Topic 1.3 · Core
Arthropod Groups
Arthropods are invertebrate animals sharing three defining characteristics: a segmented body, jointed appendages, and a hard external covering called an exoskeleton. Within that, the four required groups — myriapods, insects, arachnids and crustaceans — are told apart mainly by leg number, antennae, and body regions.
Myriapods
Myriapods have many body segments, many pairs of legs, and one pair of antennae. Their long, repeatedly segmented bodies are the giveaway — centipedes and millipedes are the familiar examples, and the sheer number of segments and legs makes them easy to tell apart from insects and arachnids.
Insects
Insects have three main body regions — head, thorax and abdomen — three pairs of legs, one pair of antennae, and often one or two pairs of wings as adults. Not every insect has functional wings, which is exactly why leg number is the more reliable feature to rely on: three pairs = six legs. An arthropod with six legs, one pair of antennae and three main body regions can be identified as an insect even without checking for wings at all.
Arachnids
Arachnids typically have four pairs of legs, no antennae, and two main body regions. Spiders and scorpions are the examples you’ll usually see. The combination eight legs + no antennae is particularly useful for telling an arachnid apart from the other three groups here.
Crustaceans
Crustaceans typically have more than four pairs of legs, two pairs of antennae, and a hard exoskeleton. Many are aquatic — crabs, lobsters and shrimps are familiar examples. Two pairs of antennae is especially useful evidence, because none of the other three arthropod groups here share it.
Comparing the arthropod groups
| Group | Legs | Antennae | Useful additional feature |
|---|---|---|---|
| Myriapods | Many pairs | 1 pair | Many repeated body segments |
| Insects | 3 pairs | 1 pair | Head, thorax and abdomen |
| Arachnids | 4 pairs | None | Two main body regions |
| Crustaceans | More than 4 pairs | 2 pairs | Many are aquatic |
When you’re classifying an unfamiliar arthropod, the most useful visible evidence usually comes in this order: number of legs → number of antennae → body regions. Several matching characteristics together always give more reliable identification than any single feature checked on its own.