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Biology 320 Invertebrate Zoology Fall 2005 Chapter 16 – Introduction to Phylum Arthropoda.

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Presentation on theme: "Biology 320 Invertebrate Zoology Fall 2005 Chapter 16 – Introduction to Phylum Arthropoda."— Presentation transcript:

1 Biology 320 Invertebrate Zoology Fall 2005 Chapter 16 – Introduction to Phylum Arthropoda

2 Phylum Arthropoda ► Horseshoe crabs, crustaceans, arachnids, insects, and millipedes ► One million described species  80% of all known animals ► Another 30 million undescribed species of insects, estimated ► Only birds, bats, and insects have the ability to fly ► Amazing adaptive diversity

3 ► Thought to be closely related to annelids  Recently thought to be more closely related to nematodes ► Two major extant taxa  Chelicerata – horseshoe crabs and arachnids  Mandibulata ► Crustacea – crabs, barnacles, and water fleas ► Tracheata – insects and centipedes

4 Segmentation ► Metamerism is the technical term for segmentation ► Individual segments are termed somites ► Certain body regions of arthropods display a fusion of several segments ► Arthropods display tagmosis (tagmatization)  Segments are organized into regions called tagma (tagmata)  Insects posses a head, thorax, and abdomen  Arachnids and crustaceans (typically) possess a cephalothorax and abdomen


6 Cephalization ► Highest degree of cephalization of any phylum we have discussed to this point ► Head possesses  Concentration of ganglia (brain)  Concentration of sensory structures ► Eyes and antennae  Mouth, feeding appendages, and legs (in those with a cephalothorax)

7 Appendages ► Segmented with articulations ► Specialized for specific functions  Some posses more than 10 types  Enormous variety ► May be simple, or have multiple branches

8 Body Wall ► Cuticle (exoskeleton) is most superficial layer  Made of chitin and protein  Provides strength and protection ► Epidermis  Monolayered  Secretes cuticle ► Thin connective tissue layer ► Specialized individual muscles, rather than sheets

9 Exoskeleton ► Cuticle has hard areas and soft areas  Allows for movement at articulations ► Some areas are sclerotized  Proteins crosslink causing cuticle to harden and darken  Typical of arachnids and insects ► Some areas are mineralized  Calcium salts are added to cuticle  Crustaceans supplement sclerotization with mineralization

10 ► Two main layers of exoskeleton  Epicuticle – thin outer layer that is water / gas resistant ► Waterproofed by different types of lipids ► Useful in dry environments ► Important for osmoregulation in FW environments  Procuticle – thick portion

11 ► Must be periodically molted in order for growth to occur  Rigid exoskeleton cannot stretch  Process known as ecdysis  Regulated by the steroid hormone ecdysone ► Even the lining of the foregut, hindgut, tracheae, etc. are shed

12 ► Has been thoroughly studied in decapods ► When animal prepares to molt  Old cuticle begins to detach  New cuticle begins to form  Layer of molting fluid (containing proteases and chitinases) forms between the two cuticles  Epicuticle of new cuticle protects it from enzymes  Calcium from old cuticle is reclaimed ► During ecdysis  Animal is very vulnerable  Usually molts inside burrow, shelter, etc.  Molting related complications / deaths are common Ecdysis

13 ► After molting  Animal takes up water or air, and swells as a result  May ingest old cuticle to reclaim calcium  Sclerotization and mineralization (crustaceans) occur  Excess water or air is eliminated  Animal now has room for growth ► Some molt periodically ► Others have a fixed number of instars (life stages) separated by molts

14 Musculature ► Lack sheets of muscles ► Small individual muscles that are specialized  Flexors  Extensors ► Often arranged antagonistically ► Control one articulation or one sclerite ► Striated, for rapid contraction

15 Locomotion ► Arthropods posses a large variety of appendages, that serve different purposes  Walking legs  Paddles – swimming  Wings - flight

16 Coelom ► Arthropods have an exoskeleton, and therefore do not rely heavily on a hydrostat ► Coelom is reduced compared to that of the annelids ► Hemocoel is the functional body cavity ► No segmental divisions

17 Hemal System ► Transports nutrients, hormones, wastes, and gasses (in some) ► Open system in most ► System consists of:  Large dorsal heart ► Tubular and contractile ► Possesses ostia  Arteries that branch off of heart  Sinuses are separated by perforated diaphragms ► Pericardial – dorsal ► Perivisceral – middle ► Perineural (in some) - ventral  Blood ► Crustaceans and chelicerates have hemocyanin  Hemocoel ► Muscles and organs are housed here

18 ► Generalized blood flow  Oxygenated blood leaves heart via arteries  Travels to sinuses and hemocoel where tissues and organs are bathed in blood  Deoxygenated / waste laden blood travels to respiratory structures (in all but tracheates) and gas exchange occurs  Blood enters heart via ostia

19 Excretion ► Two main excretory structures  Saccate nephridia  Malpighian tubules ► Saccate nephridia  Mainly found in aquatic species  Not actually used for excretion, as wastes (ammonia) diffuse across gills  Maintain ion balance and fluid volume  Go by many name, such as “green glands” in crayfish  Consists of a tube and an end sac which is bathed in blood

20 ► Malpighian tubules  Primary excretory organ of terrestrial species  Stringy tubules located at the junction of the midgut and hindgut  Create uric acid and guanine and transfer to the gut lumen  Wastes form a precipitate in this low pH environment  Nitrogenous wastes are removed with feces  Adaptation to terrestrial existence

21 Gas Exchange ► Small arthropods conduct gas exchange across body surfaces ► Larger varieties make use of specialized structures that must be moist ► Interestingly, the epicuticle of respiratory structures is not water / gas proof ► Many types of respiratory structures  Particular structures are mainly dependent on the type of habitat the animal inhabits

22 ► Aquatic invertebrates  Gills ► Already discussed  Book gills ► Flat sheets on ventral surface ► Possessed by horseshoe crabs ► Terrestrial species  Tracheae ► Tubular invaginations that branch and lead directly to cells, not to the hemal system ► Spiracle diameter regulated by a sphincter ► Opening / closing of spiracles is useful for ventilation and reducing water loss  Book lungs ► Common in arachnids ► Invagination of exoskeleton which contains many smaller invaginations (large SA) ► Exchange occurs between surfaces of book lungs and blood

23 Digestive System ► Complex, but somewhat similar to annelid system ► Epithelium at junction of foregut-midgut secretes a thin peritrophic membrane around bolus  Protects delicate walls of midgut from abrasion ► Gut lumen reabsorbs water

24 Nervous System ► Similar to that of annelids ► CNS  Brain  Ventral nerve cord ► PNS  Segmental nerves connecting to muscle and sensory organs ► Numerous types of sensory structures ► Exoskeleton is modified to accommodate sensory receptors

25 ► Mechanoreceptors  Setae ► Hollow extensions of cuticle w/ sensory neurons inside  Trichobothria ► Sensitive “hairs” that detect weak air currents ► Chemoreceptors  Sensilla ► Similar to setae ► Equilibrium  Statocysts ► Possessed by some  Ocelli ► Most use light as a point of reference for orientation

26 ► Sound detection  Tympanic membrane ► Responds to air vibrations ► Can also produce sound in some ► Useful for locating / attracting mates ► Moths use to hear bat echolocation ► Temperature – humidity detectors  Thermo-hygroreceptive sensillia ► Usually found on antennae of insects

27 Eyes ► Most have ocelli ► Many also have compound eyes  Composed of many subunits called ommatidia  15 – thousands per eye  Each unit has own lens and own field of vision  Fields of vision of adjacent ommatidia overlap  Composite image formed, similar to vertebrate eyes  Are usually motion detectors that have a range of about 20cm  Many have eyestalks  Some have color vision and respond to colored flowers

28 Reproduction ► Mostly gonochoric ► Fertilization is primarily internal in terrestrial species  Can be internal or external in aquatic species ► Most copulate and there is often selectivity on the part of the female ► Females often store sperm in a seminal receptacle ► Sperm transfer is direct in some  Penis may deliver sperm directly, or serve as an intromittant organ  Spermatophores are frequently used for indirect transfer ► Many brood and display parental care

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