Ch.14 The Arthropods

Arthropod Characteristics• 70-85% of all named animals species are

arthropods! The majority of which are insects (beetles, flies, wasps, butterflies)

• Segmented invertebrates • Bilateral Symmetry• Protostome development• 3 main body segments:

– Thorax: middle body region– Abdomen: posterior end– Cephalothorax: head

Arthropod Characteristics• Metamerism & Tagmatization• Chitinous exoskeleton-provides support & protection• Paired, jointed appendages• Grow by molting (ecdysis)• Ventral nervous system• Small coelom • Open circulatory system

– Blood released into hemocoel (tissue space)• Complete digestive system• Metamorphosis usually present

– Different body forms at different stages of development (reduces competition between immature and adult stages)

ExoskeletonProvides structural support, protection, impermeable surfaces for the prevention of water loss, and a system of levers for muscle

attachment and movement.

• Two layers:– Epicuticle – outermost layer

• Waxy lipoprotein• Impermeable to water• Barrier to microorganisms & many pesticides

– Procuticle – inner layer, thickest• Made up of Chitin (tough substance-think leather)• Hardens by sclerotization (hardening & tanning process)

Jointed Appendages• Appendages: structures, such

as legs and antennae, that grow and extend from an animal’s body.

• Arthropods have paired appendages.

• Appendages of arthropods are adapted for a variety of functions such as feeding, mating, sensing, walking, and swimming.

• Arthropod appendages are jointed-enable flexible movement.

Feeding & Digestion

• The mouthparts of most arthropods include a pair of appendages called mandibles that can be adapted for biting or chewing.

• Other arthropods have mouthparts modified like feathery strainers, stabbing needles, cutting swords, or sucking straws.

• Arthropods can be herbivores, carnivores, filter feeders, omnivores, or parasites.

• Arthropods have a complete, one way digestive system with a mouth, gut, and an anus, along with various glands that produce digestive enzymes.

Respiration1. Gills

– Most aquatic arthropods have gills that function in the same way as they do for mollusks

2. Tracheal tubes– Terrestrial arthropods, tubes branch into smaller and smaller

tubules that carry oxygen throughout the body.

3. Book lungs– Saclike pockets with highly folded walls for respiration,

folded walls increase surface area and allow for efficient exchange of gases.

• Both tracheae and book lungs open to the outside of the body of the arthropod in openings called spiracles.

Excretion• Cellular wastes are removed

from the blood through Malpighian tubules.

• Malpighian tubules are attached to and empty into the gut, which contains the undigested food wastes to be eliminated from the body.

• Crustaceans and some other arthropods do not have Malpighian tubules, they have modified nephridia, similar to those in annelids.

Response to Stimuli

Vision:– Arthropods have large, compound eyes.– A compound eye has many facets, which are

hexagonal in shape.– Each facet sees part of an image and the

brain combines the images into a mosaic.– Compound eyes can detect the movements

of prey, mates, or predators, and also can detect colors.

– Many arthropods have 3-8 simple eyes as well as their compound eyes! (Only distinguishes light and dark)

Response to Stimuli• Chemicals:

– Pheromones: chemicals secreted by many animal species that influence the behavior of other animals of the same species.

– Arthropods give off a variety of pheromones that signal behaviors such as mating or feeding.

• Hearing:– In addition to eyes, many arthropods also have another

sense organ called a tympanum-a flat membrane used for hearing.

– It vibrates in response to sound waves.– Arthropod tympanums can be located on the forelegs as

in crickets, or on the abdomen as in some grasshoppers, or on the thorax as in some moths!

Reproduction

• Most arthropods reproduce sexually and have a variety of adaptations for reproduction.

• Most arthropods have separate sexes, but a few such as barnacles, are hermaphrodites and undergo cross-fertilization.

• Most crustaceans brood, or incubate, their eggs in some way, but they do not care for their hatched offspring.

• Some spiders and insects also incubate their eggs, and some, such as bees, care for their young!

Ecdysis (molting)• Four stages

– 1. enzymes digest old procuticle

– 2. new procuticle and epicuticle secreted

– 3. old exoskeleton splits along predetermined lines when the animal stretches by air or water intake & then shedding occurs

– 4. calcium carbonate deposits & sclerotization harden new exoskeleton Cicada Molting

Metamorphism• Reduces competition between adults and

immature stages• Is a radical change in body form and physiology

as an immature stage, usually called a larva, becomes an adult.

Taxonomy of Phylum Arthropoda:

• Page 217, Table 14.1

Subphylum Trilobitomorpha• Extinct, lived in

oceans 600 million years ago

• Oval body & body could roll into a ball to protect its ventral surface!

• 3 tagmata:– Head– Thorax– Pygidium

Subphylum Chelicerata• Means “many claws”• Tagmata:

– Prosoma (Cephalothorax)• Anterior• sensory & feeding

– Opisthosoma• Posterior to prosoma• Contains organs for reproduction, digestion, respiration, and

excretion– Paired appendages – for locomotion

• Chelicerae – pincerlike, feeding• Pedipalps – sensory, feeding• Paired walking legs

Class Merostomata

1. Giant water scorpion• Extinct• six-legged • about 5 feet long &3

feet wide

Class Merostomata

2. Horseshoe crabs– Only 4 species of horseshoe

crabs live today, they were in existence 600mya

– Can be found in the Gulf of Mexico!

– 2 Chelicerae, 2 pedipalps, 6 chelate walking legs, 2 digging (or swimming) appendages

– 5 pairs of book gills – Dioecious,external

fertilization (congregate in intertidal areas to mate)

– Oviparous – lays eggs– Eat small invertebrates

Class Arachnida

• Order Scorpionida– Scorpions

• Order Araneae– Spiders

• Order Opiliones– Harvestmen (daddy longlegs)

• Order Acarina– Ticks & mites

Scorpions• Mostly nocturnal• 2 small chelicerae near

mouth, 2 large pedipalps with pincers, 8 walking legs

• Most sting like a wasp, only a few have deadly venom

• Dioecious, courtship can last hours, mating is internal

• Most Ovoviviparous – internal but, large yolky eggs

• Some Viviparous – mother provides nutrients for embryos

• Development about 1.5 years for 20 to 40 young

• After birth, young crawl onto mother’s back (one month)

Spiders• About 34,000 species (largest arachnid group)• 2 poisonous chelicerae with fangs, 2 pedipalps for sperm

transfer, 8 walking legs• 6 to 8 eyes• 6 to 8 spinnerets eject silk• Silk protein comes out as a liquid, then hardens• Ballooning – using silk line to travel hundreds of kilometers• Bite prey, paralyze them, wrap them in silk, inject enzymes, &

suck victim dry• Females attract males to web with pheromones• Internal fertilization & oviparous (egg outside body)

Black Widow Lactrodectus mactans

*Recognized by its shiny black body with a red hourglass pattern on its ventral surface

*Neurotoxin: “nerve poison” can cause paralyses, difficulty of breathing, vomiting, fever, & sweating

Brown RecluseLoxosceles reclusa

• Hemotoxin: destroys red blood cells, disrupts blood clotting, and causes tissue damage.

• Recognized by the dark brown, violin-shaped mark on the dorsal surface.

Daddy Longlegsor Harvestmen

• Long legs• Most are

omnivores, some predators

• Not harmful to humans

Mites & Ticks• Mites 1mm or less

– Free-living are herbivores & scavengers– Ectoparasites, most stay attached a few hours or

days – Chigger (redbug) larvae burrow in skin, drop off to

molt– Harmless follicle mite is permanent-most of us

actually have this mite!– Itch mite causes Scabies (skin disease that causes

extreme itching)• Ticks up to 3cm

– Ectoparasites all throughout their life– Copulation on host, females drop to ground to lay

eggs

Chigger

Scabies

Tick

Class Pycnogonida• Sea spiders• All Marine and

most common in cold water

• Predators• Dioecious and

males carry eggs until they hatch!

Subphylum Crustacea

• Class Malacostraca– Crabs, lobsters, crayfish, shrimp, krill,

isopods, amphipods• Class Branchiopoda

– Fairy shrimp, brine shrimp, water fleas• Class Maxillopoda

– Copepods, barnacles

Class MalacostracaGreek “malakos”=soft + “ostreion”=shell

• Crabs, lobsters, crayfish, shrimp, krill, isopods, amphipods• Omnivores & scavengers• Have gills• Hemocoel present• Compound eyes on movable eye stalks• Dioecious (except for barnacle)• Abdomen is reduced and is held beneath the Cephalothorax

– Head & Thorax (using crayfish as example)• 2 pairs of antennae, grinding mandibles, 2 pair maxillae, 3 pairs of

maxillipeds, 5 pairs of walking legs (1st is large and chelate), males have an extra 2 pairs of claspers), 5 pairs of swimmerets, telson with uropods (flipperlike)

Reproduction• Dioecious• Males flip

females over and deposit sperm

• “Sticky” eggs develop on tale of female

• Larval stages can lasts months

Class Branchiopoda

• Fairy shrimp & brine shrimp– Mostly freshwater– During droughts, embryos become dormant in

capsules• Water fleas (Daphnia!)

– Look like fleas with large carapace– In spring, females reproduce parthenogenetically

(without fertilization)– In winter, sexual reproduction produces winter eggs

that hatch in spring

Class Maxillopoda• Copepods

– Some or the most abundant crustaceans– Marine and freshwater– Few live on substrate and filter feed, some

are predatory, others are parasites• Barnacles

– Sessile adults– Marine only and ~1,00 species– Filter-feeders– Mostly monoecious– Many will colonize ships and boats, rocks,

and even other animals, like whales!

Subphylum Myriapoda

• 4 Classes:– Diplopoda (millipedes)– Chilopoda (centipedes)– Symphyla (symphylans)– Pauropoda (pauropodans)

Class DiplopodaMillipedes

• Have 11-100 trunk segments• 2 pairs of appendages on each

trunk segment• Worldwide in distribution and

are found in or under leaf litter or decaying logs

• Feed on decaying plant matter• Millipedes roll into ball when

disturbed

Class Chilopoda

Centipedes• Mostly nocturnal• Scurry about the surfaces of logs, rocks, or other forest-floor

debris• Single pair of long legs on each segment• 15 or more trunk segments• Fast-moving predators• Food is small arthropods, earthworms, and snails and

sometimes even frogs and rodents• Poison claws kill or immobilize prey…most venom is

harmless to humans, although many have bites comparable to a wasp and a few human deaths have been reported from large, tropical species

• Legs and segments are added with each molt