The Arthropods (Arthropod PDF is here)

The Phylum Arthropoda is the largest and most diverse of all animal phyla (Fig 1). More than three quarters of the animals on earth are arthropods, and most of these are insects. More than 900,000 species have been described, and if biologists have the opportunity to explore the rain forests before they are completely burned to the ground, a like number will probably be discovered.

Figure 1. Numerical Distribution of Named Animal Taxa.

Arthropods have their skeletons on the outside of their bodies, rather than on the inside (they have an exoskeleton; while our internal skeleton is called an endoskeleton.) The phylum's name is based on this feature (Arthropoda means "joint-foot" referring to the external joints in the skeleton that allow movement). There are both advantages and disadvantages to being inside out (although the majority opinion is clearly that it's our skeletons that are oddly situated). The advantages seem to outweigh any disadvantages, judging from the phylum's success and diversity (arthropods include in their ranks crabs, lobsters, spiders, mites, and the vast number of different insects).

More than anything, the exoskeleton has allowed the arthropods to diversify and move into niches that were previously inaccessible. They were the first beasts to successfully colonize land (the exoskeleton pre-adapted them for terrestrial habitats since it provided both support and protection from drying out). Insects flew long before anything like a bird existed. Some must have been quite impressive; relatives of today's dragonflies with wingspans of nearly a meter flew the Triassic skies (over 230 million years ago and pre-dating the first birds by nearly 100 million years). We even owe the presence of flowers to insects (most flowers are constructed to attract insects for pollination).

A short sensory PDF is here