Phylum Arthropoda

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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.

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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.

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Characteristics of the Arthropoda

• Jointed appendages. Primitively one pair per
  segment, but number often reduced.
• Appendages often greatly modified for specialized
  tasks.

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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.

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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.

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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.

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Characteristics of the Arthropoda

• Respiration occurs in multiple possible ways
• across the body surface
• via a system of tracheal tubes
• gills
• book lungs

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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.

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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).

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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

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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.

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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.

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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.

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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
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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).

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12.2
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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.

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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.

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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.

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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.

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12.3
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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.

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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.

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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.

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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

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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.

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12.4
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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.

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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.

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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.

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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.

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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

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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).

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Reproduction in spiders

• The male, if he lives long enough, inserts a
  pedipalp into a females 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.

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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.

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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.

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Hobo spider
Brown recluse spider
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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.

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Toxic spiders

• Sphingomyelinase D and other components in the
  venom apparently also turn on the patients
  inflammatory defenses against his or her own
  tissues.
• White blood cells destroy themselves releasing
  other enzymes that attack the victims own flesh
  and blood clots form in tiny vessels cutting off
  the blood supply to the bite area and causing
  necrosis.

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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.

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Toxic spiders

• Black widow spiders and the funnel web spider
  (one of the worlds most dangerous species
  naturally its from Australia!) produce potent
  neurotoxins that affect the nervous system.

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Female Black Widow spider with egg sac
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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.

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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.

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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.

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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.

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Funnel web spiders
Funnel web spider burrow site
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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.

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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.