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Phylum Arthropoda
 The Arthropoda
(from the Greek Arthron,
joint and podus, foot) are the largest group
of organisms and they occur in all
environments on earth.
 The
group includes: spiders, ticks, mites,
centipedes, millipedes, crustaceans,
insects and others.
Characteristics of the Arthropoda
 Bilaterally
symmetrical with a segmented
(metameric) body divided into head, thorax
and abdomen; cephalothorax and
abdomen; or fused head and trunk.
Characteristics of the Arthropoda
 Jointed
appendages. Primitively one pair
per segment, but number often reduced.
 Appendages
often greatly modified for
specialized tasks.
Characteristics of the Arthropoda
 Exoskeleton
of cuticle.
 Exoskelton
secreted by underlying
epidermis. Made of chitin, protein, lipid
and often calcium carbonate.
 Exoskeleton
is shed periodically (ecdysis)
as the organism grows.
Characteristics of the Arthropoda
 Muscular
system is complex and muscles
attach to the exoskeleton.
 Striated
muscles for voluntary movement
and smooth muscles for the viscera.
 Coleom
is reduced. Most of body cavity is
a hemocoel (sinuses or spaces) filled with
blood.
Characteristics of the Arthropoda
 Complete
digestive system. Mouthparts
are specialized being modified from
appendages and specialized for dealing
with various types of food.
 Open
circulatory system with a dorsal
contractile heart arteries and hemocoel.
Characteristics of the Arthropoda
 Respiration
occurs in multiple possible
ways




across the body surface
via a system of tracheal tubes
gills
book lungs
Characteristics of the Arthropoda
 Nervous
system is similar to that of
annelids. A dorsal brain connected via a
ring around the pharynx to a double
ventral nerve cord.
Characteristics of the Arthropoda
 Sexes
are usually separate with internal
fertilization.
 Can be oviparous or ovoviviparous.
 Offspring often go through process of
metamorphosis.
 Parthenogenesis occurs in a some
species (e.g. aphids).
Classification of Phylum Arthropoda
 Subphylum
Chelicarata: horseshoe crabs,
spiders, tick, mites, scorpions,
 Subphylum Crustacea: crabs, lobsters,
copepods, barnacles, pill bugs
 Subphylum Myriapoda: millipedes and
centipedes
 Subphylum Hexapoda: springtails, insects
Classification of the Arthropoda
 Subphylum

Chelicerata
Class Merostomata
• Subclass Xiphosurida: Horseshoe crabs


Class Pycnogonida: sea spiders
Class Arachnida: spiders, scorpions, ticks,
mites, harvestmen, whip scorpions.
Subphylum Chelicerata


Body is divided into two tagmata (fused
segments) head and abdomen.
Six pairs of appendages:




a pair of chelicerae, first pair of appenadges used for
feeding.
a pair of pedipalps (not in horseshoe crabs)
4 pairs of walking legs (5 in horseshoe crabs).
No mandibles or antennae.
Subphylum Chelicerata
 The
chelicrates are a very ancient group
that includes the extinct Eurypterids (giant
water scorpions (200-500 mya), which
were the largest known arthropods
reaching up to 3m long.
Eurypterus (left) from the Silurian of New York State. Eurypterus remipes was voted
the New York state fossil in 1983.
Mixopterus (right) from the Devonian reached about one meter in length
Subphylum Chelicerata
 Subclass
Xiphosurida: Horseshoe crabs.
These are an ancient group that date from
the Cambrian (543-495 mya).
 Five
living species. Limulus, which lives in
shallow waters on the Atlantic coast of the
U.S. is almost unchanged since the
Triassic (251-206 mya).
Figure 18.02
12.2
Subphylum Chelicerata: Horseshoe
crabs
 Horseshoe
crabs have an unsegmented
carapace, which is sort of horseshoe
shaped, hence the name.
 There
is also a wide abdomen with a long
spinelike tailpiece (telson).
 Flat
leaf-like gills, called book gills, are
present on the underside.
Horseshoe crabs

Horseshoe crabs feed on foods such as worms and
molluscs, such as clams, which brings them into conflict
with fishermen.

The synchonized breeding of horseshoe crabs which
come to spawn on the beaches of the mid-Atlantic coast
of the U.S. at the lunar high tides is a striking sight in
summer.

The eggs produced in the millions are an important food
source for migrating shorebirds such as Knot, which
flock to areas such as the Delaware Bay to fatten up for
migration.
Horseshoe crabs
 Horseshoe
crabs are harvested
commercially for bait and also for their
blood, which is used in laboratory testing
for endotoxins in medical products.
 Currently,
the horseshoe crab is at the
center of a series of legal battles about the
size of harvests and their sustainability.
Subphylum Chelicerata: Class
Pycnogonida: sea spiders

Another name used for these animals is
Pantopoda (“all legs”), which is an excellent
description.

The body is greatly reduced in size, whereas the
legs are long and clawed. In some species
modified legs called ovigers are used by males
to carry egg masses.

There is a long proboscis on the head, which the
pycnogonid uses to feed on soft-bodied
invertebrates, especially cnidarians.
Figure 18.03
12.3
Class Pycnogonida: sea spiders
 Most
pycnogonids are small being only a
few millimeters long, but a few reach 70cm
measured by leg spread.
 Pycngonids
are exclusively marine
(occuring from the intertidal to the deep
seas) and there are about 1000 species.
Subphylum Chelicerata: Class
Arachnida: Order Aranae: spiders

Spiders are a very large group with more than 35,000
described species that occur worldwide except in
Antarctica.

The body is divided into a cephalothorax and an
abdomen.

Spiders breathe using book lungs or tracheae. Book
lungs are unique to spiders and consist of a large
number of air pockets extending into a blood-filled
chamber. Tracheae are tubules that carry air directly
from outside to tissues. They can be closed to prevent
excessive water loss.
Spiders

Spiders and insects have an excretory system
that uses structures called Malphigian tubules.

Potassium, other salts, and other waste solutes
drain into these tubules, which empty into the
intestine. Rectal glands selectively reabsorb
most of the potassium and water leaving behind
nitrogenous wastes in the form of uric acid,
which requires little water for its excretion.
Spiders
 Spiders
possess eight eyes, but their
image forming ability is limited.
 Hairlike
setae, however, provide a lot of
information about the environment sensing
e.g. vibrations and air currents
Spiders

All spiders are predators and their chelicerae
function as fangs.

The fangs are connected via ducts to venom
glands that produce a lethal venom the spider
uses to dispatch its prey.

After killing a prey item the spider injects
digestive fluid into the organism and sucks up
the resulting soup.
Figure 18.05
12.4
Spiders
 Spiders
use a variety of techniques to
catch prey.
 Some
chase their prey or leap on it, some
ambush prey often using trip wires, and, of
course, many use webs.
 One
small group, the bolas spiders,
capture their prey by deception.
Bolas spiders

Bolas spiders at night spin a line of thread with a sticky
globule (the bolas: akin to the bolas used by South
American Gauchos to hunt) at the free end.

The bolas contains pheromones which attract certain
male moths which think they are approaching a female.
When the spider senses the vibrations in the air caused
by a nearby flying moth it twirls its bolas in response.

Once the bolas strikes the moth it sticks and the moth is
hauled in.
Silk-spinning

Spiders spin silk, which they use to make webs
to trap prey.

Webs come in a variety of shapes and sizes and
contain and are coated with an adhesive that
holds prey.

Webs are typically placed in insect flyways and
the spider sits and waits for an insect to become
entangled. The vibrations of the struggling prey
signal the spider to emerge and dispatch it.
Silk-spinning

The silk is produced by two or three spinnerets,
which are connected to abdominal silk glands.
The silk is formed from a protein secretion that
hardens on contact with air.

The silk is extremely strong (stronger e.g. than
steel of equivalent weight) and is being used as
a kevlar substitute. Unlike steel, silk can stretch
which makes it an extremely useful material.
Silk-spinning
 The
silk is used to make webs, but also for
a variety of other purposes:




Line nests
Form egg sacs
For dispersal by ballooning
To wrap prey
Reproduction in spiders
 Courtship
rituals are a major feature of
spider mating in which the (usually much
smaller) male attempts to mate without
being eaten by the female.
 Males
produce a sperm packet wrapped in
silk which he holds in a cavity in one of his
pedipalps (second pair of appendages).
Reproduction in spiders

The male, if he lives long enough, inserts a
pedipalp into a female’s genital opening and she
stores the sperm in a seminal vesicle.

The female later fertilizing eggs when she is
ready to lay them. Eggs are laid in a silk cocoon
where the young hatch and remain for a short
time and molt before departing for an
independent life.
Toxic spiders
 The
vast majority of spiders are harmless
to humans, but a handful are toxic and
potentially deadly.
 In the U.S. there are a few venomous
spiders: several black widow species
(genus Latrodectus), the brown recluse
(Loxosceles reclusa) and the hobo spider
(Tegenaria agrestis) introduced from
Europe to the Pacific Northwest.
Toxic spiders
 Spiders
eat a liquid diet by injecting
digestive juices into their prey and some
spider venoms contribute to this process
by destroying tissue.
 Bites
of such spiders can lead to tissue
necrosis. This is the type of venom
possessed by the hobo and brown recluse
spiders.
Hobo spider
Brown recluse spider
Toxic spiders
 A bite
from one of these spiders results in
a bite site that develops a painful ulcer
where tissue dies (necrosis) and in the
case of hobo spiders severe headaches.
 One
component of brown recluse venom
is sphingomyelinase D which attacks and
dissolves cell membranes.
Toxic spiders

Sphingomyelinase D and other components in
the venom apparently also turn on the patient’s
inflammatory defenses against his or her own
tissues.

White blood cells destroy themselves releasing
other enzymes that attack the victim’s own flesh
and blood clots form in tiny vessels cutting off
the blood supply to the bite area and causing
necrosis.
Toxic spiders
 Generally,
bites are not fatal but if the
venom gets into the bloodstream it may
destroy red blood cells or attack the bone
marrow, which may lead to fatal
complications.
Toxic spiders
 Black
widow spiders and the funnel web
spider (one of the world’s most dangerous
species [naturally it’s from Australia!])
produce potent neurotoxins that affect the
nervous system.
Female Black Widow spider with egg sac
Toxic spiders
 Venom
is complex and many components
only affect invertebrates, but one
substance in black widow venom called
alpha latrotoxin stimulates cells to release
neurotransmitters such as acetylcholine.
 Nerves contract uncontrollably and this
can cause small muscle rigidity and
intense, agonizing pain.
Toxic spiders
 Additional
symptoms include nausea and
vomiting, increased blood pressure, and
the heart may begin racing or slowing
significantly.
 Brain
functions may also be affected
producing anxiety, amnesia, and even
psychosis.
Toxic spiders

Black widow bites were historically often
associated with outhouses, but now usually
occur when working in the fields or clearing junk
in outbuildings.

Fatality rates have been estimated at 1-5%, but
this figure certainly is an overestimate as many
people bitten do not seek medical attention.
Those most at risk are small children and the
elderly.
Toxic spiders

Funnel web spiders (especially the Sydney
funnel web spider) are extremely dangerous.

Funnel web spiders inject a venom whose lethal
component is called atrotoxin.

The toxin travels in the lymphatic system and
binds to nerve endings all over the body where it
causes nerves to discharge wildly, especially
those of the autonomic nervous system.
Funnel web spiders
Funnel web spider burrow site
Toxic spiders
 The
constant discharging of the autonomic
nervous system results in fever, irregular
heart rhythm and wild changes in heart
rate and blood pressure that can cause
respiratory failure, coma and cardiac
arrest.
 Death
may occur anything from 15
minutes to 6 days after a bite.
Toxic spiders
 Work
by Dr. Struan Sutherland and
colleagues in Melbourne has led to
success in treating funnel web spider bites
and greatly reduced mortality.
 Dr
Sutherland pioneered the use of
pressure bandages to prevent the
spreading of the venom and led the team
that developed successful antivenins.