Invertebrate Zoology
Lecture 17: Phylum Arthropoda, Part 1
Lecture outline
 Phylum Arthropoda
Diversity/Classification
Phylogeny
Bauplan Basics
Focus: Arthropod exoskeleton
Feeding/digestion
Circulation/gas exchange
Osmoregulation/excretion
Nervous system/sensory
Reproduction
Diversity/Classification

Subphylum
Trilobitomorpha
 Two longitudinal
furrows divide body
into three sections
 Marine; Paleozoic
 Extinct (end of
Permian)
Diversity/Classification

Subphylum Crustacea: Lobsters,
crabs, barnacles, copepods, etc…
Antarctic krill
Euphausia superba
Subphylum Hexapoda: Insects
Brusca & Brusca
Dennis Kunkel
Subphylum Myriapoda:
Centipedes and Millipedes
Subphylum Cheliceriformes
Spiders, mites, horseshoe crabs,
scorpions, sea spiders et al…
Phylogeny
Based on body plan & development
 Hypothesis 1:
Bauplan basics

Features shared with Phylum Annelida
 Classic protostome development
 Segmentation; segments added posteriorly
 Nervous system architecture

Derived characters (differ from Annelida)
 Rigid cuticle: contains chitin; forms
exoskeleton
 Open circulatory system; hemocoel
 Reduced coelom


Key hypothesis: loss of functionality (i.e.
peristalsis)
Where are coelom “remnants” located?
Intermediate phyla
Why considered intermediate?
 Phylum Onychophora
 Phylum Tardigrada
Bauplan basics

Other features
 Tagmatization
 Paired jointed appendages

“Primitive condition”: appendages associated
with each segment,
 Lack motile cilia, except for some sperm

Why? Which other group has non-motile cilia?
Focus: cuticle
 Epidermis
Secretes cuticle
Dermal glands
 Variety of secretions
Sensory structures
 Procuticle
Chitin
Calcium carbonate
 Crustacea
Sclerotization
Joints
 Epicuticle
Cement layer
Wax layer
Molting
Feeding/digestion



Different modes of feeding
Variety of mouthparts
Complete digestive system
 Note digestive cecum
Circulation/gas exchange

Open circulatory system; ostiate heart
 Hemocoel: space bounded by basement
membrane of epidermis
 Re-entry of hemolymph into heart?
Circulation/gas exchange

Respiration: Subphylum Crustacea

Gills; body surface (small forms)
Circulation/gas exchange

Respiration: Hexapoda & Myriapoda
 Tracheal system (openings = spiracles)
Circulation/gas exchange

Respiration: Cheliceriformes
 book lungs; book gills
Osmoregulation & ammonia excretion

Crustacea:
 Gills (ammonia release only
 Antennal (green) gland


Entry via hemocoel channels & active transport
Selective secretion and uptake along tubule
Osmoregulation & ammonia excretion
 Hexapoda, Myriapoda,
Chelicerata:
Malpighian tubules
 Closed distally: hemocoel
Active ion transport 
fluid uptake
 Open into hindgut
Adaptive value?
Nervous system

Organization
 Brain (=cerebral ganglia)
 Subesophogeal (subenteric)
ganglion
 Ventral nerve cord (paired,
fused partly or completely)
 Ganglia: one per segment
or fewer (consolidated)
Nervous system

Crab: Note consolidated thoracic ganglia
Sensory systems: Compound eye
 Compound eyes comprised of multiple ommatidia
 Interommatidial hairs in some…
Sensory systems: Compound eye
 Key structures
Cornea
Crystalline
cone/stalk
Iris (=corneal
pigment cells)
Retinular cell
w/rhabdome
Sensory systems: Compound eye
 Light-adapted vs. dark-adapted eye
Shift in pigment position (within iris)
Alteration in light pathway
Consequences for sensitivity and acuity
Sensory systems: Compound eye
 Mosaic theory of insect vision
Each ommatidium views piece of visual field
 Adaptive advantages of compound eye
 Acuity varies with
Ommatidia concentration
Size
Flatness
 Key: angular distance
Sensory systems: Sensillae
 Mechanosensory & chemosensory
Reproduction: Overview
 Sexual reproduction
Usually gonochoristic
Some hermaphrodites
 Example: Barnacles
 Parthenogenesis in some groups
Part of aphid life cycle
Some crustaceans living in temporary ponds
 Egg production often food limited. Why?
Example: Antarctic krill
Reproduction: Male system




Testes
Vas deferens
Seminal vesicle
Accessory glands
Types of
secretions?
 Ejaculatory duct
 Gonopore
Spermatophores (in
some)
 Penis or modified
appendages for
copulation
Damselfly penis
Barnacle penis
Spider pedipalps
Reproduction: female system
 Ovaries
Ovariole structure
Egg batches
 Oviduct
 Accessory glands
Types of
secretions?
 Copulatory bursa
 Spermathecae
Function?
Bees and lobsters…
Reproduction & molting
 Mate guarding
Adaptive value for male?
Adaptive value for female?
Reproduction & cannibalism
 Hexapoda & Cheliceriformes (i.e. spiders)
Adaptive value for male?
Adaptive value for female?