ARTHROPODS Phylum Arthropoda

1 General Characteristics • Most successful and adaptable • Largest phylum in numbers • means “ Jointed foot ” • Segmented bodies. • Exhibit • Common ancestry w/annelids • Segments fused into specialized regions General Characteristics

• Exoskeleton of chitin – semi-rigid but flexible; permeable to water; waxy cuticle prevents dessication • Coelom & Hemocoel • Metamorphosis (change in form) • Molting (Ecdysis) • Huge group > 1,000,000 species.

• Estimate : 1,000,000 spp. 1,190,000 spp. Animals

• ~ 84% of all animal species • How can we explain the success of the arthropods?

• Exoskeleton! Exoskeleton Structure • Exoskeleton of chitin+protein=cuticle. Structure: – epicuticle (oily, waxy) – exocuticle (chitin & protein) – endocuticle (chitin only) – epidermis secretes cuticle Problems and Exoskeleton • Problem 1. MOVEMENT • Solution : Joints • arthro-, = joint • pod, = leg/foot • Exocuticle absent from joints; may form hinges. • Endocuticle alone allows flexibility. • Problem 2. GROWTH

• Solution: Molting Moulting I

Secretion of “Molting fluid " to dissolve old endocuticle. Molting II • New cuticle formed under old exocuticle.

• Break out of old cuticle ( breaks at line of weakness) Molting III

• Inflate with water/air to increase size while skeleton soft –but soft skeleton & gravity limit size ; –arthropods are mostly small.

• Hardening of new exocuticle. Growth Stages

• Passes thru 3-20+ growth stages in life cycle. • Stop molting as adults (, most ) • Continue to molt (crayfish, tarantulas) Metamorphosis Problem 3. SENSORY INPUT

• touch –sensory setae connected to neurons

• smell & taste –hollow sensory setae w/ chemosensitive nerve endings • Problem 3 SENSORY INPUT

• vision –clear cuticle over compound or simple eyes • Problem 3. SENSORY INPUT

• hearing –tympanum = endocuticle, vibrates like eardrum –trichobothria (right ) Benefits of Exoskeleton • Support • Locomotion –lever system • walk, swim, fly • Mechanical protection ( armor ) • Retards evaporation (in air) and/or osmosis (in water) –water balance . Benefits of Exoskeleton • Reduction of coelom & segmentation –Abandoned hydrostatic system of annelid- like ancestor) –Coelom reduced to pericardial cavity

• Segments fused = tagma, tagmata –Tagmosis –Specialization of body regions (= tagmata) –Specialization of appendages • Head (~ 4-6 segments) –feeding, sensation

• Head appendages –mandibles, –maxillae, –maxillipeds, –chelicerae –antennae Tagmosis

• Thorax (~ 3-6 segments) –locomotion, grasping.

• Thoracic appendages –walking legs, –wings –chelipeds Tagmosis • Abdomen (~8- 30+ segments) –respiration, reproduction, etc.

• Abdominal appendages –abdominal gills (aquatic insect larvae) –swimmerets (crayfish) –filtering legs (barnacles) –gonopods (crayfish, etc.) –spinnerets (spiders) Tagmosis

• Number of segments/legs in each tagma varies – Cephalothorax of 6 segments in • 1 pr. chelicerae • 1 pr. pedipalps • 4 pr. walking legs – Cephalothorax of 13 segments in Crustacea (shrimps) • 2 pr. antennae • 1 pr. mandibles • 2 pr. maxillae • 3 pr. maxillipeds • 5 pr. walking legs ( 1st pair modified as chelipeds ) Open circulatory system –Dorsal heart pumps hemolymph over brain –Hemolymph moves through hemocoel back towards heart –Ostia (holes) in sides of heart let hemolymph in to go around again. Respiratory System –Gills in aquatic/marine arthropods –Tracheal systems in terrestrial arthropods –Book lungs (modified gills) in spiders & Nervous System • Resembles that of annelid –Dorsal brain with nerves around oesophagus –Paired ventral nerve cords –Segmental ganglia (fused into 1-2 ganglia in each tagma) Distinguishing Characters • Jointed exoskeleton • Tagmosis • Compound eyes Anatomy of a Grasshopper Classification

• Trilobitomorpha • Subphylum Crustacea • Subphylum Chelicerata • Subphylum Uniramia Subphylum 1: Trilobitomorpha –Class Trilobita • Dorso-ventrally flat. • Three-lobed head & body (left, middle, right) • Diverse in Paleozoic ~540-240 MYA ( Extinct ) Limulus: A living fossil fossil Subphylum 2: Crustacea –2 pr. Antennae (antennules, antennae)

–Cephalothorax : 13 segments & appendage pairs.

–Abdomen : variable among Classes Subphylum Crustacea –Class shrimps –Class barnacles Crustraceans Anatomy of a Crayfish Subphylum 3: Chelicerata

–Cephalothorax • Jaws are chelicerae • Pedipalps • 4 pr. Walking legs

–Abdomen Subphylum Chelicerata Class Merostomata (Legs attached to mouth) • Horseshoe crabs (not true crabs) • Scorpions ?? Subphylum Chelicerata –Class: Arachnida • Lost compound eyes • Spiders • “Daddy -long -legs” • Amblypygi • & • More…Scorpions -- Class:Pycnogonida (=thick knees, Sea Spiders) Subphylum 4: Uniramia

–Legs unbranched –Class Centipedes • 1st legs are “fangs” Subphylum Uniramia –Class Millipedes • Double segments (2 pr. legs per segment) Subphylum Uniramia –Class Insects • Head, thorax, abdomen • 2 pr. Wings

• ~800,000 species, majority of all arthropods Diplopoda: Millipede Phylum Arthropoda Classes Class Crustacea - lobsters, crayfish, shrimp, crabs Class Chilopoda - “100 leggers” - centipeds; 1 pr legs per segment carnivorous; poisonous

Class Diplopoda - “1000 leggers”millipedes; 2 pr legs per segment; herbivorous Phylum Arthropoda Classes

Class Insecta - • terrestrial & aquatic insects

Class Merostomata - • horseshoe crab, king crabs; “thigh mouth” Why Arthropods are so Successful?

• Exoskeleton  protection from water loss  early colonization of land  head start.

–Arthropods were diverse and widespread on land before vertebrates! Subphylum Chelicerata - chelicera mouthparts Class Merostomata - aquatic chelicerates = horseshoe crab. Class Pycnogonida - sea spiders, usually 4 pairs of legs. Class Arachnida - scorpions, spiders, ticks, mites. Subphylum Crustacea - mandibulate, 2 pair antennae Class - aquatic gill breathers, biramous limbs. Head and thoraxfused into a Cephalothorax Subphylum Uniramia - mandibulate mouthparts, 1 pair antennae, uniramous limbs. Class Diplopoda - millipedes, 2 pair legs/segment Class Chilopoda - centipedes, 1 pair legs/segment. Class Insecta - insects, 3 pairs legs.