Phylum ARTHROPODA                                

• Learn more about arthropods  from the
  • University of California Museum of Paleontology website.
  • University of Michigan Museum of Zoology website
  • Paisley University website
• arthropods are protostomes related to annelids [Fig. 23.2]
• over 80% of all described species are arthropods, most are insects
• about 800,000 species have been described; some estimate there may be as many as 6,000,000 species
• copepods [Fig. 23.1], tiny marine crustaceans, are the most abundant animals on earth

Characteristics

1. segmented body (metamerism), but with tagmatization
2. jointed appendages
3. rigid exoskeleton (cuticle) [Fig. 23.3] of mostly chitin; thin and flexible at joints [Fig. 23.4]
4. growth by molting (ecdysis) [Fig. 23.5]
5. metamorphosis
6. open circulatory system
7. etc.,

Subphylum TRILOBITOMORPHA

• Learn more from the UCMP about these trilobed Paleozoic arthropods with biramous appendages [Fig. 23.6]

Subphylum CHELICERATA

Learn more about chelicerates  from the University of California Museum of Paleontology website.

• Characteristics
  1. two tagmata: cephalothorax and abdomen
  2. no antennae
  3. 6 pairs of limbs
     1. chelicerae
     2. pedipalps
     3. pairs of walking legs

Class MERISTOMATA

Subclass Eurypterida UCMP eurypterids   [Fig. 23.7]: Paleozoic "water scorpions"	Eurypterus remiped lacustris; Upper Silurian; semicircular carapace of head region (prosoma) with crescentic eyes: from Bertie Formation of Ridgeway Ontario; click on picture to enlarge; photos courtesy of BIODIDAC
Subclass Xiphosura horseshoe crabs [Fig. 23.8] biomedical applications, Peter Armstrong the Xiphosuran Home Page from Lyall Anderson	Dorsal and ventral views of Limulus; click on pictures to enlarge; photos courtesy of BIODIDAC

Class ARACHNIDA

Class PYCNOGONIDA: sea spiders [Fig. 23.16]

Subphylum CRUSTACEA

Characteristics

1. biramous appendages [Fig. 23.17]
2. two pair of antennae
3. one pair of mandibles
4. 40,000 species
5. learn more from Paisley University

Classification

1. Class Malacostraca
   1. decapods: lobsters [Fig. 23.18], crayfish [Fig. 23.19], crabs, shrimp, krill, 
   2. amphipods 
   3. isopods (mostly aquatic, but includes pill bugs, the only terrestrial crustaceans [Fig. 23.22])
2. Class Branchiopoda
   1. brine shrimp, cladocerans (water fleas-Daphnia) [Fig. 23.23]
3. Class Copepoda: "the sheep of the sea" [Fig. 23.1]
4. Class Cirripedia: barnacles [Fig. 23.24]
   
   Balanus sp.; acorn barnacles; Photograph by Sherry Ballard, California Academy of Sciences.  Pollicipes polymerus; Leaf Barnacle. Photograph below left by E. Eugenia Patten, California Academy of Sciences. Photograph below right by Sherry Ballard, California Academy of Sciences
   

Subphylum UNIRAMIA

• Characters
  1. one pair of antennae
  2. all appendages are uniramous
  3. includes the insects and four other classes (myriapods)
  4. more information is available from the UCMP

Class DIPLOPODA - millipedes [Fig. 24.3a] two pairs of legs per segment body round in cross section mainly herbivorous, also scavengers many can secrete HCN (hydrogen cyanide) millipedes, centipedes, and two obscure classes are sometimes grouped together as myriapods photograph by Jo-Ann Ordano, California Academy of Sciences.
Class CHILOPODA - centipedes [Fig. 24.3b] one pair of legs per segment carnivorous appendages of first trunk segment modified into poison fangs photograph by James T. Johnson, California Academy of Sciences.

Class HEXAPODA (or Insecta) [fig. 27.12]

• 750,000 named species, perhaps as many as 30 million total

• Characters
  1. Three part body - head, thorax, abdomen [Fig. 24.4]
  2. Three pairs of legs, all attached to thorax
  3. One pair of antennae
  4. usually 1 or 2 pairs wings
  5. elaborate mouthparts - mandibles, etc. [Fig. 24.6, 24.7]
  6. tracheae [Fig. 24.9]
  7. metamorphosis
     • ametabolous
     • parametabolous [Fig. 24.12]
     • holometabolous [Fig. 24.13] - egg, larva, pupa, adult
  8. Malpighian tubules [Fig. 24.11]

Orders classified by mouthparts--more than 25 orders are recognized, including

1. Thysanura: silverfish
2. Odonata: dragonflies
3. Orthoptera: grasshoppers and crickets
4. Blattaria: cockroaches
5. Isoptera: termites, social insects
6. Phthiraptera: lice
7. Coleoptera: beetles; entomology image gallery from Iowa state has photographs and movies
8. Hymenoptera: ants, bees, wasps: social insects, pollination
9. Lepidoptera: moths and butterflies, pollination
10. Siphonaptera: fleas
11. Diptera: flies

For more information

• check out this key to the orders of insects from the Wonderful World of Insects
• The Hexapoda and Insecta pages from the Tree of Life

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