Introduction to Phylum Arthropoda
The phylum Arthropoda contains a wide diversity of animals with hard exoskeletons and jointed appendages. Many familiar species belong to the phylum Arthropoda—insects, spiders, scorpions, centipedes, and millipedes on land; crabs, crayfish, shrimp, lobsters, and barnacles in water (Fig. 3.72). Arthropods are considered the most successful animals on Earth. The phylum includes more species and more individuals than all other groups of animals combined. Over 85 percent of all known animal species are arthropods (Fig. 3.73). They live in the widest range of habitats and eat the greatest varieties of food. Consider how many kinds of animals you can find in your schoolyard. The number of kinds of arthropods (ants, flies, mosquitoes, cockroaches, beetles, spiders, etc.) vastly exceeds the number of kinds of vertebrates (humans, dogs, cats, mice, rats, etc.).
The word arthropod (from the Greek root words arthro- meaning joint and -pod meaning foot) refers to a unique feature of the group—jointed legs, called appendages, which vary widely in number and function. Appendages are used for eating, feeling, sensing, mating, respiring, walking, or defense. In addition, the arthropods have developed a hard, protective exoskeleton (outer shell). Chitin (pronounced KY-tin) is the main component in arthropod exoskeletons. Some groups, such as crabs and barnacles, secrete calcium carbonate into the exoskeleton, making it thick and hard. To grow, arthropods must molt (shed) their exoskeletons periodically. During the molt, they form a larger exoskeleton to allow for expansion. The combination of an exoskeleton and jointed appendages is analogous to a suit of armor. The muscles for movement are attached to the inner surface of the exoskeleton Fig. 3.74 A. Vertebrates such as fish and humans have an internal skeleton, called an endoskeleton, with muscles attached to its outer surface (Fig. 3.74 B).
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An arthropod’s body is divided into segments, like we saw with the annelids. In species like millipedes and centipedes, the segments are quite similar to each other (Fig. 3.75 A). In other species, like an ant, the segments are clustered in major body regions. Insects have an abdomen of several segments and a separate head and separate thorax (Fig. 3.75 B). Most segments have a pair of attached jointed appendages. The posterior pairs commonly function as swimming legs (swimmerets), the middle pairs as walking legs, and the anterior pairs as food-getting apparatuses (chelicerae, or maxillae and mandibles) or sensory organs (antennae).
Arthropods have a complete digestive system and a true coelom. Arthropods have a nervous system with a large ventral nerve cord that branches into many smaller nerve fibers that innervate the body. The ventral nerve cord leads to a small brain in the cephalic or head segment of the body. Many arthropods have image forming compound eyes and excellent chemosensory abilities. The circulatory system of arthropods is open, as it is in molluscs. While vessels carry blood from the heart into the body cavity, blood returns into the heart through small pores. Another special feature among arthropods relates to their musculature. While most invertebrate musculature is of the smooth type (similar to the muscles lining the digestive tract of humans), arthropod muscles are primarily striated (like the skeletal muscles of humans). Striated muscle has a much faster contraction rate than smooth muscle, and it is this feature that probably enabled the development of flight in many insects. Arthropods are generally dioecious (meaning they have two separate sexes), and in many species development is indirect, which means that the immature form is a larva that appears very different from the parent and undergoes a process called metamorphosis to change to the adult body plan. The caterpillar to butterfly transformation of the lepidopteran insects is a dramatic example of this type of life cycle.
Subphylum Chelicerata
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Chelicerata (from the Greek word chela meaning claw) is the subphylum of arthropods that includes spiders, mites, scorpions, and horseshoe crabs. Unlike other arthropods, chelicerates lack antennae. They also do not have the mandible food processing appendages used by other arthropods for tearing and grinding food. They instead possess a set of clawed appendages called chelicerae that are used to grab and shred food. Most chelicerates are terrestrial; exceptions are the sea spiders and the horseshoe crabs.
Not to be confused with true crabs, horseshoe crabs have their head and thorax segments are fused into a single segment called the cephalothorax that is covered by a carapace, an unjointed piece of exoskeleton (Fig. 3.76 A). This region of the body contains the walking legs and chelicerae. The abdomen segment of the horseshoe crab has the reproductive appendages and five paired gill flaps, each of which contains approximately 150 book gills, used for gas exchange. The book gills are named because they are arranged like the pages in a book. Horseshoe crabs are considered living fossils because modern horseshoe crabs are similar to those appearing as fossils from 360 million years ago.
Sea spiders are known as pycnogonids (pycno- means closely packed and gonid refers to gonidia, which is a group of asexually reproductive cells). Sea spiders are in the class Pycnogonida. They are not true spiders, or even arachnids. Sea spiders have long legs with relatively small bodies (Fig. 3.76 B). They are found across the ocean, with over 1300 species. Most are small (even as small as 1 mm) and live in relatively shallow depths. However some deep and cold water species grow to over 90 cm.
Subphylum Myriapoda
The subphylum Myriapoda is represented by land-dwelling arthropods such as centipedes and millipedes. This group contains over 13,000 species. There are no known marine myriapod species. They have a single pair of antennae and mouthparts roughly similar to those found in chelicerates. The main distinguishing feature of myriapods is the multiple jointed legs that extend from their elongated worm-liked bodies. The subphylum name comes from Greek root words meaning countless feet. Myriapods have between 10 to 750 legs.
Subphylum Hexapoda
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The class Insecta is another primarily terrestrial group of arthropods, although insects have been reported from nearly all environments with the exception of deep-sea habitats. Insects are the most diverse members of the subphylum Hexapoda (Fig. 3.77). Hexapods have a distinct body plan, which includes three large sections: a head, thorax, abdomen with three pairs of thoracic legs and one pair of antennae (Fig. 3.75 B). Hexapods have three sets of jaws called mandibles, maxillae, and labium (Fig. 3.80). Insects breathe by taking in air through spiracles into trachea tubes. Unlike in the arachnids, these tubes do not terminate at book lungs, but branch into smaller networks of tubes called tracheoles that branch directly into the tissues of the insect for gas exchange (Fig. 3.78). There is no active pumping of air, but any small movement in the insect body leads to airflow throughout the trachea.
Along with their close relatives the centipedes and millipedes, insects have appendages that are unbranched. Their bodies are developed into three distinct segments of head, thorax and abdomen. The wings (usually two pairs) are contained on the thorax segment along with three pairs of legs. Insect wings and legs are modified in many ways depending upon the lifestyle of the insect. In most insects, though, the legs include sensory receptors. Other sensory receptors are found on the body, and insects have compound eyes and light sensing ocelli on their heads.
Subphylum Crustacea
Most marine arthropods belong to the subphylum called Crustacea (Fig. 3.79). Most crustaceans live in the ocean, where they are so abundant that they are often called “insects of the sea.” Crabs, shrimp, and lobsters commonly live along the shoreline. Some species of shrimp, called krill, spend their lives as plankton, drifting in the surface waters of the open oceans. Some species of crustaceans, such as crayfish, are common in freshwater lakes and streams; a few—isopods and pill bugs, for example—live on land. Many crustaceans are used as food, both by larger animals and by humans. While most crustaceans are mobile, one exception is the barnacle. Barnacles are free swimming as larvae, but when they metamorphose, the head reduces and is cemented to the substrate with a strong chemical glue. The barnacle secretes a hard protective shell around itself and filter feeds from inside this shell, using elongated appendages to collect food particles. Due to their lack of mobility, barnacles are one of the few arthropods that are hermaphroditic.
The crustacean body is clearly divided into a head, an abdomen, and a thorax. While the insects have a flexible joint between the head and thorax, crustaceans tend to have the two sections fused together. Crustaceans have two antennae on the head, followed by mandibles, maxillae, and maxillipeds, all structures for handling food (Fig. 3.80). The thorax contains the walking and swimming legs. The front pairs of appendages are often modified into claw-bearing appendages. The abdominal section contains appendages called swimmerets (Fig. 3.80). These appendages produce water currents that pass over the gills for respiration. Female crustaceans typically use swimmerets to hold onto eggs for brooding, or keeping the eggs with them until they are ready to hatch. The abdomen also contains the tail appendages.
Lobsters and crayfish have external structures that make it possible to identify their sex. Males and females can be identified by their swimmerets, the appendages on the abdomen. The female swimmerets have hairs, making them look like feathers; the male swimmerets are relatively hairless (Fig. 3.81). In the male, the sperm openings are at the base of the last walking legs. During mating the male transfers the sperm by using the first pair of its abdominal swimmerets (behind the walking legs). In males, the swimmerets are large and point toward the front of the body. When brought together in the midline of the body, these two appendages form a tube through which sperm pass to the female. The female keeps the sperm in a saclike receptacle whose opening is between the third pair of walking legs. The openings of the female reproductive tract are at the base of the second pair of walking legs. Some days or weeks after receiving the sperm, the female releases several hundred fertilized eggs. These eggs are attached to her swimmerets by a glue-like secretion. The eggs hang like grapes on a stem for weeks to months until hatching. In crabs the abdomen is much smaller than in lobsters and crayfish, and the reproductive structures are not as easy to see. In males the abdomen is narrow and fits into a groove on the under side of the thorax. In females the abdomen is flat and broad for sheltering eggs, covering most of the underside of the thorax (Fig. 3.82).
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