THE BIOLOGY BOOK

c. 570 Million BCE

Arthropods are the most successful animals on the planet, having populated land, sea, and air, from the highest mountain to the deepest sea, from the poles to the tropics. They make up over three-quarters of all living and fossil animals, and it has been estimated that one billion billion (10¹⁸) are currently living on earth, representing over one million species that have been described, with many millions more, living in tropical rainforests, yet to be identified. Their size ranges from microscopic insects and crustaceans to blue king crabs in the Bering Sea, with a leg span extending beyond 6 feet (1.8 meters) and often weighing more than 18 pounds (8 kilograms).

The origin and evolution of arthropods are clouded in controversy because many of the earliest members did not leave fossil remains. It is generally believed that all arthropods evolved from a common annelid ancestor—a marine worm—some 550–600 million years ago. Scientists are not of a single mind whether all arthropods evolved only once or multiple times from this common antecedent. The earliest fossil remains are of the now-extinct marine trilobites, dating back over 530 million years. The first land animals were myriapod arthropods (centipede-related) appearing some 450 million years ago.

Arthropods, the most diverse phylum, are invertebrates that are categorized into five major groups—insects, spiders, scorpions, crustaceans, and centipedes—all of which have common characteristics: They are bilaterally symmetrical (as are humans), that is, the left half of the body is the mirror image of the right half. They are surrounded by a cuticle, an exoskeleton (external skeleton) composed of chitin (a carbohydrate polymer), which provides protection, points of attachment for muscles that move their appendages, and prevents water loss from the body. Insect bodies are segmented and their appendages are jointed (arthropod means “jointed feet”), permitting them to move their legs, claws, and mouthparts, although their body is encased in the inflexible exoskeleton. Appendages have evolved to become fewer in number and more specialized in function, such as for locomotion (walking or swimming), feeding, defense, sensory perception (which is well developed), and reproduction.

SEE ALSO: Insects (c. 400 Million BCE).

Three-quarters of all living and fossil animals are classified as arthropods, including such crustaceans as the lobster. This watercolor, Hawaiian Lobster, was painted in 1819 by a sixteen-year-old French artist named Adrien Taunay the Younger.