lecture 17/18: Phylum Arthropoda

body plan of arthropods

- bilateral symmetry

- chitinous exoskeleton

- externally segmented bodies

- modified appendages for feeding

- hemocoel

- no cilia

- ventral nerve chord and dorsal brain

Phylum Arthropoda

- has two subphylums: Mandibulata and chelicerata

Subphylum Mandibulata

has well-developed mandibles

subphylum Chelicerata

has well-developed chelicerae

segmentation

- centipedes and millipedes have head and trunk

- spiders and some crustaceans have cephalothorax and abdomen

- insects and many crustaceans have head, thorax, and abdomen

cephalothorax

head and thorax are fused together

Exoskeleton

- basement membrane is deepest

- epidermal cells are intermediately placed

- cuticle is most superficial

Cuticle

consists of three layers: Epicuticle, exocuticle, and endocuticle

epicuticle

lipids and polyphenols

exocuticle

produced by sclerotization (tanning process to harden the layer)

endocuticle

flexible/recyclable

Exoskeleton

- schlerites

- sulci (doubling of layer, less flexible)

- membrane

- pore canals

- setae

Arthropod size limitations/advantages

- smaller size means:

• less time and energy to develop

• less energy to sustain life

• protection from predators and environment

• more habitats to exploit

• muscular action more efficient

• solar heat/random dispersal

• gravity has less effect

disadvantages to small size

- predation

- physical factors (can get trapped in droplets of rain, carried by wind, etc.)

- great potential for water loss (transpiration)

arthropod classes

- Arachnida

- chilopoda

- crustacea

- diplopoda

- Insecta

class arachnida

- chelicerata

- lack antenna

- cephalothorax and abdomen

- 4 pairs of legs and adults

common arachnid orders

- acari (ticks, mites)

- araneae (spiders)

- opiliones (harvestmen)

- pseudoscorpiones (pseudoscorpions)

- Scorpiones (scorpions)

- solifugae (sun spiders)

subphylum Chelicerata

- 2 or 3 segments

- chelate condition (opposable thumb-like structure)

- unchelate condition (thumb is lacking)

- poison glands in spiders empty through a duct in chelicerae

pedipalpi

variously modified and segmented sensory structures

silk

- spun to cover eggs

- used to build retreats

- ballooning (wind transport)

- used to capture prey (abdominal spinnerets)

feeding habits of arachnids

- carnivorous

- Parasites (ticks and some mites)

medical concerns

- rocky mountain spotted fever (ticks)

- Lyme disease (ticks)

- scabies (mites)

- asthma (dust mites)

- poisonous bites (several spiders)

- poisonous stings (scorpions)

respiration

- small arachnids respire through exoskeleton or minute tracheal tubes

- large spiders have book lungs

order chilopoda

- centipedes

- head, long antennae, mandibles, and maxillae

- multisegmented trunk - single pair of legs

- terminal segment - 2 pairs of legs

- eggs are singular or in groups for reproduction

- female guards eggs

- anamorphic development or epimorphic development

- active at night

- predaceous on arthropods, worms, and slugs

- bite can be painful but rarely dangerous

anamorphic development

primitive; segments added with each molt

epimorphic development

more advanced; same number of segments of adults

order diplopoda

- millipedes

- head, antennae, mandibles, gnathochilarium (fused maxillae)

- trunk with diplosegments

- genital pore on the anterior

- eggs deposited in groups

- larvae have only 3 pairs of legs

- 1-7 years required for life cycle

- molt in earthen chambers

- found in rotting logs, leaf litter, humus

- feed on decaying matter and fungi

- locomotion via waves of leg action

- roll into tight coil when disturbed

- most secrete noxious fluids

anamorphosis

diplosegments added after molts

order crustacea

- many shapes and sizes

- amphipoda, isopoda, ostracoda, decapoda

- most marine, some freshwater, some terrestrial

- head, thorax, abdomen (or cephalothorax and abdomen)

- biramous appendages

- mandibles

- antennae

- maxillae

- have two larval stages: naupilus (simplest larva) and zoea larva (more advanced crustacea)

- anamorphic development

- food varies according to size

- planktivores, carnivores, scavengers, and parasites

Order insecta

- 3 body regions: head, thorax, and abdomen

- 1 pair of antennae

- all feed by mandibles (some modified)

- are the only arthropods with wings

- epimorphic (except protura): no new segments after hatching

- metamorphosis may be incomplete or complete

- several weeks to 17 years of development

- most terrestrial, some aquatic

- varied feeding strategies

- varied habitats

- very diverse and successful group