DICK'S VIRTUAL ARTHROPOD ZOO
"Sounds interesting, but -- uh -- what, exactly, is an arthropod?"
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Maybe the easiest way to get started is to name some examples. Insects (ants, for example, or beetles) are arthropods; so are arachnids (for example, spiders and scorpions). Then there are crustaceans (for example, shrimps), centipedes, and millipedes. These you know -- you've seen most of them around. (If you live in or near the Sonoran Desert, you're more likely than some people to have seen a scorpion or a centipede, but folks from elsewhere may be surprised to learn that there are also crustaceans hereabouts. The specimens in our "virtual zoo" are all from the Sonoran Desert.)
"Yes, I suspected something like that, but what, exactly, is an arthropod? What do all these creatures have in common that justifies lumping them under one name? And if millipedes qualify, why don't worms?"
All right, you asked for it. Webster defines "arthropod" as follows:
any member of Arthropoda, a phylum of invertebrates with segmented bodies and jointed limbs.
It's the "jointed limbs", actually, that originally supplied the name; "arthropod" comes from the Greek "arthro", meaning jointed, and "poda", meaning foot. "Foot", though, is too restrictive a term; an arthropod has jointed appendages in general.
The name may come from the jointed appendages, but the segmented bodies are every bit as distinctive. And there are further arthropod characteristics that Webster's definition doesn't mention: in particular, an arthropod has an exoskeleton -- an outer framework made of a tough material called chitin (As often as not, the material is mentioned along with the framework: "chitinous exoskeleton".) An arthropod has no bones, but its exoskeleton serves the same purpose of structural support; in addition, it serves as body armor.
Many arthropods (for example, butterflies) spend a part of their lives as larvae, during which time their bodily form is rather wormlike. To see at a glance that they are arthropods, you may have to wait until they grow up; however, if you look closely, you can see that even a caterpillar has little legs. (They may look like nothing more than bumps on the underside, but they assist in locomotion, and the front three pairs develop into the adult's legs. )
Arthropods have some other characteristics in common, but not all of them are visible on the outside. To look at them, you'd have to dissect the poor creature. If you're curious, here's a fuller, more formal list of arthropod characteristics.
Now let's bring on the arthropods.
To learn more about arthropods after traveling through our virtual arthropod zoo, check out more Sonoran Arthropod Studies Institute Web Pages!