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The Arthropods: Blueprint for Success Chapter 14 Zoology What is the most abundant animal on Earth? • Answer: Copepods, a type of marine Arthropod. • 1-2 mm planktonic crustacean • Basis of marine food webs. • One of the smallest creatures is food to one of the largest – blue whales. Number of Species by Phylum Mollusca Chordata Platyhelminthes Nematoda Arthropoda Porifera Annelida Echinodermata Sarcomastigophora Apicomplex Ciliophora What is an Arthropod? • Arthro – joint + podas – foot • Crayfish, lobsters, spiders, mites, scorpions, and insects. • + 1 million species described. • Estimated to be 30 – 50 million undescribed species. • Most successful Phylum in the Animal Kingdom. Arthropoda - Jointed Legs Arthropods - Most Successful Animals • • • • Number of species Diversity Distribution Longevity Reasons for Arthropod Success • Versatile exoskeleton • Segmentation • Oxygen piped directly to cells (terrestrial) • Highly developed sensory organs (compound eye) • Complex behavior • Metamorphosis General Characteristics of Arthropods • Triploblastic Protostome • Bilateral Symmetry • Metamerism – segmented body with tagmatization • Jointed Appendages • Exoskeleton • Ecdysis or molting • Ventral Nervous System • Open Circulatory System • Complete Digestive Tract • Metamorphosis Phylogeny of Arthropods Arthropoda Annelids (worms) Onychophorans (worms w/legs) Chelicerates (spiders) Crustaceans (lobsters) Trilobites (extinct) Worm-like Ancestor Insects (butterflies) Metamerism and Tagmatization • Metamerism – segmented body (like Annelida) but specialization of body regions for specific functions – Tagmatization. • Segmented externally, but no internal septa to divide each segment; organ systems are not divided. • Internal metamerism is not needed; exoskeleton replaces the hydrostatic compartments of annelids for support. Tagmatization • Body regions are called tagmata - specialized for feeding, locomotion, and visceral functions. • Examples are: head, thorax, and abdomen • Most internal organs are not segmented. The Exoskeleton • Arthropods enclosed in an external, jointed skeleton – exoskeleton or cuticle. • Provides: structural support, protection, prevents water loss, attachment points for muscle movement. • It is a nonliving mixture of lipids, proteins and a tough polysaccharide called – chitin. • Made by a deeper layer of tissue – epidermis or hypodermis. Two Layers of The Exoskeleton • Epicuticle – waxy, outermost layer; impermeable to water, bacteria, and pesticides. • Procuticle – deeper, thicker layer of chitin and protein; outer portion may darken and harden by sclerotization (proteins) or with calcium carbonate. • Hard, but flexible armor. Hardened Procuticle Softer Procuticle 13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Modifications of Exoskeleton Soft and flexible at joints for movement Invaginations for gas exchange From A Life of Invertebrates, Copyright © 1979, W. D. Russell-Hunter. Growth with an Exoskeleton • Growing arthropods must periodically shed the exoskeleton by molting – ecdysis. • Triggered by hormones (ecdysone) – which causes the old procuticle to breakdown and separate from the epidermis. • A new epicuticle and procuticle are made underneath by the epidermis. • Old exoskeleton splits open as the animal stretches with air or water intake and wriggles out of old exoskeleton. How do Arthropods grow? Ecdysis in my tarantula! They grow up so fast! Old Exoskeleton New Exoskeleton and Animal Crawling Out How big can something with an exoskeleton get? The Hemocoel • An internal cavity for the open circulatory system of arthropods; results from no internal segmentation. • Internal organs are bathed by body fluids (hemolymph) for exchanges of nutrients, wastes and gases. • Because of an exoskeleton, the coleom is no longer used as a hydorstatic skeleton. Review of Circulatory Systems Open Circulatory System Ostia Closed Circulatory System Metamorphosis • Changes in body form and physiology as arthropods grow and develop. • Immature stages (larvae) often are radically different than adult forms. • These differences reduce competition between adult and young (different body forms, behaviors, and habitats). • Many types of metamorphosis (discussed more with insects). Metamorphosis Metamorphosis in a Monarch Butterfly Larva – Caterpillar eats leafy vegetation Chrysalis Adult – Butterfly eats nectar Phylum Arthropoda Groups • Subphylum Trilobitomorpha (trilobites) • Subphylum Chelicerata – Class Merostomata (horseshoe crabs) – Class Arachnida (spiders,scorpions,ticks) – Class Pycnogonida (sea spiders) • Subphylum Crustacea – Class Malacostraca (lobsters, crabs, shrimp) – Class Branchiopoda (brine shrimp, water fleas) – Class Maxillopoda (barnacles, copepods) • Subphylum Hexapoda (insects) • Subphylum Myriapoda (millipedes and centipedes) Subphylum Trilobitomorpha • Trilobites, dominant life form that went extinct 345 million years ago. • Oval, flattened bodies divided into 3 tagmata (head, thorax, pygidium). • All body parts could roll up (like a rollie-pollie). • Appendages – two lobes or branches (biramous) – one as a walking leg and the other for digging or as a gill). • Crawled on the bottom feeding on annelids, molluscs, and decaying matter. Subphylum Chelicerata • chele – “claw”; spiders, mites, horshoe crabs,etc. • Two Tagmata: 1. Prosoma (Cephalothorax) - “head” for sensory, feeding, locomotion; often eyes but NEVER antennae. 2. Opisthosoma – “abdomen” for digestion, reproduction, excretion, and respiration. Prosoma has all the appendages! • First two pairs of appendages for feeding: 1. Chelicerae – first pair that are pincerlike (chelate) used in feeding; maybe hollow fangs. 2. Pedipalps – second pair that are for sensory, feeding, movement, or reproduction. 3. Paired walking legs are posterior to the pedipalps; number of legs varies. Opisthosoma or Abdomen Prosoma or Cephalothorax Chelicerata Legs 1st Pair - Chelicerae 2nd Pair - Pedipalps Posterior Pairs – Walking Legs Class Merostomata • Ancient group of 4 species of horseshoe crabs and giant water scorpions (extinct 280 mya). • Horseshoe crabs are benthic scavengers and predators. • Hard, horseshoe-shaped carapace covers the prosoma. • Has cheilicerae, pedipalps and first three pairs of walking legs are chelate – used in walking and feeding. • Last pair of legs for walking and digging – clams, annelids, and other invertebrates. Dorsal Ventral Opisthosoma of Horseshoe Crabs • Long, unsegmented tail – telson – used to self-right the animal. • First Pair of Appendages – genital opercula - cover genital pores. • The remaining Five Pairs of Appendages – book gills – platelike gills used in gas exchange between blood and water. Horseshoe Crab • Live up to 19 years; they are dioecious, may take 9-12 years to sexually mature. • Mates during spring and summer full and new moons, onto ocean beaches. • Females lay up to 30 thousand eggs, which males fertilize before burying them in the sand • These eggs provide a major food source for migrating birds along the Atlantic coast. Those that are not eaten hatch during the next high tide, and the tiny larvae are carried away to sea. Class Arachnida • arachne – “spider”; includes mites, ticks and scorpions. • Some of the first terrestrial animals – exoskeleton helps retain water; many other adaptations for land. • Have 4 pair of walking legs. • 2 body segments (prosoma and opisthosoma) – Except mites & ticks • All are dioecious. Digestion • Most are carnivores. • Some inject enzymes into prey with their chelicerae (venomous) to help digest food. • Suck digested material into pharynx. • Gut tract is in three parts: foregut, midgut, and hindgut- lined with cuticle so the lining is shed as the exoskeleton. Excretory Organs for Terrestrial Arachnids Excretion • Arachnids use coxal glands and/or Malpighian tubules to excrete waste and reabsorb nutrients. • Coxal Glands – paired, thin-walled sacs to remove nitrogenous wastes; leave via excretory pores at base of posterior appendages. • Uric acid is the main nitrogenous product of land arachnids. Malpighian Tubules • In insects and other terrestrial arthropods, Malpighian tubules – Remove nitrogenous wastes from hemolymph and function in osmoregulation Digestive tract Rectum Intestine Midgut (stomach) Salt, water, and nitrogenous wastes Malpighian tubules Feces and urine Malpighian tubule Rectum HEMOLYMPH Hindgut Reabsorption of H2O, ions, and valuable organic molecules Anus Malpighian Tubules K+ Hemolymph Water and K+ K+ Water and waste K+ Na+/K+-ATPase Hindgut Conc. Waste Respiration • To reduce water loss, some arachnids have book lungs – paried, leaf-like, infoldings on the ventral surface. • Air enters the book lung through slit opeing and circulates between lamellae. • Other arachnids have a tracheal system for gas exchange. • Exchange gases with the hemolymph fluid. • Remember…no blood vessels! Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Arachnid Book Lung Tracheal Systems in Insects • The tracheal system of insects – Consists of tiny branching tubes that penetrate the body Air sacs Tracheae Spiracle (a) The respiratory system of an insect consists of branched internal tubes that deliver air directly to body cells. Rings of chitin reinforce the largest tubes, called tracheae, keeping them from collapsing. Enlarged portions of tracheae form air sacs near organs that require a large supply of oxygen. Air enters the tracheae through openings called spiracles on the insect’s body surface and passes into smaller tubes called tracheoles. The tracheoles are closed and contain fluid (blue-gray). When the animal is active and is using more O2, most of the fluid is withdrawn into the body. This increases the surface area of air in contact with cells. • The tracheal tubes – Supply O2 directly to body cells. Body cell Tracheole Air sac Trachea Tracheoles (b) This micrograph shows cross sections of tracheoles in a tiny piece of insect flight muscle (TEM). Each of the numerous mitochondria in the muscle cells lies within about 5 µm of a tracheole. Body wall Air Mitochondria Myofibrils 2.5 µm Generic Arthropod Nervous System • Ladder-like nervous system designed for a segmented body plan – similar to annelids. • Ventral longitudinal nerve cord. NERVOUS SYSTEM Lateral view of body showing relative position of circulatory (yellow), digestive (green), and nervous (blue) systems. Sense Organs • Sensilla - modified extensions of the exoskeleton act as receptors (mechano- and chemoreceptors). • May be slits or membraneous drums. Photoreceptors • Arachnids have one or more pairs of eyes. • Detect movement, light intensity or images (hunting spiders). • Two Types: 1. Simple or Pigment – cup Ocelli – median eyes. 2. Compound Eyes – lateral eyes. Pigment – cup Ocelli • highly variable within the arthropods. • all the receptor units (retina cells) share a common lens. • concavity of the cup is oriented toward the light source. • pigment layer prevents the entry of light from any other direction. Compound Eyes • Most have one pair of compound eyes. • Each compound eye is made of thousands of lightreceiving units called ommatidia. • Ommatidium is the individual, self-contained, independent light-detecting unit. • Each includes focusing system (cuticular cornea and crystalline cone) Compound Eye Lens Crystalline cone Pigment cells Facet Visual cells Nerve fibers from visual cells Ommatidia Optic nerve Compound Eye Ommatidium Anatomy Arachnid Reproduction • All are dioecious • Genital openings are ventral; sperm usually transferred indirectly. • Male may package sperm in a spermatophore which he either places into the female or the female crawls over the spermatophore and picks it up. • May use modified pedipalps to transfer sperm. (spiders) Order Scorpionida • Scorpions • Tropical and temperate climates • All are predatory • Prosoma fused into hard carapace • Small chelicerae, but enlarged pedipalps • Opisthosoma is divided into preabdomen and postabdomen • Sting and venomous gland located on postabdomen Scorpion Anatomy 3 Types of Birth in Scorpions • Oviparous – females lay eggs that develop outside of body. • Ovoviviparous – young develop in large, yolky eggs held internally in body, then are born, fully developed. • Viviparous – mother provides nutrients to embryos; eggs develop in special chambers close to female digestive tract. After birth, young crawl onto mother’s back for about 1 month. Viviparous Scorpion Seeing is believing! Order Araneae • +34,000 species spiders; largest group of arachnids • Chelicerae are modified with poison and fangs. • Pedipalps are leg-like; males are modified for sperm transfer. • 6-8 eyes • Spinnerettes at the posterior end that make silk using silk glands. Tarantula Feeding What kinds of prey can they eat? Latrodectus mactans • Black widow spider • Neurotoxin Loxosceles reclusa • Fiddle back spider – Brown recluse – Brown – Violin • Necrotoxin- causes tissue death. • Warning!!! The following pictures may be distrubing! Loxosceles reclusa • Necrosis of tissue Day 3 Day 4 Day 5 Day 6 Day 9 Day 10 Order Acarina • Mites and ticks • Many are ectoparasites, others are free-living. • Most serious to human health • Prosoma and opisthosoma are fused and covered by a single carapace. • Chelicerae and pedipalps are variously modified for piercing, biting, etc. and have 4 pair of walking legs. Rocky Mountain Spotted Fever • • • • • Ticks are vector High fever Headache Muscle pain Rash – BEGINS ON EXTREMETIES • 25% fatal without antibiotics Dermacentor variabilis Dog tick Dermacentor andersoni Wood tick Dermatophagoides • Dust mite • Allergies to fecal products • 1 gram of dust holds 250,000 droppings Trombicula • • • • Chigger mite Larvae feed on skin Dermatitis Adults are feed on insect eggs. Sarcoptes scabiei, the human itch mite See it in action! Class Pycnogonida • Sea Spider • Marine arthropods- found especially in the Mediterranean and Caribbean Seas, as well as the Arctic and Antarctic Oceans • 1300 known species • Walking legs is usually eight (four pairs), but species with five and six pairs exist. • small size and slender body and legs, no respiratory system is necessary for the Sea Spiders, with gases moving by diffusion. Subphylum Crustacea • Lobsters, shrimp, crayfish, copepods, barnacles • Have two pairs of antennae • Biramous appendages two distal projections • Medial ramus – endopodite • Lateral ramus - exopodite Class Malacostraca • Crabs, lobsters, shrimp; largest class of crustraceans. • Possess mandibles and maxillae – modified appendages for chewing and grinding food. Dorsal Ventral Internal Anatomy of Crustaceans Excretory Organs Crayfish 87 Who cares about crustaceans? The Krill Some of the biggest aquatic crustaceans! Order Isopoda Pillbugs- A terrestrial crustacean. A Strange Isopod Parasite! Cymothoa exigua See it in action! Class Branchiopoda • Branchio – gill + podos – foot; fairy and brine shrimp, water fleas; freshwater. • Flattened appendages for respiration, filter-feeding, and swimming. • Females may reproduce parthenogenetically. Brine Shrimp Class Maxillopoda • Subclass Copepoda – copepods; kope – oar + poda – foot; most abundant crustacean; marine and freshwater. • Cylindrical body and one compound eye. • First antennae are modified for swimming and no other appendages. • Most are planktonic and filter feeding. Copepods Class Maxillopoda • Subclass Thecostraca – +1000 species of barnacles; sessile marine crustaceans. • Most are monecious. • A larval form develops a bivalved carapace and first antennae become used for attachement. • Gut tract becomes U-shaped and thoracic appendages used for filter feeding. • Attach to rocks, ships, whales • Some are parasitic Barnacle Cirri Testis Penis Anus Mouth Stomach Ovary Cement gland Barnacles The End? Fig. 19.24a 100