BIO 385 — Invertebrate Zoology
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Invertebrate Diversity
Phylum Arthropoda

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Arthropod Characteristics

  • Body segmented, with regional specialization (at minimum, head and trunk)
  • Hard, chitinous exoskeleton (cuticle) that is molted (ecdysis)
  • Pair of jointed appendages on each body segment
  • Usually with pair of compound eyes and multiple ocelli (lost in some)
  • Reduced coelom forms hemocoel (open circulatory system)
  • Well developed digestive, nervous systems
  • Extraordinarily diverse: > 1 million species!

Subphylum Chelicerata

  • Body divided into prosoma (fused head and thorax) and opisthosoma (abdomen)
  • First pair of appendages modified for feeding: chelicerae
  • Appendages biramous
  • Lack antennae
  • Trilobites may belong in this clade

Class Merostomata, Order Xiphosurida — Horseshoe Crabs

Characteristics

  • Prosoma covered by large, U-shaped carapace
  • 5 Pairs of walking legs
  • Abdominal appendages flattened into "book gills"
  • Long posterior tail (= telson)
  • 5 Living species, on ocean bottoms
  • Some biologists no longer use Merostomata as this may not be a monophyletic group
Horseshoe Crab
Atlantic Horseshoe Crabs, Limulus polyphemus; mating aggregation; DE
Juv. Horseshoe Crab
Atlantic Horseshoe Crab, Limulus polyphemus, juvenile
Horseshoe Crab Ventral Side up
Atlantic Horseshoe Crab, Limulus polyphemus, ventral side
See also labeled photo.

Order Eurypterida — Sea Scorpions

Characteristics

  • Sea Scorpions (extinct)
  • One pair of appendages form large, flat paddles
  • Prosoma covered in a carapace
  • Posterior telson either flattened (for swimming) or stinger-like
  • Mostly marine, some in fresh water, and may have been able to briefly move onto land
  • Some reached over 2 m long
See additional fossil chelicerates (merostomates) here
Sea Scorpions
Sea Scorpion models: Pterygotus (left), Eurypterus (center) & Acutiramus (right); Silurian Period
Sea Scorpions
Sea Scorpion, Eurypterus sp., head plate; Silurian Period
Sea Scorpion
Sea Scorpion fossil, Balteurypterus tetragonophtalmus; Late Silurian Period, Ukraine

Class Arachnida

Characteristics

  • Additional feeding appendages: pedipalps & maxilla
  • 4 pairs of walking legs
  • Lack abdominal appendages (except for spinnerets in some)
  • Nearly all are terrestrial
  • Respiration via book lungs or trachea

Go to Arachnid Diversity Page
Arachnid Composite


Class Pycnogonida — Sea Spiders

Characteristics

  • Small bodies with proportionately long legs (>3x body length)
  • Body not differentiated into regions
  • Unique proboscis
  • Usually 4 pairs of walking legs
  • Marine predators on slow-moving or sessile prey
  • Larvae are parasitic (mostly on Cnidarians)
Pycnogonum
Anemone Sea Spider, Pycnogonum sp. (preserved specimen)
SeaSpider
Sea spider, Ammothea hilgendorfi; LaJolla, CA
Sea Spider specimen
Sea Spider, preserved specimen
See also labeled photo.


Subphylum Myriopoda

Characteristics

  • Body elongated, with multi-segmented trunk
  • Little regional differentiation: only cephalon (head) distinct
  • Walking legs on nearly all segments
  • Four Orders (Pauropoda not covered)

Class Diplopoda

  • Millipedes
  • Pairs of segments fused together
  • Appear to have 2 pairs of legs/segment
  • Slow moving detritivores
North American Millipede
North American Millipede, Narceus americanus; NY
Polydesmid Millipede
Polydesmid Millipede, Pachydesmus sp.; KY
Unidentified Millipede
Sucking Millipede, Siphonophora sp.; Belize
Crested Millipede
Crested Millipede, Abacion magnum; KY
Millipede swarm
Swarm of unidentified juvenile red millipedes; Kenya
Pill Millipede
Pill Millipede, dried specimen

Class Chilopoda

  • Centipedes
  • Segments not fused; 1 pair of legs/segments
  • > 15 pairs of legs
  • First pair of legs modified into poison claws
  • Last pair of legs modified into sensory appendage
  • Fast moving predators
Giant Desert Centipede
Giant Desert Centipede, Scolopendra heros (Order Scolopendromorpha); Camp Verde, AZ
Giant Desert Centipede head
Giant Desert Centipede, Scolopendra heros, ventral view of head
See also labeled photo.
Centipede
Tropical Centipede, Scolopocryptops sp., guarding eggs (Order Scolopendromorpha); CA
House Centipede
House Centipede, Scutigera coleoptrata (Order Scutigeromorpha); PA
Stone Centipede
Stone Centipede, Lithobius sp.? (Order Lithobiomorpha); PA
Soil Centipede
Soil Centipede, (Order Geophilomorpha); preserved specimen

Class Symphyla

  • Garden centipedes or pseudocentipedes
  • Small; 12 pairs of legs (fewer in juveniles)
  • Lack pigment (white or translucent)
  • Lack eyes
  • Occur in soils
Symphylan
Symphylan (preserved specimen)
Symphylan
Symphylan (whole mount)


Subphylum Crustacea

Characteristics

  • Head (cephalon) with 5 pairs of appendages; including 2 pairs of antennae
  • Trunk usually divided into thorax and abdomen
  • Appendages mostly biramous
  • Have compound eyes (often on stalks) and ocelli
  • Predominantly marine; some fresh water or terrestrial
  • Unique nauplius larva

Go to Crustacean Diversity Page
InsectComposite



Subphylum Hexapoda

Characteristics

  • Body with three distinct tagmata: Head (derived from 5 segments); Thorax (3 segments); & Abdomen (11 segments)
  • 3 pairs of uniramous legs (on thorax)
  • Antennae present
  • Trachea (respiration)
  • Malphigian tubules (excretion)

Class Entognatha

Characteristics

  • Mouthparts enclosed within head (not externally visible)
  • Ametabolous development (no structural differences between juveniles and adults)
  • All are wingless
  • Eyes reduced or absent
  • Most live in soil

Order Collembola

  • Springtails
  • Furcula: forked jumping appendage at posterior of abdomen
  • Small size, <6 mm
Springtail
Springtail, Tomocerus nigritus; NY
Springtail ventral view
Springtail, Tomocerus nigritus, ventral view showing furculum; NY
Collembolan
Springtail, whole mount slide

Orders Protura & Diplura

  • Both lack eyes
  • Proturans: coneheads
    • conical head
    • 1st pair of legs held elevated as sensory structure
    • Very small, <2 mm
  • Diplurans: bristletails & forcepstails
    • Pair of posterior appendages (either filamentous or pincer-like)
    • Small size, <5 mm
Proturan
Proturan; whole mount slide
Forcepstail
Forcepstail (Family Japygidae), AZ; whole mount slide

Class Insecta

Characteristics

  • Insects
  • Mouthparts external (usually visible)
  • Either hemimetabolous or holometabolous development
  • Most have 2 pairs of wings (a few are primitively wingless and some are secondarily wingless)
  • Eyes and other sensory structures well developed
  • Most diverse class; nearly 1 million species described

Go to Insect Diversity Page
Insect Composite

Go to Insect Coloration Page
Color Composite

This page last updated 15 Decembr 2022 by Udo M. Savalli ()
Images and text © Udo M. Savalli. All rights reserved.