids are a group of animals that we most commonly associate with spiders. Here we will define arachnids, describe their physical characteristics, and review some examples. At the end, you can test your knowledge with a short quiz.


Many people have a strong fear of spiders. Perhaps yourself included! Spiders are just one type of animal found in the category of arachnids, a class of invertebrates including spiders, ticks, mites, and scorpions. Though it can often be easy to confuse arachnids with insects, who do belong in the same phylum, they are not themselves insects and belong to a different class. The different classes within the same phylum are kind of like cousins to each other. Arachnids are very commonly seen in the fossil record and have had great evolutionary success during Earth's history, namely due to their unique survival characteristics. As of now, there are over 100,000 known species of arachnids!


Arachnids are classified in the Phylum Arthropoda, which also consists of crustaceans, insects, centipedes, and millipedes. This phylum is defined by the following key characteristics:

  • Body segmentation - usually consisting of a head, thorax, and abdomen
  • Jointed appendages
  • A firm but flexible exoskeleton
  • An open circulatory system - blood is free-flowing in the body, not contained in vessels
  • Specialized appendages - including claws (crustaceans) or wings (insects)

Arthropods are sometimes nicknamed the 'swiss army knives of living things', for their dexterity and array of appendage functions.

Arachnids' specific class within Phylum Arthropoda is called Class Arachnida.

Arachnids possess the same general characteristics that define all arthropods, but have a few other unique distinctions:

  • No antennae, claws, or wings, but instead have mandibles : structures for biting and chewing prey
  • Eight appendages
  • A fused head and thorax, called a cephalothorax

Physiology & Anatomy

The specific characteristics of arachnids allow for effective movement, hunting, and self-defense techniques. Having jointed appendages means flexibility and quick escapes from predators. The exoskeleton acts like a shield of armor, but also provides rigidity for holding internal organs and maintaining pressure within the body.

The mandibles allow for chewing of prey, which in the case of arachnids, is usually other arthropods, but certainly can include larger animals. Some arachnids are venomous and have fangs with which to inject venom into prey, or predators trying to attack it. Scorpions, for example, have a poisonous stinger on the end of a long, whip-like tail.

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