Arthropod Review Sheet The Phylum Arthropoda is the largest and most diverse of all animal phyla (Fig 1). More than three quarters of the animals on earth are arthropods, and most of these are insects. More than 900,000 species have been described, and if biologists have the opportunity to explore the rain forests before they are completely burned to the ground, a like number will probably be discovered. Figure 1. Numerical Distribution of Named Animal Taxa. Arthropods have their skeletons on the outside of their bodies, rather than on the inside (they have an exoskeleton; while our internal skeleton is called an endoskeleton.) The phylum's name is based on this feature (Arthropoda means "joint-foot" referring to the external joints in the skeleton that allow movement). There are both advantages and disadvantages to being inside out (although the majority opinion is clearly that it's our skeletons that are oddly situated). The advantages seem to outweigh any disadvantages, judging from the phylum's success and diversity (arthropods include in their ranks crabs, lobsters, spiders, mites, and the vast number of different insects). More than anything, the exoskeleton has allowed the arthropods to diversify and move into niches that were previously inaccessible. They were the first beasts to successfully colonize land (the exoskeleton preadapted them for terrestrial habitats since it provided both support and protection from drying out). Insects flew long before anything like a bird existed. Some must have been quite impressive; relatives of today's dragonflies with wingspans of nearly a meter flew the Triassic skies (over 230 million years ago and pre-dating the first birds by nearly 100 million years). We even owe the presence of flowers to insects (most flowers are constructed to attract insects for pollination). Trilobites Trilobites are the most primitive of the arthropods. Although they are now extinct, their hard exoskeletons are so well-preserved and plentiful in the fossil record that much is known about their anatomy, diet, embryology, and even behavior (about 4000 species are described). Examination of their appendages and general body structure show that most were marine bottom-dwelling organisms. The structure of their mouthparts and X-rays of their digestive systems indicate that their diets included seaweeds, coelenterates, and mollusks. Some were probably scavengers, feeding on organic debris in the muck much as some annelids do today. Trilobites reached their height of distribution and abundance during the Cambrian and Ordovian periods
(about 500 million years ago). The Chelicerates Members of the Subphylum Chelicerata have no antennae and feeding appendages called chelicerae. Their body is divided into two main sections: an anterior cephalothorax and a posterior abdomen. Class Merostomata. This class is represented by only four species of animals commonly known as "horseshoe crabs". Horseshoe crabs are common inhabitants of the coastal marine waters off the North American Atlantic coast and along the Gulf of Mexico. They spend most of their time burrowing through soft mud looking for worms, mollusks, and other tasty tidbits. Examine a living or preserved specimen of the horseshoe crab, Limulus polyphemus. The anterior cephalothorax is covered with a horseshoe-shaped carapace, from which the animals derive their common name. Two large compound eyes and a pair of small simple eyes (at the anterior end of the median ridge) are on the dorsal carapace. The abdomen (posterior to the carapace) bears rows of short spines and a single large telson. Despite its formidable appearance, the telson is used neither for offense nor defense, and living animals can be safely handled. When annoyed, horseshoe crabs fold their abdomen under the carapace to protect the more delicate ventral appendages. Turn your animal over to expose the ventral surface. The smallest of the pincers are the chelicerae, feeding appendages that are characteristic of this subphylum. The mouth is located directly behind them. There are five pair of walking legs. Book gills (the page-like appendages on the abdomen) are used for respiration and swimming. The anus is just anterior to the telson.
Class Arachnida. The arachnids are the most successful group in this subphylum. They include spiders, scorpions, mites, and ticks and were among the first animals to move onto land. Although this group is probably the most unpopular of all invertebrates, they are ecologically important (spiders, for example, devour more insect pests than do birds). Scorpions: Scorpions are most easily recognized by the stinging barb at the tip of their abdomen. Gently remove a preserved scorpion from its vial. The body is divided into an anterior prosoma and the posterior abdomen. As in the horseshoe crabs, the prosoma is covered with a dorsal shield (the carapace). Two pincher-like appendages (the pedipalps) are used to manipulate prey (mostly insects and spiders). There are four pair of walking legs. Spiders: The body is divided into an anterior cephalothorax and a posterior abdomen. From a dorsal view, eight eyes are clustered at the front of the cephalothorax (the eyes are the shiny black dots). On the ventral side note the fangs at the base of the movable chelicerae. Nearby you will find two small leg-like appendages (the pedipalps). In adult or penultimate ("teenage") males the last segment of the pedipalps is expanded into a copulatory organ. The pedipalps are followed by four pair of walking legs. The last segment of each leg is called the tarsus (it serves as a foot). Trace your way back toward the body and identify the metatarsus, tibia, patella, femur, and a small coxa at the base of the leg. The sternum is the flat shield between the eight coxae on the ventral surface. Identify the entrance to the book lungs (two slits on the anterior part of the abdomen) and the spinnerets (finger-like appendages at the posterior tip of the abdomen). Silk glands in the abdomen are connected to the spinnerets. Examine the other spiders on display. Mites and ticks. The abdomen of mites and ticks has lost any external evidence of segmentation and is fused to the prosoma. The carapace therefore extends over the entire body. Mites are a group of small (<1 mm), mostly parasitic arachnids (although free-living mites can inhabit both terrestrial and aquatic ecosystems). Ticks, on the other hand, are usually larger and are obligate external blood-sucking parasites throughout their entire lives.
Crustaceans Crustaceans are an extremely diverse group of primarily aquatic arthropods with two pair of antennae. The class includes such culinary delights such as shrimp, lobsters, crayfish, and barnacles. Although most are free-living, a few are parasitic. Primitive crustaceans have three main body parts: a head, a thorax, and an abdomen. There is a tendency, however, for the head and thorax to fuse and form a single cephalothorax. Major organs and appendages of the water flea, Daphnia
Crayfish external anatomy. Crayfish mouth parts and associated structures
Biramous Appendages of a Crustacean.
Internal Anatomy of a Crayfish. Uniramian Superclasses Superclass Hexapoda: The Insects. As previously mentioned, insects are the most successful of all animal phyla. They differ from crustaceans by having a single pair of antennae (rather than two) and legs based on a uniramus plan (with unbranched appendages). The exoskeleton, though similar in structure to that of crustaceans, lacks significant calcium deposits. Characteristics that separate them from the other arthropods include three pairs of legs and (usually) two pairs of wings. Examine the preserved and living insects on display to appreciate the variability in their body plans and adaptations.
Bee legs Grasshopper disection Superclass Myriapoda. Myriapods include the centipedes and millipedes (the term "Myriapoda" means "many footed"). Like the insects, they are primarily terrestrial in habits, have a single pair of antennae, uniramus appendages, and mouthparts based on mandibles. Unlike the insects, the myriapod body is divided into only two segments, a head and trunk (instead of an insect's three segments- the head, thorax, and abdomen) and the eyes are not as complex. The trunk segments may range from a few to more than a hundred. Class Chilopoda- the Centipedes. Centipedes have one pair of legs per body segment and poison glands (most, however, are not dangerous to humans). They prefer moist places and feed on earthworms and insects. Examine the centipedes on demonstration so that you can recognize their general structure. Class Diplopoda- the Millipedes. Millipedes have two pairs of legs per body segment and do not possess poison glands. Like centipedes, they prefer moist habitats, but they are either herbivorous or scavengers.