Chapter 28 Arthropods Section 2 Diversity of Arthropods

Arachnids  Class: Arachnida  spiders, scorpions, mites and ticks  Have only two body regions – a cephalothorax and an abdomen  Do not have antennae  Have six pairs of jointed appendages

Arachnids  Class: Arachnida  Have six pairs of jointed appendages  Chelicerae  First pair of appendages  Located near the mouth  Often modified pincers (for holding food) or fangs (for injecting poison into prey)  Pedipalps  Second pair of appendages  Adapted for handling food and for sensing  Four remaining pairs of appendages are adapted for locomotion

Arachnids  Spiders  Have extracellular digestion of food  Lack mandibles for chewing, so…  Digestive enzymes from the spider’s mouth liquefy the internal organs of the captured prey  The liquefied food is then sucked up by the spider  All spin silk  Silk is secreted by silk glands in the abdomen  Spun into thread by structures called spinnerets that are located at the rear of the spider

Arachnids  Ticks  Have only one body section  Feed on blood from reptiles, birds, and mammals  Can expand to more than three times their original size after ingesting blood  Can spread diseases

Arachnids  Mites  Have only one body section  Feed on fungi, plants, and animals  Can often not be seen with the unaided eye (tiny)  Can spread diseases

Arachnids  Scorpions  Have many abdominal body segments and enlarged pincers  Have a long tail with a venomous stinger at the tip  Used to paralyze large prey  Live in warm, dry climates  Eat insects and spiders

Crustaceans  Class: Crustacea  crabs, lobsters, shrimps, crayfishes, water fleas, pill bugs, and barnacles  Only group of arthropods that have two pairs of antennae for sensing  Have either 2 or 3 body sections  Have mandibles that open and close from side to side in order to crush food  Typically have two compound eyes that are often located on moveable stalks

Crustaceans  Class: Crustacea  Many have 5 pairs of walking legs  First pair of walking legs are often modified into strong claws for defense  Other pairs can be used for walking, seizing prey, and cleaning  Most are aquatic and use gills for gas exchange  Sow bugs and pill bugs are terrestrial but must live in moist environments in order to carry out gas exchange

Centipedes  Class: Chilopoda  centipedes  Have flattened bodies  Have Malpighian tubes for excreting wastes  Have tracheal tubes  Are carnivorous  Eat soil arthropods, snails, slugs, and worms  Bites can be painful to humans

Millipedes  Class: Diplopoda  millipedes  Have cylindrical bodies  Have Malpighian tubes for excreting wastes  Have tracheal tubes  Eats mostly plants and dead materials on damp forest floors  Do not bite but can spray foul-smelling fluids from defensive stink glands

Horseshoe Crabs  Class: Merostomata  Horseshoe crabs  Considered to be living fossils  Have an extensive exoskeleton  Live in deep coastal waters (currently off the east coast of North America or the Asian tropics)  Forage on the ocean bottom for algae, annelids, and mollusks  Migrate to shallow waters in the spring in order to mate at night

Insects  Class: Insecta  insects  Largest and most successful arthropod class  There are more species of insects than all other classes of animals combined  Have three body segments and six legs

Insects  Reproduction  Most insects mate once during their lifetime  Eggs are usually fertilized internally  Although some insects can also be reproduce from non-fertilized eggs  Many females are equipped with an appendage that can pierce through the surface of the ground or into wood in order to lay eggs  A large number of eggs are laid in order to increase chances of offspring survival

Insects  Reproduction  After eggs are laid, insect embryos develop and the eggs hatch  Some wingless insects (such as silverfish) hatch directly into mini forms of the adult and then go through successive molts to reach the adult size  Other insects hatch into forms that do not resemble the adult and most go through metamorphosis - a series of changes that are controlled by chemical substances in the animal

Insects  Complete Metamorphosis  Includes four stages : egg, larva, pupa, and adult  Undergone by more than 90% of insects  Advantageous because adults and larva do not compete for the same food source  Stages:  Embryo (Egg)  Larva  Free-living, wormlike stage (caterpillar)  As the larva eats and grows it molts several times  Pupa  Period of reorganization in which the tissues and organs of the larva are broken down and replaced by adult tissues  Usually the insect does not move or feed during this time  Adult

Insects  Incomplete Metamorphosis  Includes only three stages of development: egg, nymph, and adult  The nymph has the same general appearance as the adult but is smaller, may lack or have appendages seen/not seen in the adults, and cannot reproduce  As the nymph eats and grows it molts several times, becoming more like the adult with each molt

Success of Arthropods  Why are arthropods so successful?  Have varied life cycles, high reproductive output, and many structural adaptations such as small size, hard exoskeleton, and jointed appendages

Origins of Arthropods  How do we know about the origin of arthropods?  Insects have many hard parts that fossilize easily  What do we know about the origin of arthropods?  Most likely evolved from an ancestor of the annelids (segmented worms)  As arthropods evolved body segments fused and became adapted for specific function