Dressed For Success

One way to measure the success of any group of animals is to look at biodiversity (BI-o-dih-VUHR-seh-tee), or the variety of species in a particular place. Another way is to count the numbers of individuals of a particular species. By either measure, arthropods are the most successful group of animals on Earth. The physical features that make them so successful are the size and structure of their bodies. Most arthropods range in length from 0.04 to 0.4 inches (1 to 10 millimeters). This allows them to live in numerous small habitats where larger animals cannot hope to make a living. Furthermore, their bodies are wrapped in a hard, protective, external covering called the exoskeleton (EHK-so-SKEH-leh-tin). Small and armored, arthropods are perfectly suited for living and reproducing on land or in the water.

A suit of armor

The exoskeleton works both as skin and skeleton. It protects the animal from harm as it swims, crawls, burrows, or flies through the habitat, and it provides a means of support for the muscles and internal organs inside. The exoskeleton is made up of several layers that are composed mostly of chitin (KYE-tehn), a complex material that is made of fibers and combines with a protein to make the exoskeleton light, tough, and flexible, just like fiberglass. The surface of the exoskeleton is covered with small pits, spines, and hairlike structures called setae (SIH-tee). Some setae are sensitive to touch and sometimes help to protect the body from injury. In most insects and spiders, a waxy layer covering the exoskeleton helps to maintain the moisture levels inside the body. Millipedes, centipedes, and crustaceans do not have this protection.

The exoskeleton is divided into two or three body regions. Each region is made up of a series of armored plates that are sometimes closely joined together to increase strength or distinctly segmented to maintain flexibility. The appendages (mouthparts, antennae, legs) are jointed, or divided into segments to increase their flexibility. In fact, the name arthropod means "jointed foot." All plates and segments are joined together by a thin, flexible membrane of pure chitin.

The mouthparts of arthropods are made up of two to four pairs of appendages. These appendages come in a variety of forms and are used as lips, jaws, and fangs. Insect jaws may antennae ocelli frons -clypeus labrum antennae ocelli frons -clypeus labrum

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A lateral view showing the major features of an insect. (Illustration by Bruce Worden. Reproduced by permission.)

form piercing-sucking or lapping mouthparts for drinking plant or animal fluids.

The eyes, if present, are either compound or simple. Compound eyes are made up of several to tens of thousands of individual lenses and are used for seeing images. Some species, as well as all larval insects or young, have only simple eyes, which have just one lens for each eye. Simple eyes are used primarily to distinguish light and dark. One or two pairs of antennae (an-TEH-nee), or sense organs, are covered with setae that are especially sensitive to touch and often have special pits for detecting certain odors.

Adults have three or more pairs of jointed legs and, in many insects, one or two pairs of wings. The legs come in a variety of shapes and are used for running, jumping, climbing, digging, swimming, and grasping prey. The abdomen contains the in

Different types of skeletons: external (snail), hydrostatic (earthworm), and jointed (scorpion). (Illustration by Kristen Workman. Reproduced by permission.)

ternal and reproductive organs, as well as special appendages used for defense (as in scorpions), steering (in horseshoe crabs), or spinning silk (as in spiders). The abdomen is distinctly segmented in most arthropods but not in ticks, mites, and nearly all spiders.

Different types of skeletons: external (snail), hydrostatic (earthworm), and jointed (scorpion). (Illustration by Kristen Workman. Reproduced by permission.)

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