Lab 4 – Ecdysozoa - Arthropoda


The systematic definition of the Arthropoda continues to be changeable. Traditionally the phylum Arthropoda has been divided into 4 subphyla: the Trilobita (extinct), Uniramia (insects), Chelicerata (spiders and scorpions), and Crustacea (crabs, shrimp).  However, other schemes that include such groups as the Onychophora and Tardigrada within the Arthropoda have also been proposed. For the purposes of this lab we will follow the traditional breakdown into 4 classes.


  1. Trilobita – The Trilobita are an extinct group of chelicerate-like arthropods. Like all arthropods they are segmented and have jointed appendages on some of the body segments. The Trilobita were widespread in marine habitats before going extinct some 245 myr bp.



Classification: The Trilobita is divided into two subclasses: the Agnostida and the Polymerida.


Morphology: The Trilobita are segmented animals with a chitinous exoskeleton and jointed appendages. The body is divided into three anterior to posterior regions; the head region or cephalon that is a region of fused segments, the mid-region or the thorax that has multiple segments, and the posterior end is called the pygidium. The body of a Trilobite is also divided into three lateral regions the left Pleural lobe, the middle or axial lobe, and the right pleural lobe. It is from these three lobes that Trilobita takes its name.


Fossil record: An extensive fossil record. The Trilobita originated approximately 540 myr bp and went extinct 245 myr bp.



Read more about the Trilobita on the UCMP web site

Trilobita on the Palaeos web site

More on Trilobita

Even more on Trilobita

Drawings and photos of Trilobite fossils


  1. Chelicerata – The Chelicerata are composed of the horseshoe crabs (Xiphosura), the sea scorpions (Eurypterida), the Arachnida (spiders, scorpions, mites, and others), and the sea-spiders (Pycnogonida). The Eurypterida are all extinct.   The Eurypterida were marine predators that reached 2 meters in length! They originated in the Ordovician and went extinct in the Permian. There are approximately 86,000 described species of Chelicerata. The Xiphosura, Pycnogonida, and Eurypterida are all marine in distribution, however, the Arachnida are almost entirely terrestrial.


Sea Spider (Pycnogonida)

Horseshoe crab (Xiphosura)

Solpugid (Solfugae)                                             Pseudoscorpion (Pseudoscorpiones)


                                                Water Spider         (Araneae)                                                              Crab Spider (Araneae)


Scorpion (Scorpiones)                                                                        Spider (Araneae)

Pictures courtesy of BIODIDAC


Classification – The Chelicerata are divided into four main groups: horseshoe crabs (Xiphosura), the extinct sea scorpions (Eurypterida), sea –spiders (Pycnogonida), and the spiders, scorpions, mites, etc. (Arachnida). The Xiphosura differ from all other Chelicerata in having compound eyes. There are only 4 extant species in two genera. The Arachnida is divided into 12 groups that include, the spiders (Araneae), scorpions (Scorpiones), mites (Acari), daddy-longlegs (Opiliones), whip-scorpions (Thelyphonida), pseudoscorpions (Pseudoscorpiones), and others.


Morphology – The body of chelicerates is divided into two regions. The prosoma (head) in the anterior of the animal bears six pairs of appendages. One pair of these appendages forms the typical chelicerate claws, while other pairs are modified for feeding or for reproductive functions. Chelicerates are the only group of arthropods that lack antennae. The second, posterior region of the body is known as the opisthosoma. Appendages on the opisthosoma are reduced, and in Xiphosura are modified into external gills.



Fossil record – Mid Cambrian (545-525 myr bp)


Read more about the Chelicerata on the UCMP web site

Chelicerata on the Tree of Life web site

Images of spiders and scorpions on the University of Nebraska website

More on Chelicerata

Chelicerata on the AnimalDiversity Web

Chelicerata on the Palaeos web site


  1. Crustacea – The Crustacea includes the crabs, shrimp, lobsters, barnacles, and many other groups. There are approximately 38,000 described species of Crustaceans. Most Crustacea are marine in distribution, but there are also freshwater and terrestrial species.


Gammarus pulex (Amphipoda)                                          Daphnia pulex (Cladocera)

Pictures courtesy of BIODIDAC


Isopoda                                                                                 Callinectes sapidus (Decapoda)

John R. Meyer


Lobster (Decapoda)                                             Cyclops sp. (Copepoda).


Pictures courtesy of BIODIDAC


Classification and Morphology – The Crustacea are divided into five major groups: the Remipedia, the Cephalocarida, the Branchiopoda, the Maxillopoda, and the Malacostraca.


The Remipedia consist of only 7 described species. They have a head and elongate body consisting of 32 similarly shaped segments with laterally oriented appendages.


The Cephalocarida are tiny benthic species that feed on detritus. Only 9 species have been described.


The Branchiopoda are the fairy shrimp (Anostraca), water fleas (Cladocera), tadpole shrimp (Notostraca), and clam shrimp (Conchostraca). The Branchiopoda consist of 800 described species. Body form and appendages are quite variable within this group, but the thorax and abdomen are fused to indistinguishable. Most species live in freshwater to brackish water habitats. 


The Maxillopoda include the mussel shrimp (Ostracoda), the Mystacocarida, the copepods (Copepoda), the fish lice (Branchiura), barnacles (Cirripedia), and the Tantulocarida. The Ostracoda are small, mostly freshwater and marine species that have five pairs of appendages on their head and only three pairs on the rest of the body. They occur in sediments in lakes and oceans, but some species occur in the water column. A few species are found in terrestrial habitats where they feed on decaying plant material around mosses. There are 8000 described species of Ostracods. The Copepoda are small largely marine species and are extremely abundant in the oceans. There are planktonic species (found in open water) benthic species (found in or on the sediments on the ocean bottom), and parasitic species. There are approximately 9,000 described species of Copepods. The Branchiura are ecto-parasitic on the outside of fish. There are approximately 130 described species.  The Cirripedia are marine in distribution and sessile in habitat. There are free-living species that attach to rocks, whales, boat bottom, dock and pilings, and parasitic species that are ectoparasites of anemones or endoparasites of decapod crustaceans. There are approximately 1,000 described species of Cirripedia.


The Malacostraca include the shrimp, crayfish, and crabs (Decapoda), beach hoppers (Amphipoda), krill (Euphausiacea), mantis shrimp (Stomatopoda), and pill bugs (Isopoda) and other allied groups. There are approximately 25,000 described species of Malacostraca.



Fossil record – The first Crustaceans are found in the Cambrian between 544 – 505 myr bp.


Crustacea on the UCMP web site

Crustacea on the Tree of Life web site

Crustacea on the AnimalDiversity web

More on Crustacea

Crustacean pictures


  1. Uniramia – The Uniramia is the largest group of Arthropods and it contains the insects, millipedes, centipedes, and allied groups.


Millipede (Diplopoda)                                         Centipede Scolopendra sp. (Chilopoda)

Pictures courtesy of BIODIDAC



Classification and Morphology – The Uniramia are divided into two major groups the Myriapoda (centipedes, millipedes, symplylans, and pauropods) and the Hexapoda (insects). The Uniramia are arthropods that have appendages with only one branch. All other arthropod groups have appendages with two branches (biramous). Adult Uniramia lack external gills. They have a series of  internal tubes (trachea) that open to the air through small opening in the exoskeleton called spiracles.


The Myriapoda contains approximately 13,000 described species distributed among the centipedes (Chilopoda), the millipedes (Diplopoda), the Symphyla, and the Pauropoda.  All the Myriapoda have many legs, unlike the Hexapoda, which has only three pairs of legs. The Symphyla and Pauropoda are small and live in the leaf litter and soil in terrestrial habitats. The Chilopda have a single pair of legs on each body segment, while the Diplopoda have two pairs of legs on each body segment.


The Hexapoda and divided into two groups the Entognatha which includes a few small groups such as the Springtails (Collembola – about 6,000 species), Protura (about 500 species), Diplura (about 800 species), and the enormous group the Insecta.


Entognatha – The Springtails or Collembola are soil dwelling and feed on decaying plant material.  They possess a springing mechanism, which allows them to jump substantial distances. The Protura are soil arthropods. They are small, eyeless and without antennae (less than 2 mm in length) and probably feed on fungi. The Diplura are also soil arthropods with long antennae and long filiform cerci extending from the tip of the abdomen.


Lepidoptera (butterfly)                                                        Coleoptera (beetle)


Orthoptera (grasshopper)                                                   Homoptera (aphid)


Trichoptera (caddisfly)                                                       Diptera (true fly)


Insecta – The Insecta contains 29 extant orders and 9 extinct orders. The most diverse orders include the Coleoptera (beetles), Hymenoptera (ants, bees, wasps), Diptera (true flies), and Lepidoptera (Moths and butterflies). Several orders of insects have complete metamorphosis (holometabolous) with a morphologically distinct juvenile and adult stage such as in the Lepidoptera, Diptera, Coleoptera, and Hymenoptera. Other orders have incomplete metamorphosis (hemimetabolous) where juveniles resemble the adults, but have non-functional wings and are not sexually mature (Hemiptera, Homoptera, Orthoptera and others).   Insects occur widely in terrestrial and freshwater habitats, but are largely absent from saltwater environments. The body of an insect is divided into three regions (head, thorax, and abdomen). Each insect has 3 – pairs of legs on the ventral side of the thorax, and usually two pairs of wings extending from the dorsal side of the thorax.


Fossil record – The earliest fossils are mid-Cambrian (545 – 525 myr bp).



Uniramia on the UCMP web site

Uniramia on the AnimalDiversity web

Insecta on the Animal Diversity Web



Lab 4 Exercise


  1. List traits that distinguish 10 insect orders (no dichotomous key necessary).
  2. Find photos for 5 insect orders not pictured on the lab web page.
  3. Find an estimate of the number of species of insects.