Arthropods are the most diverse animal group on Earth, including insects, spiders, crabs, centipedes, and many other familiar animals. Their success comes from a body plan built around a hard exoskeleton, segmented body regions, and jointed appendages. These features let arthropods move efficiently, protect their soft tissues, sense their environment, and adapt to many habitats.
Studying arthropod diversity helps explain how one basic body plan can evolve into millions of species.
Key Facts
- Arthropods have a segmented body, a hard exoskeleton, and jointed appendages.
- The exoskeleton is made mostly of chitin and must be shed during molting for the animal to grow.
- Insects usually have 3 body regions, 6 legs, 1 pair of antennae, and often wings.
- Arachnids usually have 2 body regions, 8 legs, no antennae, and include spiders, scorpions, ticks, and mites.
- Crustaceans usually have 2 pairs of antennae, many legs, and often live in aquatic habitats.
- Species richness ratio = number of species in a group / total number of arthropod species.
Vocabulary
- Arthropod
- An invertebrate animal with a segmented body, jointed appendages, and an external skeleton.
- Exoskeleton
- A hard outer covering that supports and protects an arthropod's body.
- Molting
- The process of shedding an old exoskeleton so a larger new one can expand and harden.
- Appendage
- A body part that extends from the main body, such as a leg, antenna, claw, or mouthpart.
- Compound eye
- An eye made of many small visual units that can detect movement and form a broad field of view.
Common Mistakes to Avoid
- Calling all arthropods insects is wrong because insects are only one major arthropod group with six legs and three main body regions.
- Counting spider legs as six is wrong because arachnids have eight legs and no antennae.
- Thinking the exoskeleton is an internal skeleton is wrong because it is outside the body and protects the animal like armor.
- Forgetting molting when explaining growth is wrong because a rigid exoskeleton cannot simply stretch as the animal gets larger.
Practice Questions
- 1 A survey finds 120 insects, 30 arachnids, 20 crustaceans, and 10 myriapods in a habitat sample. What fraction and percent of the arthropods are insects?
- 2 An arthropod has 8 legs, no antennae, and two main body regions. How many pairs of legs does it have, and which major arthropod group does it most likely belong to?
- 3 Explain why an exoskeleton and jointed legs helped arthropods become successful in many different environments.