ABSTRACT

Arthropods Arthropods comprise approximately 78Q of all known species of animals. They

possess jointed appendages and a segmented body that is covered with a chitinized exoskeleton. There are two major groups of arthropods, chelicerates and mandibulates. The chelicerate arthropods include arachnids (spiders, scorpions, mites, and ticks), xiphosurids (king crabs), and pycnogonids (sea spiders). These organisms differ markedly from one another, but generally they possess chelicerae as the first pair of appendages, and they lack antennae. Mandibulate arthropods, which include insects, crustaceans, millipedes, and centipedes, typically possess antennae and mandibles, but lack chelicerae. Some of the characteristics distinguishing mandibulates and chelicerates are given in Table 1.