Millions of arthropod species have already been described, and many more are waiting to be discovered. They have been found in all environments and all use legs (and sometimes wings) to get about - they walk, fly, and jump – swim, burrow, and crawl.
Arthropod Biology
All arthropods (phylum Arthropoda) need to shed their ‘skin’ (exoskeleton) in order to grow in size, and all have a body composed of a number of segments (where each segment is a ‘repeated body unit’). The exoskeleton of all arthropods contains chitin, although it may be strengthened by the addition of other chemicals, as with the crabs and lobsters.
Centipedes show their segmentation clearly, with many of the body segments bearing a pair of legs, and the specialised head segments carrying antennae and mouthparts.
The ancestral arthropod has been imagined as something worm-like, with a pair of legs on each of its many segments. The legs of each segment have become highly specialised over time and the analogy ‘Swiss Army Knife’ has been used to illustrate the idea.
Arthropod Classification
Traditional classification puts all arthropods into one of five groups (often referred to as ‘sub-phyla’):
- Trilobites are all extinct, but they were extremely successful aquatic arthropods, with over 17,000 described specie. Very little is known for certain about their soft parts, since it is usually only the tough armour on the back that is fossilised.
- Chelicerates (all the spiders, mites, scorpions and their relatives) are distinguished by their peculiar claw-like mouthparts. They have four pairs of walking legs.
- Myriapods (the centipedes and millipedes) have many pairs of legs – one pair per segment in the centipedes, and two in the millipedes.
- Hexapods have three pairs of legs. The springtail, bristletails, and proturans are not well known by non-specialists, but the insects are familiar to most. Over a million species of insect (class Insecta) have been described, and the vast majority are land animals, large enough to be seen clearly with the naked eye.
- Crustaceans are almost all aquatic and they are as numerous in water as the insects are on land. There are many different types, ranging from the planktonic copepods to the well-known crabs and lobsters. The number of legs is not diagnostic, but all crustaceans have a similar ‘nauplius’ larva – indeed it was the possession of this type of larval stage that first put the barnacles into the group (they had been thought of as molluscs until the larva was seen).
Molecular Analysis and Arthropod Relationships
Details of arthropod classification are very complex and controversial, and the exact evolutionary relationships of many types is still poorly understood, but DNA and RNA studies have shown that they are all directly related to the tardigrades and onychophorans. The nematodes and Nematomorpha also now thought to be related to the arthropods, but much more distantly.
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