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Arthropods Chapter 28 Animal Classification Invertebrates Sponges Cnidarians Worms Mollusks Arthropods Echinoderms -Class Arachnida -Class Crustacea -Class Insecta Animals (cold-blooded) Fishes Amphibians Reptiles Endotherms Birds Mammals Ectotherms Vertebrates (warm-blooded) 28-1: Characteristics of Arthropods Characteristics Arthropod comes from the Greek words “arthron” – joint “pod” – foot A typical arthropod is: Segmented A coelomate Has bilateral symmetry Has an exoskeleton Characteristics Appendages – any structure (a leg or an antenna) that grows out of the body of an animal Characteristics Arthropods are the earliest known invertebrates to exhibit jointed appendages Joints are advantageous because they allow more flexibility and powerful movements Arthropod Exoskeletons Provide Protection Exoskeleton is made of protein and chitin (KI tun) Chitin is also found in cell walls of fungi The exoskeleton protects and supports internal tissues and provides places for attachment of muscles Arthropod Exoskeletons Provide Protection The weight of the exoskeleton limits the size of arthropods The larger an arthropod is, the thicker and heavier its exoskeleton must be to support its larger muscles Why Arthropods Must Molt Exoskeletons cannot grow, so they must be shed periodically Molting – shedding the old exoskeleton Before an arthropod molts, a new, soft exoskeleton formed beneath the old one Cicada Segmentation in Arthropods Arthropods do not have as many segments as segmented worms In most groups, arthropods are consisted of three segments Head Thorax Abdomen Segmentation in Arthropods Some arthropods have two sections Cephalothorax – A fused head and thorax Abdomen Arthropods Have Efficient Gas Exchange Three types of respiratory structures have evolved in arthropods: 1. Gills – Aquatic arthropods exchange oxygen and carbon dioxide through gills Arthropods Have Efficient Gas Exchange Land arthropods have either: 2. Tracheal tubes – branching networks of hollow air passages that carry air throughout the body Arthropods Have Efficient Gas Exchange 3. Book lungs – air-filled chambers that contain leaf-like plates Arthropods Have Acute Senses Movement, sound, and chemicals can be detected with great sensitivity by antennae Antennae are also used for sound and odor communication; can detect pheromones Atlas moth Arthropods Have Acute Senses Pheromones – chemical odor signals given off by animals Some pheromones are used as scent trails (example: the group-feeding behavior of ants) Many pheromones are important in the mating behavior of arthropods Arthropods Have Acute Senses Most arthropods have one pair of large compound eyes and three to eight simple eyes A simple eye is a visual structure with only one lens that is used for detecting light Arthropods Have Acute Senses A compound eye is a visual structure with many lenses Compound eyes can detect the movements of prey, mates, or predators, and can also detect colors Arthropods Have Acute Senses Arthropods have hairlike structures used to detect movement Ex. Fly and flyswatter Arthropod Nervous Systems are Well Developed The nervous system consists of a double ventral nerve cord, an anterior brain, and several ganglia Ganglia act as control centers for the body sections in which they are located Arthropods Have Other Complex Body Systems Arthropods have an open circulatory system and a complete digestive system Mouth, stomach, intestine, and anus Arthropods Have Other Complex Body Systems Mandibles – the mouthparts of most arthropod groups include one pair of jaws The mandibles are adapted for holding, chewing, sucking, or biting Arthropods Have Other Complex Body Systems Malpighian tubules – used to excrete wastes into the intestine Arthropods Reproduce Sexually Some species (bees, ants, and wasps) exhibit parthenogenesis – a form of asexual reproduction in which a new individual develops from an unfertilized egg In bees, drones are developed from unfertilized eggs 28-2: Diversity of Arthropods Diversity of All Animals About 85% of all known animals are arthropods Additional information http://evolution.berkeley.edu/evolibrary/article/_0_0/arthropods_intro_05 Arachnids Spiders, scorpions, mites, and ticks belong to the class Arachnida Spiders are the largest group of arachnids An arachnid has a cephalothorax and an abdomen with six pairs of jointed appendages Arachnids Chelicerae [kuh-lis-er-uh] – the first pair of appendages located near the mouth (modified pinchers or fangs) Pincers are used to hold food, and fangs inject prey with poison Arachnids Pedipalps – the second pair of appendages that are adapted for handling food and for sensing In male spiders, pedipalps are used to carry sperm during reproduction The four remaining appendages are adapted as legs Arachnids As silk is secreted, it is spun into thread by structures called spinnerets, located at the rear of the spider Crustaceans Crustaceans are the only arthropods that have two pairs of antennae for sensing Some crustaceans have three body sections, and other have only two Crustacean mandibles open and close from side to side The blue crab has 3 body sections Crustaceans Class Crustacea include crabs, lobsters, shrimps, crayfishes, pill bugs, and barnacles Most crustaceans are aquatic and exchange gases over gills Centipedes and Millipedes Centipedes and millipedes have tracheal tubes for gas exchange Centipedes are carnivorous and eat soil arthropods, snails, slugs, and worms These arthropods can bite Centipedes and Millipedes A millipede eats mostly plants and dead material on damp forest floors Millipedes do not bite, but they can spray foul-smelling fluids from stink glands Insects Class Insecta is the largest group of arthropods Insects have three body segments and six legs Insect Reproduction Insects usually mate once during a lifetime Some insects exhibit parthenogenesis Most insects lay a large number of eggs, which increase the chances that some offspring will survive long enough to reproduce Metamorphosis Metamorphosis – the series of changes, controlled by chemical-substances in the animal Two types of metamorphosis: 1. Complete metamorphosis consists of four stages: Egg Larva – the free-living, wormlike stage of an insect Pupa – a period of reorganization in which the tissues and organs of the larva are broken down and replaced by adult tissues Adult Metamorphosis Examples: ants, beetles, flies, and wasps Complete metamorphosis is an advantage for arthropods because larvae do not compete with adults for the same food Metamorphosis 2. Incomplete metamorphosis – insects go through only three stages of development Egg Nymph – hatches from an egg, has the same general appearances the adult but is smaller Adult With each molt, it begins to resemble the adult more Grasshoppers and cockroaches undergo incomplete metamorphosis Picture of incomplete metamorphosis
