Biology 320
Invertebrate Zoology
Fall 2005
Chapter 16 – Introduction to
Phylum Arthropoda
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
► 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
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
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)
Appendages
►
Segmented with
articulations
►
Specialized for specific
functions
 Some posses more than 10
types
 Enormous variety
►
May be simple, or have
multiple branches
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
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
► 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
► 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
Ecdysis
►
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
►
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
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
Locomotion
► Arthropods
posses a
large variety of
appendages, that
serve different
purposes
 Walking legs
 Paddles – swimming
 Wings - flight
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
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
► 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
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
►
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
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
►
Aquatic invertebrates
 Gills
► Already
 Book gills
discussed
► 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
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
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
►
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
►
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
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
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