BIO 385 Invertebrate Zoology
Go To: Udo Savalli | BIO 385 |

Invertebrate Diversity
Phylum Arthropoda

(Click on any image for larger version)

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 Trilobitomorpha)
Class Trilobita


  • Extinct, but diverse fossil forms 540-250 mya
  • Body divided into median and 2 lateral lobes by 2 longitudinal furrows
  • Anterior cephalon and posterior pydigium covered by platelike shields
  • Segmentation visible in middle section (thorax)
  • Appendages biramous (branched)
Trilobite fossil, Cyphaspis sp.; Devonian Period, Morocco
Trilobite fossil, Asaphus expansus; Ordovician Period, Russia
Diacalymene Trilobite
Trilobite fossil, Cambropallas telesto; Cambrian Period, Morocco
Trilobite fossil, Elrathia kingi; Cambrian Period, UT
Reedops Trilobite
Trilobite fossil, Crotalocephalina gibbus; Devonian Period, Morocco
Agnostid Trilobite fossil, Ptychagnostus sp.; Cambrian Period, UT

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

Class Merostomata Horseshoe Crabs


  • 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
  • Also includes the now-extinct Eurypterids (giant sea scorpions)
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.
Sea Scorpions
Sea Scorpion models: Eurypterus sp. (left) & Pterygotus sp. (right); Silurian Period
Sea Scorpions
Sea Scorpion, Eurypterus sp., head plate; Silurian Period

Class Arachnida


  • 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


  • 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)
Anemone Sea Spider, Pycnogonum sp. (preserved specimen)
Sea spider, Ammothea hilgendorfi; LaJolla, CA
Sea Spider specimen
Sea Spider, preserved specimen
See also labeled photo.

Subphylum Mandibulata

  • 3rd pair of cephalic appendages modified into mandibles
  • Antennae usually present
  • Appendages uniramous or biramous

Class Myriopoda


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

Order 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
Unidentified Millipede; Belize

Order 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; Camp Verde, AZ
Tropical Centipede, Scolopocryptops sp., guarding eggs; CA
House Centipede
House Centipede, Scutigera coleoptrata; PA

Order Symphyla

  • Pseudocentipedes
  • Small; 12 pairs of legs (fewer in juveniles)
  • Lack pigment (white or translucent)
  • Lack eyes
  • Occur in soils
  • Not represented by specimens in lab
Symphylan (preserved specimen)

Class Crustacea


  • 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

Superclass Hexapoda
Class Insecta


  • Body with three distinct regions: Head (5 segments); Thorax (3 segments); & Abdomen (11 segments)
  • 3 pairs of uniramous legs (on thorax)
  • Trachea (respiration)
  • Malphigian tubules (excretion)
  • Most species have 2 pairs of wings (not homologous to other appendages); capable of flight
  • Direct (hemimetabolous; with adult-like nymphs) or indirect (holometabolous, with distinct larval stage) development
  • Some primitive flightless forms sometimes placed in separate class Entognatha within Superclass Hexapoda
  • Most diverse class; nearly 1 million species described (with millions more awaiting discovery!)
  • 36 Orders

Go to Insect Diversity Page

This page last updated 21 August 2016 by Udo M. Savalli ()
Images and text Udo M. Savalli. All rights reserved.