Arthropod Classification: Classes, Characteristics, and Diversity
Diversity and Success of Arthropods
The most successful animals on the planet are the arthropods. They live on land, in the sea, and in the air, making up over three-fourths of all currently known living and fossil organisms—over 1 million species in all.
Arthropods exhibit a vast distribution, ranging from the deep sea to mountain peaks, and vary greatly in size, from the king crab with its 12-foot armspan to microscopic insects and crustaceans.
Key Characteristics of Phylum Arthropoda
- Arthropods have a stiff cuticle made largely of chitin and proteins, forming an exoskeleton that may or may not be further stiffened with calcium carbonate.
- The phylum takes its name from its distinctive jointed appendages, which may be modified in a number of ways to form antennae, mouthparts, and reproductive organs.
Class Arachnida (Spiders, Scorpions, Mites, Ticks)
Spiders constitute the largest group within Class Arachnida.
General Characteristics of Arachnids
- All arachnids have two body segments: the cephalothorax and the abdomen.
- They possess six pairs of jointed appendages:
- The first set, called chelicerae, are located near the mouth and are often modified into pincers or fangs.
- The second set, pedipalps, are used for handling food and sensing; some males use them to carry sperm.
- The four remaining pairs of appendages are walking legs.
- Arachnids have no antennae.
- They lack mandibles for chewing. Food is captured, enzymes are injected, and the spider sucks out nutrients from the body of the prey.
Ticks
- Possess one fused body segment.
- Feed on blood from reptiles, birds, and mammals.
- Small, but can grow up to 1 cm after a large blood meal.
- Known for spreading diseases, such as Lyme disease.
Mites
- Possess one fused body section.
- Often too small to be seen by the human eye.
- Bites can be painful and itchy.
Scorpions
- Characterized by many abdominal body sections and pincers.
- Have a long tail with a venomous stinger used to paralyze prey.
- Thrive in warm, dry climates and feed on insects and spiders.
Class Crustacea (Crabs, Lobsters, Shrimps, Barnacles)
- Most are aquatic and use feathery gills to respire.
- They are the only arthropods with two pairs of antennae for sensing.
- Possess mandibles and two compound eyes.
- Many have five pairs of walking legs; the first pair is often modified into defensive claws.
Class Chilopoda (Centipedes)
- They use Malpighian tubules to secrete wastes and respire through tracheal tubes.
- Centipedes are carnivores that eat soil arthropods, snails, slugs, and worms.
- Their bite can be painful to humans.
Class Diplopoda (Millipedes)
- They use Malpighian tubules to secrete wastes and respire through tracheal tubes.
- Millipedes are herbivores/detritivores.
- They do not bite, but can spray foul-smelling fluids for defense.
Class Merostomata (Horseshoe Crabs)
- Known as Living Fossils, they are largely unchanged since the Cambrian period (500 mya).
- They are similar to extinct trilobites, possessing a heavy exoskeleton; young horseshoe crabs resemble trilobites.
- They forage on ocean bottoms for seaweed, worms, and mollusks.
Class Insecta (Insects)
Insects make up the largest group of arthropods, outnumbering all other animals combined (including all other arthropods).
Insect Characteristics and Reproduction
- Possess three body segments and six legs on the thorax.
- Usually have one or two pairs of wings.
- They usually mate once during their life; eggs are internally fertilized.
- Some hatch into miniature adults, while others undergo metamorphosis.
Metamorphosis Life Cycle
- Egg
- Larva (usually a caterpillar)
- Pupa
- Adult