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9.10: Arthropods

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    What does this lobster have in common with a wasp?

    You might notice that their bodies have segments. And they both have a hard outer layer. Because they share these and other features, they are both classified as arthropods.

    What are Arthropods?

    How often do you think you see an arthropod? Well, have you ever looked up close at an ant? A spider? A fly? A moth? With over a million described species (and many more yet to be described) in the phylum containing arthropods, chances are, you encounter one of these organisms every day, without even leaving your house. Arthropods are a very diverse group of animals. In fact, they are the biggest group of animals on the planet, with upwards of 5 million distinct species.

    Types of Arthropods

    Arthropods belong to the phylum Arthropoda, which means “jointed feet,” and includes four living subphyla.

    • Chelicerata, which includes spiders (Figure below), mites, and scorpions. In these animals, the first pair of appendages are often modified as fangs or pincers, and are used to manipulate food. Spiders have eight legs.
    • Myriapoda, which includes centipedes and millipedes. All of these animals live on land, and can have anywhere from ten to nearly 200 pairs of appendages.
    • Hexapoda, which includes the insects. These animals dominate the land. All hexapods have three pairs (six appendages) of walking appendages.
    • Crustacea, which includes lobsters, crabs, barnacles, crayfish, and shrimp. These animals dominate the ocean, and usually have a set of anterior appendages that are modified as mandibles, which function in grasping, biting, and chewing food.
    Spiders are one type of arthropod
    Figure \(\PageIndex{1}\): Spiders are one type of arthropod.

    Characteristics of Arthropods

    Characteristics of arthropods include:

    1. A segmented body (Figure below) with a head, a thorax, and abdomen segments.
    2. Appendages on at least one segment. They can be used for feeding, sensory reception, defense, and locomotion. In addition to legs, antennas and mouth parts are considered modified appendages.
    3. A nervous system.
    4. A hard exoskeleton made of chitin, which gives them physical protection and resistance to drying out. In order to grow, arthropods shed this outer covering in a process called molting.
    5. An open circulatory system with hemolymph, a blood-like fluid. A series of hearts move the hemolymph into the body cavity where it comes in direct contact with the tissues. Hemolymph is involved with oxygen distribution.
    6. A complete digestive system with a mouth and an anus.
    7. Aquatic arthropods use gills to exchange gases. These gills have a large surface area in contact with the water, so they can absorb more oxygen.
    8. Land-living arthropods have internal surfaces that help exchange gasses. Insects and most other terrestrial species have a tracheal system, where air sacs lead into the body from pores in the exoskeleton. These pores cover a large part of their external body surface. Others use book lungs, gills modified for breathing air, as seen in species like the coconut crab. Some areas of the legs of soldier crabs are covered with an oxygen absorbing skin. Land crabs sometimes have two different structures: one used for breathing underwater, and another used to absorb oxygen from the air.
    The blue American lobster illustrates the segmented body plan of the arthropods
    Figure \(\PageIndex{2}\): The blue American lobster illustrates the segmented body plan of the arthropods.

    Summary

    • The arthropods include four living subphyla: chelicerates, including spiders, mites, and scorpions; myriapods, including centipedes and millipedes; hexapods, including insects; and crustaceans.
    • Arthropods are characterized by a segmented body, a hard exoskeleton, and appendages used for feeding, sensory structures, defense, and locomotion.

    Explore More

    Use the resources below to answer the questions that follow.

    Explore More I

    • Marine Arthropods: A Successful Design at vimeo.com/37289745 (9:28)
    1. Why are jointed limbs significant for Arthropods?
    2. How are appendage adaptations and segmentations key to the success of Arthropods as a group?
    3. Soft shell crabs are delicacies in some restaurants. Where do soft shell crabs come from?
    4. What aspect of horseshoe crabs' behavior do scientists feel gives clues to why Arthropods first left the ocean?

    Review

    1. What are three examples of arthropods?
    2. What are three distinguishing features of the arthropods?
    3. Describe the arthropod circulatory system.
    4. Describe how insects obtain oxygen.