Subphylum Chelicerata
Body divided into cephalothorax (prosoma) and abdomen (opisthosoma). Antennae absent. Generally 8 pairs of appendages. First pair of appendages modified as chelicerae. Second pair of appendages modified as pedipalps.

Class Merostomata
5 or 6 pairs of abdominal appendages modified as gills. Posterior telson.
Limulus polyphemus - horseshoe crab [juvenile] [adult]

Class Arachnida
Unsegmented prosoma generally covered dorsally by a solid carapace. Waxy epicuticle. Book lungs and tracheae.
Order Scorpiones
Scorpions. Segmented preabdomen and postabdomen terminating in a stinging apparatus. One pair of large median eyes and 2-5 pairs of lateral eyes. Chelicerae small and chelate. Pedipalps enlarged to form pincers.
Order Uropygi
Whip scorpions (vinegarroons). Segmented postabdomen. Prosoma covered by dorsal carapace. One pair of anterior median eyes and 3-4 pairs of lateral eyes. Chelicerae 2-segmented, with distal hook. Pedipalps short and stout.
Order Araneae
Spiders. Prosoma with convex carapace generally bearing 8 anterior eyes. Sternum on ventral surface, with small labium preceding it anteriorly. Chelicerae composed of a fang and and basal segment. Globose abdomen unsegmented and connected to prosoma by a narrow pedicel.Spinnerets at posterior end of abdomen.
Order Pseudoscorpiones
False scorpions. Superficially resemble scorpions, but lacking the long abdomen and sting. Poison gland(s) in chelate pedipalps.
Order Opiliones
Harvestmen or daddy longlegs.Prosoma broadly joined to short, segmented abdomen with no constriction. Chelicerae thin and chelate. Pedipalps short.
The Acari
Diverse assemblage containing 7 orders of mites and ticks. Abdomen without segmentation and fused to prosoma. Mouthparts modified as capitulum.
Order Ixodida
Ticks. Mouth with recurved teeth for piercing. Tracheal spiracle located near 4th pair of coxae.
Ixodes sp. - hard tick (Ixodidae) [6-legged larva] [adult]
Dermacentor sp. - hard tick (Ixodidae)
Argas sp. - soft tick (Argasidae)
Order Prostigmata
Mites with one or 2 spiracles located near capitulum.

Class Pycnogonida
Sea spiders. Cephalon with 4 dorsal eyes and anterior proboscis. Chelicerae reduced.
[1] [2] [3]
 

Subphylum Crustacea
Calcareous exoskeleton. Head with 5 appendage bearing segments (including 2 pairs of antennae), trunk with 2 to >65 segments, and terminal telson. One to 7 trunk segments fused to head to form cephalothorax. Appendages typically biramous. Head with median ocelli or lateral (often stalked) compound eyes. Excretion by antennal or maxillary glands. Gonochoristic. Fertilization by copulation. Development generally indirect with nauplius larva.

Class Branchipoda
Small crustaceans with foliaceous trunk appendages. Mostly freshwater.
Order Anostraca
Fairy shrimp. Trunk with 11-18 segments.
Order Notostraca
Tadpole shrimp. Thorax with appendages, abdomen without appendages. Thorax covered by carapace.
Order Cladocera
Water fleas. Trunk enclosed in bivalved carapace. Head with single, median, compund eye. Second antennae elaborated for swimming. 4-6 pairs of trunk appendages. Mostly freshwater.
Daphnia laevis [1] [2]
Moina macrocopa [1] [2]
Pleuroxis denticulatus
Ceriodaphnia megalops
Ceratia sp.

Class Malacostraca
Trunk with 14 appendage bearing segments. First 8 segments constitute the thorax, and last 6 compose abdomen. Compound eyes present in most species.
Subclass Hoplocarida
Order Stomatopoda
Mantis shrimp. Second pair of thoracic appendages large and subchelate. First antenna with three flagella. Carapace does not cover last 2 thoracic appendages. Compound eyes stalked.
Subclass Eumalacostraca
Antennae without 3 flagella. Abdomen without 7th segment.
Superorder Peracarida
Eggs brooded in marsupium. First thoracic segment fused to head. At least first pair of thoracic appendages modified as maxillipeds.
Order Amphipoda
Laterally compressed. Sessile compound eyes. No carapace. Thorax with one pair of maxillipeds and 7 pairs of pereopods. Last 3 pairs of abdominal appendages modified as uropods.
Caprella sp.
Gammarus sp.
Order Isopoda
Dorsoventrally flattened. Sessile compound eyes. No carapace. Thorax with one pair of maxillipeds, and 7 pairs of pereopods. Some abdominal segments might be fused with telson. Some or all pleopods modified as gills.
Oniscus sp. - pill bug
Ligia sp.
Order Mysidacea
Thorax covered by unfused carapace. Eyes stalked.
Order Tanaidacea
Anterior 2 thoracic segments covered by and fused with carapace. Second pair of thoracic appendages are gnathopods.
Superorder Eucarida Carapace fused with all with all thoracic segments. Gills at base of thoracic appendages. Eyes stalked.
Order Decapoda
Ten appendage bearing segments. First 3 pairs of thoracic appendages are maxillipeds. Gills enclosed by carapace.
zoea larva - crab
megalops - crab
Callinectes sapidus - blue crab [1] [2]
Paguroidea - hermit crab

Class Maxillopoda
No more than 11 trunk appendages. First pair of trunk appendages modified as maxillipeds.
Order Cirripedia Barnacles. Sessile parasitic and free-living crustacea. Body enclosed in bivalved caraace. Antennae reduced or absent.
barnacle nauplius larva
Order Copepoda
Cylindrical body. Trunk with thorax bearing 5 pairs of biramous appendages. Abdomen without appendages. First antennae longer than second. No compound eye. Naupliar eye present.
nauplius [1] [2]
Orthocyclops modestus - cyclopoid copepod
Diaptomus pallidus - calanoid copepod [1] [2]
Ergasilidae - parasitic poecilostomatoid copepods [1] [2]

Class Ostracoda
Body (including head) enclosed in bivalved carapace. Reduced trunk with no more than 2 appendages.
Cypridopsis spp. [1] [2] [3] [4] [5]
 

Subphylum Uniramia
Single pair of antennae. Unbranched appendages. Tracheae. Malpighian tubules.

Superclass Myriapoda
Elongated trunk with many leg-bearing segments. Head with antennae and ocelli.

Class Chilopoda
Centipedes. One pair of legs per segment. Mouth appendages covered by forcipules (poison claws).

Class Diplopoda
Millipedes. Fusion of somites to form diplosegments, giving the appearance of 2 appendages per segment.

Superclass Insecta
Insects. Three pairs of legs. Generally 2 pairs of wings. Single pair of antennae. Single pair of coumpound eyes.

Class Collembola
Springtails. Apterygote insects. Posterior furculum held forward by a clasping structure.

Class Pterygota
Winged insects.

Subclass Exopterygota
Hemimetabolous insects.
Order Ephemeroptera
Mayflies.
Hexagenia sp. - mayfly nymph
Order Odonata
Dragonflies (Anisoptera) and damselflies (Zygoptera).
Libellulidae - anisopteran naiads [1] [2] [3]
Coenagrionidae - zygopteran naiad
Order Plecoptera
Stoneflies.
Order Orthoptera
Grasshoppers, crickets, and katydids.
Order Phasmida
Stick-insects.
Order Blattodea
Cockroaches.
Order Mantodea
Mantids.
Order Isoptera
Termites.
Order Mallophaga
Biting or chewing lice.
Order Anoplura
Sucking lice.
Pediculus corporis - human body louse
Pthirius pubis - human crab louse
Order Homoptera
Cicadas, aphids, leaf-hoppers, tre-hoppers, scale insects, mealybugs, spittlebugs.
Order Hemiptera
True bugs.
Cimex sp. - bedbug

Subclass Endopterygota
Holometabolous insects.
Order Coleoptera
Beetles. (NOTE: giant water bug at center of picture is a hemipteran, not a coleopteran.)
Order Diptera
Flies.
Culex sp. - mosquito [adult] [larva]
Order Siphonaptera
Fleas.
[1] [2]
Order Trichoptera
Caddis flies.
Order Lepidoptera
Butterflies and moths.
caterpillar
Order Hymenoptera Wasps, ants, bees, sawflies, etc.