Biology 320 Invertebrate Zoology Fall 2005
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Transcript Biology 320 Invertebrate Zoology Fall 2005
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