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Characteristics of ArthropodsFIGURE8Arthropod CharacteristicsThis Sally lightfoot crab shows the tough exoskeleton, the segmented body, and the jointed appendages that are characteristic of arthropods.Arthropods are classified into four major groups.The major groups of arthropods are crustaceans, arachnids, centipedes and millipedes, and insects.All arthropods share certain characteristics.Arthropods are invertebrates that have an external skeleton, a segmented body, and jointed attachments called appendages.Wings, mouthparts, and legs are all appendages. Jointed appendages are such a distinctive characteristic that arthropods are named for it.Arthrosmeans “joint” in Greek, andpodosmeans “foot” or “leg.”Arthropods share some characteristics with many other animals, too. They have bilateral symmetry, an open circulatory system, and a digestive system with two openings. In addition, most arthropods reproduce sexually.Outer SkeletonIf you were an arthropod, you would have a waterproof covering. This waxy covering is called anexoskeleton,or outer skeleton. It protects the animal and helps prevent evaporation of water. Water animals are surrounded by water, but land animals need a way to keep from drying out. Arthropods may have been the first animals to live on land. Their exoskeletons probably enabled them to do this because they keep the arthropods from drying out.As an arthropod grows larger, its exoskeleton cannot expand. The growing arthropod is trapped within its exoskeleton, like a knight in armor that is too small. Arthropods solve this problem by occasionally shedding their exoskeletons and growing new ones that are larger. The process of shedding an outgrown exoskeleton is calledmolting.After an arthropod has molted, its new skeleton is soft for a time. During that time, the arthropod has less protection from danger than it does after its new skeleton has hardened.
FIGURE9A Molting CicadaThis cicada has just molted. You can see its old exoskeleton hanging on the leaf just below it.Applying ConceptsWhy mustarthropods molt?Segmented BodyThe bodies of arthropods are segmented. A segmented body plan is easiest to see in centipedes and millipedes, which have bodies made up of many identical-looking segments. In fact, their bodies look something like the bodies of earthworms. You can also see segments on the tails of shrimp and lobsters. In some groups of arthropods, several body segments become joined into distinct sections. An arthropod may have up to three sections—a head, a midsection, and a hind section.Jointed AppendagesJust as your fingers are appendages attached to your palms, many arthropods have jointed appendages attached to their bodies. The joints in the appendages give the animal flexibility and enable it to move. If you did the Discover activity, you saw how important joints are for allowing movement. Arthropod appendages tend to be highly specialized tools used for moving, obtaining food, reproducing, and sensing the environment. For example, arthropods use legs to walk and wings to fly. In addition, most arthropods have appendages called antennae (singularantenna). Anantennais an appendage attached to the head that contains sense organs.DiversityScientists have identified more species of arthropods—over one million—than all other species of animals combined! There are probably many others that have not yet been discovered. Look atFigure 10to compare some characteristics of the four major groups of arthropods.CrustaceansIf you’ve ever eaten shrimp cocktail or crab cakes, you’ve dined oncrustaceans(krusTAYshunz).Crayfish and lobsters are other familiar crustaceans. Crustaceans thrive in freshwater lakes and rivers, and even in puddles that last a long time. You can find them in the deepest parts of oceans and along coastlines. A few, like the pill bug, live in damp places on land.Body Structure