Arthropod Characteristics
- Body segmented, with regional specialization (at minimum, head and trunk)
- Hard, chitonous 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!
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(Subphylum Trilobitomorpha)
Class Trilobita
Trilobite fossil, Elrathia kingi; Cambrian Period, UT
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Characteristics
- Extinct, but diverse fossil forms 540-225 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)
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(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
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Class Merostomata — Horseshoe Crabs
Atlantic Horseshoe Crabs, Limulus polyphemus; mating aggregation; DE
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Atlantic Horseshoe Crab, Limulus polyphemus; preserved specimen
See also labeled photo.
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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
- Also includes the now-extinct Eurypterids (giant sea scorpions)
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(Subphylum Chelicerata)
Class Arachnida
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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
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(Subphylum Chelicerata)
Class Pycnogonida — Sea Spiders
Unidentified Sea Spider; LaJolla, CA
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Sea Spider, preserved specimen
See also labeled photo.
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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)
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(Subphylum Mandibulata)
- 3rd pair of cephalic appendages modified into mandibles
- Antennae usually present
- Appendages uniramous or biramous
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Class Crustacea
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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
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(Subphylum Mandibulata)
Class Myriopoda
Characteristics
- Body elongated, with multi-segmented trunk
- Little regional differentiation: only cephalon (head) distinct
- Walking legs on nearly all segments
- Four Orders (Pauropoda & Symphyla not covered)
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Order Diplopoda
- Millipedes
- Pairs of segments fused together
- Appear to have 2 pairs of legs/segment
- Slow moving detritivores
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North American Millipede, Narceus americanus; NY
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Polydesmid Millipede, Pachydesmus sp.; KY
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Order Chilopoda
- Centipedes
- Segments not fused; 1 pair of legs/segment
- First pair of legs modified into poison claws
- Fast moving predators
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Tropical Centipede, guarding eggs; CA
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House Centipede, Scutigera coleoptrata; PA
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(Subphylum Mandibulata)
Class Insecta (Hexapoda)
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Characteristics
- 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
- Most diverse class; nearly 1 million species described (with 10s of millions awaiting discovery!)
- 36 Orders
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