Arthropods

Main 9 characteristics of Arthropods:

  1. Metameric with tagmatization

    1. segments/appendages with specialized functions

    2. Head, thorax, abdomen OR Cephalothorax, abdomen

  2. Chitinous exoskeleton

    1. Chitin is a polysaccharide (a multi-chained sugar [carb]) (INNERMOST)

    2. Exoskeleton is hard/crunchy for protection and so the arthropod won’t dry out

  3. Paired, jointed appendages

    1. 8 legs, 4 on each side OR 6 legs, 3 on each side

  4. Ecdysis

    1. Ecdysozoans (has a cuticle and sheds outer shell)

    2. Loss of epidermal cilia

  5. Ventral nervous system

    1. the ventral nerve cord

  6. Coelom reduced to cavity around gonads (and sometimes excretory system)

  7. Open circulatory system

  8. Complete digestive tract

  9. Metamorphosis often present

    1. the animal moves between stages by moulting its exoskeleton

Arthropoda is monophyletic (its own phylum) and part of the protostome clade Ecdysozoa

Four subphyla

  1. Chelicerata

  2. Crustacea

  3. Hexapoda (insects)

  4. Myriapoda (a lot of extensions)

  • Trilobitomorpha (entirely extinct)

Metamerism and Tagmatization: segments or appendages with specialized functions

Metamerism evident externally

  • segmental body wall

  • segmental appendages

Metamerism reduced internally

  • no septa

  • most organs are obvious are not metameric

Tagmatization obvious

  • Specializations for:

    • Feeding

    • Sensory Perception

    • Locomotion

    • Visceral functions

The Exoskeleton

  • Functions

    • structural support

    • protection

    • prevents water loss

    • system of levers for muscles attachment and movement

  • Covers all body surfaces and invaginations of the body wall

  • Nonliving, secreted by epidermis (hypodermis)

  • Outermost layer

    • lipoprotein

    • impermeable to water

    • barrier to microorganisms and pesticides

  • Innermost layer

    • Chitin - polysaccharide (a multi-chained sugar)

    • outer procuticle hardened by sclerotization or deposition of calcium carbonate

    • inner procuticle less hardened, more flexible

      • articular membranes at joints

  • Modifications include sensory receptors

    • sensilla

  • Growth accompanied by ecdysis A STEP BY STEP PROCESS

    • 1. Enzymes from hypodermal glands begin digestion old procuticle

    • 2. New procuticle and epicuticle secreted

    • 3. Old exoskeleton splits

    • 4. Calcium carbonate deposition and/or scelrotization hardens new exoskeleton

Metamorphosis

  • Ecdysis is a form of metamorphosis

  • Radical change in body form and physiology as an immature stage (larva) becomes adult

    • Reduces competition between adult and immature stages

Subphylum Trilobitomorpha

  • Was very dominant millions of years ago, but is now entirely extinct

  • Burrow/leaf eaters (bottom feeders)

  • Tagmata as such:

    • Head

    • Thorax

    • Abdomen (Pygidium)

  • Jointed segments so they could roll into a ball and shield the inner (ventral) surface from predators and other dangers

  • Dual branches from appendages

Subphylum Chelicerata

  • Spiders, mites, ticks, horseshoe crabs, spea spiders

  • TWO TAGMATA

    • Cephalothorax - fused head and chest

      • Eyes (multiple)

      • Chelicerae

        • often pincher-like

        • usually feeding appendages

      • Walking legs

    • Opisthosoma (abdomen)

      • contains digestive, reproductive, excretory, and respiratory organs

Class Merostomata

  • Subclasses

    • Eurypterida

      • ENTIRELY EXTINCT giant water scorpions

    • Xiphosura

      • Horseshoe crabs

        • Genus limulus (Atlantic Ocean and Gulf of Mexico)

        • Book gills

          • gas exchange between blood and water

        • Reproduction

          • Dioecious

          • External Fertilization

        • Blue blood due to copper levels

        • Limulus Amebocyte Lysate (LAL)

          • used by pharmaceutical and medical device manufacturers to test their products for the presence of endotoxins, bacterial substances that can cause fevers and even be fatal to human

Class Arachnida

  • Spiders, mites, ticks, scorpions

  • Arose from ancient eurypterids

  • Very early terrestrial groups

    • 280-400 mya

    • Exoskeleton was preadaptation for water conservation (cuticle)

  • Form and Functions

    • Carnivores

      • Chelicerae to hold onto prey as fangs

      • Gut - multi chamber (STARTING TO GET TO A MORE ADVANCED DIGESTIVE TRACT)

        • Foregut

          • Cuticular

          • Pumping stomach

        • Hind Gut

          • Cuticular

          • Water reabsorption

        • Midgut

          • Non Cuticular

          • Secretion and absorption

    • Excretion

      • Coxal glands

        • paired sacs bathed in blood of body sinuses (open circulatory system)

        • homologous to nephridia (same structure)

        • excretory pores at base of posterior appendages

      • Malpighian tubules

        • blind ending diverticula (pits) in gut tract

        • empty via digestive tract

      • Major excretory product is uric acid which is semisolid and results in little water loss (similar to bird poop)

    • Gas Exchange

      • Book lungs (main respiratory organ)

        • paired with ventral invaginations of body wall

        • gas exchange between air and blood across book lung lamellae

        • “pages” for surface area

      • Tracheae

        • branched chitin-lined tubes

        • open at spiracles along abdomen

    • Circulation

      • open with dorsal contractile vessel

      • pumps blood into tissue spaces of hemocoel

        • blood soaks organs

      • returns to dorsal vessel via ostia

    • Nervous system

      • ventral with fusion of ganglia

    • Senses - very specialized

      • Mechanoreceptors (movement detection)

        • modifications of exoskeleton

        • sensilla respond to displacement

      • Chemical sense

        • pores in exoskeleton

      • Vision

        • eyes detect movement and changes in light intensity (bigger field of vision)

      • Reproduction

        • dioecious

        • indirect sperm transfer

          • male deposits spermatophores, which are transferred to the female

            • female has special pockets for sperm packets that the male gives to the females

        • courtship rituals are common

        • copulation occurs in spiders via modified pedipalp of male

      • Development

        • direct - embryo develops into a mature individual without involving a larval stage

Order Scorpionida

  • Scorpions - generally found in drier environments

    • Prosoma

      • shieldlike carapace

    • Opisthosoma

      • preabdomen

      • postabdomen (“tail” with a sting)

    • Courtship prior to mating

    • Oviparous (egg), ovoviviparous (eggs hatch within body), or viviparous (live young)

Order Araneae

  • Spiders

    • Prosoma

      • Chelicerae with venom glands and fangs

      • Pedipalps leglike

        • Sperm transfer

        • 6-8 eyes

    • Opisthosoma

      • connected to prosoma vid pedicel

      • swollen or elongate

      • visceral functions and spinnerets for silk production

        • liquidizes prey

    • Silk

      • Protein - strongest natural protein

      • repeating sequence of glycine and alanine

      • beta sheet

      • stored as gel prior to spinning

      • chemical modification when forced through spinnerets

    • Functions of Silk

      • webs

      • line retreats

      • safety lines

      • wrapping eggs

      • dispersal of young (ballooning)

    • Feeding

      • insects and other arthropods

      • hunt or capture in webs

      • paralyze prey

        • may wrap in silk

      • inject enzymes into prey body wall

    • Two spiders that are the most venomous to humans are THE BLACK WIDOW AND THE BROWN RECLUSE SPIDER

    • Reproduction

      • Complex behaviors

        • chemical, tactile, and visual signals

      • Males pedipalps enlarged into embolus (penis-like structure)

        • male deposits sperm on web and collects with pedipalps

        • transfers sperm to female during mating

      • Female deposits eggs in silk case

        • Seals in webbing then places in a retreat or carries with her

Order Opiliones

  • Harvestmen or daddy longlegs

    • closer related to scorpions than actual spiders

    • not necessarily venomous

  • Prosoma broadly joins opisthosoma

  • Legs long and slender

    • can break off legs and still be fully functional

      • Stotting walking (7 legs)

      • Bobbing walking (6 legs)

  • Omnivores

  • External and internal digestion

Order Acarina

  • Mites

    • prosoma and opisthosoma fused and covered by a single carapace

    • 1mm or less

    • Free-living

      • herbivores or scavengers

        • many pest species

    • Ectoparasites

      • Chigger (Trombicula)

      • Follicle mite (Demodex)

  • Ticks

    • Ectoparasites in all life stages

    • Up to 3cm

    • Females lay eggs after engorging with blood

    • Important in disease transmission

      • Rocky Mountain spotted fever

        • a bacterial disease spread through the bite of an infected tick

        • Most people who get sick with RMSF will have a fever, headache, and rash. RMSF can be deadly if not treated early with the right antibiotic. Transmission.

      • Lyme Disease

        • transmitted to humans through the bite of infected blacklegged ticks. Typical symptoms include fever, headache, fatigue, and a characteristic skin rash called erythema migrans.

Subphylum Myriapoda

  • Four classes

    • Diplopoda

    • Chilopoda

    • Symphyla

    • Pauropoda

  • Body consisting of two tagmata

  • Uniramous appendages

  • All modern myriapods are terrestrial

Class Diplopoda

  • Millipedes

  • Two pair legs per apparent segment

  • Usually round in cross section

  • Decaying plant matter

  • Repugnatorial glands produce hydrogen cyanide to repel animals

  • Sperm transfer via gonopods or in spermatophores

Class Chilopoda

  • Centipedes

  • One pair of legs per segment

  • Flattened in cross section

  • Fast moving predators

  • Reproduction

    • Courtship

    • Male places spermatophores in silk web

    • Spermatophores is picked up by female

Classes Pauropoda and Symphyla

  • Pauropoda

    • forest-floor litter

    • feed on fungi and decaying organic matter

    • think, moist exoskeleton

  • Symphyla

    • soil and leaf mold

    • lack eyes

    • feed on decaying vegetation

    • 12 leg bearing segments

Further Phylogenetic Considerations

  • Diverse body forms and lifestyles of Arthropoda have allowed members to be very successful

  • Chelicerata

    • among first terrestrial animals, quickly joined by myriapods

    • account for evolution of many water conserving features of the phylum

      • exoskeleton, excretory, and respiratory adaptations

  • Hexapods (insects) dwarf other terrestrial arthropods in number of species and individuals

  • Crustacea have not generally ventured onto the land

    • Dominant arthropods in marine and freshwater environments