This will acquaint you with the major orders within the various classes of the Phylum Arthropoda. Some of the characteristics that were present in the first arthropod and retained in virtually all descendants are: segmented body with some tagmosis (specialization and joining of segments), chitinous exoskeleton, segmented legs, and internal respiration (gills, trachea, spiracles).

Subphylum TRILOBITA (or Trilobitomorpha). Extinct. Antennae present, other appendages uniramous (superficially appear to be biramous) and undifferentiated. Authorities are still uncertain to which extant group trilobites are most closely related=4,000 extinct "species."


Subphylum CHELICERIFORMES. First pair of appendages are chelicerae (pincer-like appendages), while the second pair are leg-like sensory or sometimes prehensile structures called pedipalps. Antennae absent. Cephalothorax present, i.e., no head-thorax division.

Class CHELICERATA. Scorpions, spiders, "sea spiders," etc.


Subclass MEROSTOMATA. Continuous dorsal carapace present; a pair of compound and a pair of simple eyes present; abdomen with gills and a posterior pointed telson at tip. Mostly extinct.

Order Xiphosura. King or "horseshoe" crabs (Limulus). Cephalothorax wide, arched. 5 species.

Order Eurypterids. Extinct. Cephalothorax narrow. Giant water scorpions.


Subclass ARACHNIDA. Arachnids. Abdomen without gills. Four pairs of walking legs.

Order Palpigradi. Palpigrades. Minute, primitive arachnids. 60 species.

Order Uropygi. Whip scorpions and vinegaroons. 100 species.

Order Amblypigi. Rare. Tailless whip scorpions and whip spiders. 70 species.

Order Ricinulei. Rare. Riniculeids. 35 species.

Order Scorpiones. True scorpions. Cephalothorax and abdomen broadly joined, abdomen segmented. Chelicerae three-segmented. 1,200 species.

Order Pseudoscorpionida. Pseudo- or false scorpions. Chelicerae with silk glands and pedipalps with poison glands. 2,000 species.

Order Solpugida. Sun spiders. 900 species.

Order Araneae. Spiders. Cephalothorax and abdomen narrowly joined, not fused; abdomen unsegmented; chelicerae 2-segmented. 35,000 species.

Order Opiliones (= Phalangida). Daddy longlegs or harvestmen. Cephalothorax and abdomen broadly joined, fused; abdomen faintly segmented. 5,000 species.

Order Acari. Ticks and mites. Cephalothorax and abdomen broadly joined, fused; abdomen unsegmented. 30,000 species.


Class PYCNOGONIDA. Sea spiders. Cephalothorax greatly dwarfs abdomen. Gills. 1,000 species.


Subphylum CRUSTACEA. First postantennal somite (segment) bearing a pair of antennae (hence two pairs of antennae total); other appendages biramous and differentiated. Predominantly aquatic. > 40,000 species (10% freshwater).


Order Isopoda. Woodlice, Pillbugs. dorsoventrally flattened, small (<20mm) mostly marine, some freshwater and terrestrial. ca. 100 species in North America.


Subphylum UNIRAMIA. A pair of preoral antennae present, thought to be homologous with the antennae of trilobites. Body divided into a distinct cephalon and metameric trunk. Three pairs of postoral appendages on head; mandibles on second postantennal segment.

Class MYRIAPODA. One pair of antennae present (preoral); more than three pairs of walking legs present. Coxae of legs with single articulation with sternum.

Subclass CHILOPODA. Centipedes. Each body somite bearing a single pair of legs. Genital pore on posterior end of body. 2,500 species.

Subclass DIPLOPODA. Millipedes. Each apparent dorsal segment with two pairs of legs; genital pore between 2nd and 3rd pairs of legs. 10,000 species.

Subclass SYMPHYLA. Each of the 12 dorsal body segments with one pair of legs; genital pore between the 4th pair of legs. 120 species.

Subclass PAUROPODA. Pauropodans. Branched antennae, eyeless, mouthparts poorly developed. 9-11 leg-bearing trunk segments. Soils and leaf litter. 500 species.


"Superclass" HEXAPODA. One pair of (preoral) antennae present; three pairs of legs, one on each thoracic segment. An ambiguous taxon, used differently by different authors.

Class COLLEMBOLA (Oligentomata). Springtails and snow fleas. Abdomen of 6 segments, bearing a spring apparatus derived from paired appendages. 6,000 species, 11 families.

Class PROTURA (Myrientomata). Abdomen of adult with 11 segments; anamorphosis (addition of body segments during growth) present. 260 species, 3 families.

Class DIPLURA (Diplurata). 4 pairs of thoracic spiracles; antennal flagellum with intrinsic musculature. 675 species, 7 families.

Class INSECTA. 11 abdominal segments; no anamorphosis; 2 pairs of thoracic spiracles; antennal flagellum lacking intrinsic muscles. > 800,000 species described, 2-15 million undescribed.

Subclass Apterygota (Zygentomata). The primitively wingless insects. Orders
Microcoryphia [or Archeognatha] (bristletails) and Zygentoma [or

Thysanura] (silverfish and firebrats).


Subclass Pterygota. The winged insects.
Infraclass Paleoptera. "Ancient-winged" insects. Wings are so hinged to the thorax that they cannot be placed (folded) along the abdomen. Orders
Ephemeropta (mayflies) and Odonata (dragon- and damselflies).

Infraclass Neoptera. "New-winged" insects. Wings so hinged to thorax that they can be folded back along the abdomen at rest.

Division Exopterygota. Neoptera with incomplete metamorphosis -- the Hemimetabola. Includes orthopteroid and hemipteroid insects.

Division Endopterygota. Neoptera with complete metamorphosis -- the Holometabola, possessing an intermediate pupal stage between larval and adult stages. Includes Neuroptera, Megaloptera, Mecoptera, Diptera,Trichoptera, Lepidoptera, Coleoptera and Hymenoptera, among others.


Arthropod phylogeny: Surprising as it may seem, authorities have not reached a consensus regarding the relationships among the major arthropod lineages. Molecular phylogenetics has breathed new life into this area of research, although some of the new findings have been no less palatable (for example, Ballard et. al.'s (1992) findings suggested the Onychophora are actually modified arthropods). Manton published arguments for a polyphyletic Arthropoda (e.g. Tiegs and Manton 1953)--a position that has been rejected by the majority. There has been and continues to be a great deal of research attempting to determine the relationships among arthropod lineages.

One of the more recent shifts has dealt with a three taxon problem consisting of the Hexapods, Crustacea and the Myriapods. Traditionally the Myriapods were thought to be the Hexapods closest relatives, however, now there is strong evidence suggesting the Crustacea are the stock from which the Hexapoda arose (i.e. Hexapods and Crustacea form a monophyletic group with the Myriapods external). The characteristics uniting Myriapoda with Hexapoda might have evolved independently as adaptations to terrestrialism (Telford & Thomas 1995, Popadic et. al. 1996). Note that the above classification reflects the traditional ordering.


The above figure gives you an idea of the age and relationship of the Hexapod lineages. The Arthropods first appear in the fossil record during the Cambrian, although there are some fossils of possible arthropods from the late Vendian (PreCambrian).



Literature Cited


Ballard, J. W. O., G. J. Olsen, D. P. Faith, W. A. Odgers, D. M. Rowell, P. W. Atkinson. 1992. Evidence from 12S Ribosomal RNA sequences that Onychophorans are modified Arthropods. Science 258:1345-1348.

Popadic, A., D. Rusch, M. Peterson, B. T. Rogers, T. Kaufman. 1996. Origin of the arthropod mandible. Nature, 380:395.

Telford, M. J., and R. H. Thomas. 1995. Demise of the Atelocerata? Nature 376:123-124.

Tiegs, O. W. and S. M. Manton. 1958. The evolution of the Arthropoda. Biological Reviews 33:255-337.