Where There's a Whip, There's a Scorpion

As our understanding of the higher relationships between organisms has improved vastly in recent decades, the arachnids have remained an intransigent bunch. Proposed connections between the various historically recognised orders have remained poorly supported and, even now, there are few that do not continue to jump about with gleeful abandon with each successive analysis. One small bastion of reliable support, however, has been been the tropical clade known as the Pedipalpi.

Whip spider Phrynus exsul, copyright Michel Candel.

Members of the Pedipalpi have traditionally been divided between two or three distinct orders: the whip spiders or tailless whip scorpions of the Amblypygi, the whip scorpions of the Uropygi, and the micro-whip scorpions of the Schizomida (alternative classifications have combined the last two in a single order Uropygi or Thelyphonida). All have a broad distribution in tropical and subtropical regions of the world. Representatives of the Pedipalpi are active hunters, united by the possession of large, raptorial pedipalps used in the capture of prey. All three groups also have the first pair of legs modified to become elongate and whip-like (Shultz 2007). These legs are not used in walking but are held forwards to function like antennae. The Uropygi and Schizomida are further united by the possession of a terminal appendage on the body, the 'whip' of a 'whip scorpion'. There is also a general agreement in recent years that the Pedipalpi are in turn the sister lineage to the spiders. Some researchers have argued for a closer relationship of the Amblypygi to the spiders rather than the whip scorpions, reflecting their (among other things) similar habitus, but this remains a minority view.

Syntype (one of the original described specimens) of Paracaron caecus, from Garwood et al. (2017). Scale bar = 5 mm.

Globally, the Pedipalpi are not a hugely diverse lineage, with a bit more than 600 known species overall. About 190 species belong to the Amblypygi, the whip spiders. As noted above, these arachnids are quite spider-like in appearance owing the lack of a terminal flagellum and the presence of a well-defined waist between cephalothorax and abdomen, but they lack the poison fangs and spinnerets of a spider. Most whip spiders have a distinctly flattened habitus, allowing them to enter narrow spaces under bark or between rocks. They also have the most remarkably elongate first legs among the Pedipalpi. Living whip spiders can be divided between two lineages, referred to as the Paleoamblypygi and Euamblypygi (Garwood et al. 2017). The Paleoamblypygi are represented in the modern fauna by only a single known (but little known) species, Paracharon caecus, a blind inhabitant of termite nests in western Africa. Paracharon caecus differs from other living whip spiders in retaining a vertical plane of motion of the pedipalps, like those of whip scorpions. In the Euamblypygi, the orientation of the pedipalps has shifted so they move in a horizontal plane only. In some whip spiders, the pedipalps have become remarkably long, perfect for clasping prey in a fatal hug.

Giant whip scorpion Mastigoproctus giganteus, copyright David Bygott.

The whip scorpions of the Uropygi are the least diverse of the three lineages of Pedipalpi, with about 110 known species. They are large, robust arachnids characterised by their long, filamentous terminal flagellum. Glands at the base of the flagellum produce noxious chemicals used in defense, giving some species the alternative name of 'vinegaroons'. The Schizomida are the most diverse subgroup of the Pedipalpi, including about half the known species. Some species have become widespread as a result of human transportation in association with greenhouses whereas others have even been collected among ice and snow in California (Harvey 2003). Schizomids are smaller and softer-bodied than the Uropygi and the terminal flagellum is shorter (as in Uropygi, the flagellum is flanked by repugnatorial glands). In male schizomids, the flagellum is often distinct in shape from that of the females, becoming bulbous. Schizomids also differ from most other arachnids in the presence of visible dorsal divisions between the segments of the cephalothorax.

Female schizomid Hubbardia briggsi, copyright Marshal Hedin.

In all subgroups of the Pedipalpi, reproduction involves mating displays in which the male deposits a spermatophore on the ground and then guides the female over it (Harvey 2003). The exact manner in which the male guides the female differs between subgroups. In schizomids, the female grasps onto the male's flagellum and he leads her. In Uropygi, the male grasps the female's fore legs with his pedipalps before turning to face the same direction as her with himself in front, and pulls her over the spermatophore. He then turns, embraces her abdomen with his pedipalps, and manually inserts the spermatophore into her genital operculum. Amblypygi have perhaps the most graceful option of the three: the male stands facing the female then gently beckons her forward, allowing her to approach and collect the spermatophore of her own volition.

REFERENCES

Garwood, R. J., J. A. Dunlop, B. J. Knecht & T. A. Hegna. 2017. The phylogeny of fossil whip spiders. BMC Evolutionary Biology 17: 105.

Harvey, M. S. 2003. Catalogue of the Smaller Arachnid Orders of the World: Amblypygi, Uropygi, Schizomida, Palpigradi, Ricinulei and Solifugae. CSIRO Publishing.

Shultz, J. W. 2007. A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150 (2): 221–265.