Phrynus whitei

Whip spiders belong to the arachnids (Arachnida) an form an own order which is most closely related to the group comprising the whip scorpions and shorttailed whip scorpions (these two orders are part of the grouping termed Uropygi) * and the spiders (Aranae) (Dunlop, 2010; Shultz, 2007; Weygoldt, 2000).
Even if the english name whip spider may suggest that animals of this this group are spiders it has to be said, that these form the distinct order Amblypygi within the Arachnida that is separated from spiders (Aranae), whip scorpions (Thelyphonida) and short tailed whip scorpions (Schizomida) (Dunlop, 2010; Shultz, 2007; Weygoldt, 2000).
There are different conceptions regarding the relationships between these three taxa. According to one conception the Amblypygi and the Uropygi are sister-taxa, which can be grouped in the phylogenetic entity Pedipalpi, which itself is regarded as the sister-taxon of the Aranae (Giribet et al., 2002; Shear et al., 1987; Shultz, 1999; Shultz, 1990; Shultz, 1989). The most evident arguments for this hypothesis are the presence of spined raptorial pedipalps and elongated antenniform first walking legs (Shear et al., 1987).
Another hypothesis is that of a closest relationship between the Amblypygi and the Aranae, which together form the phylogenetic grouping of the Labellata, which itself is the sister taxon of the Uropygi (Weygoldt & Paulus, 1979; Wheeler & Hayashi, 1998). This hypothesis is mostly based on the presence of a sucking stomach and a narrow waist between pro- an opisthosoma, which both taxa have in common (Weygoldt & Paulus, 1979; Wheeler & Hayashi, 1998). However the grouping of the Amblypygi and Uropygi in the Pedipalpi is favored, because both taxa have a large number of deviated sceletomuscular characters in common (Dunlop, 2010; Shultz, 2007; Shultz, 1999; Shultz, 1990; Shultz, 1989).
The characteristic trait that gave the whip spiders their name is the extremely elongated pair of first walking legs, which in contrast to the situation in spiders cannot be used for locomotion. An elongation of the first walking leg pair also occurred in the Uropygi, but by far not in the same degree as it is the case in the whip spiders. The perpetual motion of the first pair of elongated walking legs may remember of the slash of whips (Weygoldt, 2000).
Also characteristic for whip spiders is the presence of raptorial spined pedipalps, that can attain a considerable length in many species; raptorial spine pedipalps can also be found in the Uropygi.
One of the most prominent traits that distinguish the whip spiders from the Uropygi is that the latter possess a “tail“ that consists of an extremely elongated multisegmented telson, whereas the whip spiders do not (Weygoldt, 2000).
In whip spider's patella is reduced in a considerable amount and adopts the function of a breaking point for autotomy (act by wich an animal severs an own appendage) in case a leg should be incarcerated or grabbed by a predator (Weygoldt, 2000).
Whip spiders neither possess spinnerets nor poison glands (Weygoldt, 2000).
The Amblypygi are the only arachnids where females and males continue moulting after reaching sexual maturaty and keep increasing size by this process (Weygoldt, 2000).
The body of whip spiders is flattened dorso-ventrally, due to which they fit in cracks and scailing off bark during the day leaving this hideouts by night (Weygoldt, 2000).
The order of the Amblypygi comprises a relatively low amount of species (Harvey, 2003; Weygoldt, 2000). Whip spiders are mainly distributed in tropical and subtropical areas and only a few species occur in temperate zones. In Europe whip spiders are solely represented by the species Charinus ionnaticus that occurs on the greek islands Kos and Rhodos (Harvey, 2003; Weygoldt, 2000).

* The term Uropygi s. l. (=Camarostomata) in this case refers to the phylogenetic grouping of the two orders Thelyphonida (whip scorpions) and Schizomida (shorttailed whip scorpions). Given that for a long time it was a matter of debate if the whip scorpions and shorttailed whip scorpions are suborders of the order Uropygi (Rowland & Cooke, 1973) and that the term Uropygi is and was used for designating a grouping that comprises the whip scorpions and shorttailed whip scorpions (Dunlop, 2010; Schultz, 2007), or for a grouping that only comprises the whip scorpions and excludes the shorttailed whip scorpions (Harvey, 2003), misunderstandings are easily provoked. In recent publications (Dunlop, 2010; Schultz, 2007) the term Uropygi was used to designate the phylogenetic entity of a group comprising the two orders of the whip scorpions and the shorttailed whip scorpions and this designation will be used here. In these publications the order of the whip scorpions is designated as Thelyphonida which will also be adopted here. To avoid misunderstandings one can use Petrunkevitch's (1949) term Camarostomata for the phylogenetic grouping of the Thelyphonida (whip scorpions) and Schizomida (shorttailed whip scorpions) that admittedly is only rarely used.

References

Dunlop, J. A. (2010). Geological history and phylogeny of Chelicerata. Arthropod structure & development , 39, 124-142.

Giribet, G., Edgecombe, G. D., Wheeler, W. C., & Babbit, C. (2002).Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data. Cladistics, 18, 5-70.

Harvey, M. S. (2003) Catalogue of the Smaller Arachnid Orders of the World. CSRIO Publishing, Collingwood.

Rowland, J. M. & Cooke J. A. L. (1973). Systematics of the Arachnid Order Uropygida (=Thelyphonida). Journal of Arachnology, 1, 55-71.

Petrunkevitch, A. (1949). A study of Palaeozoic Arachnida. Transactions of the Conneticut Academy of Arts and Sciences, 37, 69-315.

Shear, W. A., Selden, P. A., Rolfe, W. D. I., Bonamo, P. M., & Grierson, J. D. (1987). New terrestrial arachnids from the Devonian of Gilboa, New York (Arachnida, Tricgonotarbida). American Museum Novitates, 2901, 1-74.

Shultz, J. W. (1989). Morphology of locomotor appendages in Arachnida: evolutionary trends and phylogenetic implications. Zoological Journal of the Linnean Society, 97, 1-56.

Shultz, J. W. (1990). Evolutionary morphology and phylogeny of Arachnida. Cladistics, 6, 1-31.

Shultz, J. W. (1999). Muscular anatomy of a whipspider, Phrynus longipes (Amblypygi), and its evolutionary significance. Zoological Journal of the Linnean Society, 126, 81-116.

Shultz, J. W. (2007). A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society, 150, 221-265.

Weygoldt, P. (2000). Whips Spiders (Chelicerata: Amblypygi). Their Biology, Morphology and Systematics. Apollo Books, Stenstrup, DK.

Weygoldt, P., & Paulus, H. F., 1979. Untersuchungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata. Zeitschrift für zoologische Systematik und Evolutionsforschung, 17, (85-116), 177-200.

Wheeler, W. C., & Hayashi, C. Y. 1998. The phylogeny of the extant chelicerate orders. Cladistics, 14, 173-192.

F. Schramm, authored 2011