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What is an arthropod?

Arthropods are a motley crew: cockroaches, crabs, butterflies, beetles, centipedes, scorpions, shrimp, spiders, lobsters, lice, ticks, termites, potato bugs, and sea monkeys (a.k.a., brine shrimp) — they're all examples of arthropods. These evolutionary cousins can trace their family history back to a common ancestor that lived in the ancient oceans 600 million years ago, before vertebrates or land plants even existed.

On the surface, the arthropods may not seem related, but the family resemblance is easier to see if you compare them to other major groups of organisms (like mammals, mollusks, or bacteria). Arthropods can be distinguished from other organisms by a suite of characteristics, which include:

  • A segmented body.

    A centipede's body clearly shows the segmentation common to all arthropods.

  • Segmented legs (arthro = jointed, pod = foot).

    an arthropod leg
    A typical arthropod leg has several joints.

  • Bilateral symmetry (i.e., the left and right sides of the organism are mirror images of each other).

    The butterfly's left and right sides are symmetrical.

  • An exoskeleton (i.e., a hard covering that encapsules each segment of the body).

    All arthropods have a hard exoskeleton.

  • Molting (i.e., the shedding of the exoskeleton which enables growth).

    cicada shedding its exoskeleton
    A cicada (the light-colored insect in the center of the image), emerges from its old exoskeleton (the brown shell in the lower left of the image).

  • A body layout that is the reverse of ours. An arthropod's brains is in its head, but its nervous cord runs along its belly and its food passage (stomach, intestines, etc.) runs along its back. Since the brain (above the mouth) connects to the nervous cord (below the mouth), the brain actually surrounds the esophagus. When an arthropod eats, its food passes right through its brain! This may not be great "design," but — as we will see — evolution does not "design."

    an arthropod's nervous and digestive systems
    An arthropod cross-section, showing nervous and digestive systems.

Centipede photo © 2005 William Leonard; Beetle photo by Robert Potts © California Academy of Sciences; Cicada photo by Gladys Lucille Smith © California Academy of Sciences