Arthropoda is treated here as a monophyletic clade of genetically diverse but evolutionarily linked species. Some zoologists, however, maintain that this alleged phylum is actually an artificial, polyphyletic grouping of similar taxa evolving multiple times from different prearthropod ancestors. Much of this debate has centered on evolutionary relationships between the phyla Arthropoda and Onychophora. Classified within Arthropoda are one extinct subphylum (sometimes called super class), the Trilobitomorpha (trilobites), and four living subphyla: Chelicerata (spiders, mites, horseshoe crabs, and sea spiders), Myriapoda (millipedes and centipedes), Hexapoda (springtails, bristletails, beetles, flies, true bugs, etc.), and Crustacea (crayfish, barnacles, water fleas, pill bugs, etc.). Sometimes the number of extant subphyla is reduced to three (Chelicerata, Uniramia, and Crustacea) or even two groups (Chelicerata and Mandibulata).
Molecular studies of arthropod phylogeny present a reasonably clear picture of relationships among three of the four living subphyla. Chelicerates are evolutionarily distinct from insects and crustaceans, and they differ from all other living arthropods in lacking a tagma for either a "head and trunk" or a "head, thorax, and abdomen." Instead, they possess an anterior prosoma without a distinct head and a posterior opisthosoma. Another major clade evident from gene sequences is Mandibulata, composed of the other three extant subphyla. Morphological observations of appendages would seem then to link Myriapoda and Hexapoda into a group (Uniramia) of taxa with only one branch to each appendage and distinct from the biramous Crustacea, but molecular evidence is inconclusive on this point. In some gene sequence trees, myriapods are tightly linked with insects, while other molecular analyses show the millipedes and centipedes as deeply entangled within other genetic branches.
Accurate estimates of both relative and absolute diversities of arthropods are often problematic because of the enormous species richness, large number of unexplored habitats, greater emphasis on studies of economically important taxa, and increasingly serious lack of qualified taxonomists. For those reasons, the literature is replete with divergent estimates of the total number of species in most groups, especially the insects and mites. Table I lists the classes of Arthropoda and includes estimates of taxonomic diversity.
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