11.0.7 11.9: Arthropods are the most diverse group of all animals.

When it comes to species diversity, the arthropods take the cake, outnumbering all other forms of life. In all, the phylum contains almost one million recognized species, or approximately 75% of all animal species and 60% of all species on earth (FIGURE 11-16). Taken together, there are more than a billion billion (1 followed by 18 zeroes) arthropods alive at any given time! And about 80% of these are insects. Put another way, on earth, right now, there are more than 150 million insects for every single human. And there are about six times as many species of beetles as there are species of all birds, mammals, fishes, amphibians, and reptiles combined.

The phylum of arthropods (Arthropoda) contains bilaterally symmetrical, protostome invertebrates that have a distinct head, midsection (called the thorax), and abdomen, a rigid external covering (called an exoskeleton) made of a stiff carbohydrate known as chitin, and legs with joints (think of all the joints in a lobster’s claw) (FIGURE 11-17). The four major lineages of arthropods are:

• Millipedes and centipedes
• Chelicerates (including horseshoe crabs, spiders, mites, ticks, and scorpions)
• Crustaceans (including lobsters, crabs, shrimp, and barnacles)
• Insects

The arthropod phylum is so large that, even if we excluded all the insects—the most diverse arthropod group—it would still contain more species than any other phylum.

Millipedes and Centipedes The names millipede and centipede translate to “a thousand feet” and “a hundred feet” and refer to the enormous numbers of legs on these long, skinny animals. They don’t really have a thousand—or even a hundred—feet, but millipedes do have more legs than centipedes, and that is the easiest way to tell them apart. Millipedes and centipedes have long, segmented bodies that seem almost worm-like, but their jointed legs and hard exoskeletons are characteristics of arthropods. Both live among fallen leaves, especially in forests where the leaf litter layer is usually cool and moist. Millipedes feed on decaying plant material, while centipedes are predators that use fangs equipped with venom to kill insects and even snakes and small mammals. Centipedes use their jaws to tear prey into pieces that are small enough to swallow.

Chelicerates There are about 60,000 species of chelicerates, a group that includes four horseshoe crab species and the arachnids, land-dwelling arthropods that include spiders, scorpions, mites, and ticks. Most have four pairs of walking legs and a specialized feeding apparatus. Arachnids are distinguished from other arthropods in that they have only two body regions instead of three.

Arachnids are predators. Many spiders construct webs by spinning silk fibers (which consist of interconnected protein molecules). Orb-weaving spiders construct intricate spiral-shaped webs that trap (and sometimes even lure) flying insects. When a spider captures prey, it uses its fangs to inject venom that contains two types of enzymes. The first set of enzymes disrupts the prey’s nervous system and paralyzes it, then other enzymes dissolve its internal organs. When the prey’s innards have liquefied, the spider sucks out the contents.

The venom of most spiders is usually harmless to animals as large as humans, but two North American spiders are dangerous. The black widow spider is easily identified by a red hourglass marking on the bottom surface of the female. The brown recluse spider lacks distinctive markings, so it is harder to identify—but it is more dangerous. Both species are likely to move indoors, constructing webs in little-used spaces such as closets and cabinets and behind furniture. Brown recluse bites can create open wounds that can last for months or years and sometimes require repeated skin grafts to close.

Scorpions look dangerous to humans, but their sting is no worse than a bee’s. Scorpions are lethal to insects, hunting them at night. They seize prey with their claws and use their stinger only if the prey struggles. Some of the scorpion’s mouth structures are covered with spines that grind the prey, while glands secrete enzymes that liquefy it, and the scorpion sucks up the soup.

Crustaceans If arachnids are the most feared arthropods, the aquatic crustaceans—lobsters, crayfish, crabs, and shrimp—may be the most desirable arthropods, at least at meal time. In the United States, the top three fishery crops, by annual market value, are shrimp, crabs, and lobsters. There are about 52,000 species of crustaceans, and although crustacean species can look very different—from sessile barnacles to mobile, tiny krill—they have one feature in common: five pairs of appendages extending from the head. Three of these appendages are used for feeding, and two are antennae that sense the environment.

Crustaceans have many pairs of legs, and their legs are modified for many purposes. Shrimp and barnacles (which are closely related to shrimp, but spend their adult lives within a shell, fastened to a rock) have legs with comb-like projections that they use to capture plankton. Free-living crustaceans (shrimp, crabs, lobsters) have modified limbs in the abdominal region that are used to hold eggs or nephelany hatched young.

Although most crustaceans are aquatic, the inconspicuous wood lice (also known as pill bugs, sow bugs, or roly-polies) are terrestrial forms that you can find when you turn over a rock in your garden. Wood lice are herbivores and play an important role in recycling dead plant material.

Insects Mostly terrestrial, insects have three pairs of walking legs, and most also have one or two pairs of wings extending from the thorax. Insects abound in nearly all terrestrial habitats and are, by far, the most diverse group of arthropods. Every species of plant, for example, is eaten by at least one insect species. Wings and the ability to fly, as well as the developmental process of metamorphosis, have played important roles in the great success and diversification of insects. We explore these features in more detail in Section 11-11.