Arthropods

Arthropods are animals with an external skeleton and jointed legs. An external skeleton is also known as an exoskeleton. Exoskeletons give animals protection from the outside environment and strong points for muscle attachment on the inside. The hard exoskeleton does not grow or stretch. As the animal grows, it therefore needs to shed (moult) its old exoskeleton a number of times. After the old exoskeleton has been moulted, the soft covering underneath hardens and becomes the new exoskeleton.

Arthropods represent close to 90% of all animal species. Some examples of arthropods are: grasshoppers, ants, bees, wasps, cicadas, cockroaches, fleas, butterflies, spiders, scorpions, ticks, centipedes, millipedes, crayfish, lobsters, prawns and slaters.

All arthropod legs have joints (as shown in the diagram) and it is this feature which gives them their name. Arthropod means ‘joint footed’ in Greek (arthros: joint and pous: foot).

Arthropods are divided into four classes:

• insects
• arachnids
• crustaceans
• myriapods.

The scientific criteria used for classifying arthropods is based on the number of legs, antennae and body parts that they have. This information is summarised in the following table.

Arthropod legs
The number of legs that an arthropod has is a very useful characteristic for classifying them into different classes. Insects always have six legs (three pairs) and arachnids have eight legs (four pairs). Crustaceans have at least eight legs and often more, while myriapods can have up to a hundred legs!

The legs of arthropods may be used for walking, while others may be used to assist with swimming, hopping, digging, or catching prey. For example, praying mantids have six walking legs but the front pair are also adapted for catching and holding prey. Some different arthropod leg types are shown below.

Arthropod antennae
Antennae are sensory structures which are used for feeling and smelling. They are attached on the head between the eyes and their size and shape can vary. Some antennae are very long and obvious, like those of cockroaches and butterflies, while others are quite short like those of flies and cicadas. Antennae are found on insects, crustaceans and myriapods but not on arachnids. Insects and myriapods have two antennae (one pair) and crustaceans have four antennae (two pairs). Some examples of different insect antennae are shown below.

Arthropod body parts
When looking at arthropod body parts (major divisions of the body), scientists include the head but not the appendages (legs, arms, antennae, mouthparts).

Arthropod classes have different numbers of body parts. The insects and arachnids have the most obvious body part divisions. Insects always have three body parts: the head, thorax and abdomen, but it can be difficult to see these divisions on some insects. An easy way to locate the thorax is to find the region that the legs are attached to. The part in front of this region is the head and the part behind is the abdomen. Wings (if present on the insect) are also attached to the thorax. Examples of insect body parts are shown below.

Arachnids, like this spider, have two body parts: the cephalothorax (a combined head and thorax appearing as one unit) and the abdomen.

Crustaceans, like this prawn, have a variable number of body parts, although many of the commonly encountered crustaceans (like the prawn above) have two: a cephalothorax (combined head and thorax) and an abdomen.

Myriapods, like the centipede, have two body parts: the head and a long segmented trunk.