Taxonomy and Phylogeny
[By Paula E. Cushing, Prashant Sharma, Siddharth Kulkarni, Goran Shikak and Warren Savary]
Spanning over a century, the works of solifuge researchers in taxonomy and systematics have described 15 families in this order of arachnids along with various fossilized extinct lineages. Today, this order comprises 15 families, over 140 genera, and over 1200 described species. Five monotypic fossil genera have been described (Petrunkevitch 1913; Poinar & Santiago-Blay 1989; Lebrun 1996; Mursch & Steffan 1996; Selden & Shear 1996; Selden & Dunlop 1998; Dunlop & Rössler 2003; Dunlop et al. 2004). Solifugae are classified into two suborders – Boreosolifugae and Australosolifugae – with a largely Laurasian and Gondwanan distribution respectively (Kulkarni et al. 2023, 2024). Early attempts to produce a suprageneric classification were presented in the mid-late 1800s (Koch 1842; Simon 1879; Kraepelin 1899), but were based upon what are generally considered unreliable morphological characters that show great variability within genera and even, often, within species.
The order Solifugae is in urgent need of a higher-level phylogenetic analysis and monographic, family-scale revision to begin to sort out the confusion. Recent studies using molecular data have robustly resolved the higher-level phylogenetic relationships of Solifugae lineages (Kulkarni et al. 2023, 2024); however, among the families, only the North American Eremobatidae has received significant phylogenetic attention (Cushing et al. 2015). Kulkarni et al. (2023) suggested that all families except Daesiidae and Ammotrechidae are monophyletic and proposed two new families. However, there is a dire need for a comprehensive morphological assessment to establish family and generic diagnoses and putative synapomorphies in conjunction with denser sampling to test the cohesion of these groups.
The described species diversity of Solifugae, just slightly lower than that of another arachnid group – the order Scorpiones – is almost certainly an underestimate of the world fauna. The regions where solifuges are most diverse (e.g., southern Africa, southwestern U.S.A.) have been best documented by past solifuge specialists (e.g., R.F. Lawrence, M.H. Muma, J.O. Brookhart). However, these regions have not by any means been thoroughly surveyed: new species and distribution records continue to be discovered, e.g., in the southwestern U.S.A. (Brookhart & Cushing, 2002, 2004, 2005). And very few records exist in some parts of the world, such as the Asian tropics, because there are no scientists focused on this group of arachnids in that region. Many solifuge species are cryptic, seasonal, habitat-specific, and difficult to collect without appropriate methods (Cushing & González Santillan 2018; Graham et al. 2018) and most regions where solifuges occur have not been surveyed in appropriate seasons or using appropriate methods. Many species are known from one or a few specimens (often only the types), usually collected serendipitously at a single locality; hence little is known about their distributions. The world’s solifuge fauna will likely increase when all appropriate habitats are thoroughly surveyed.
Species delimitation in many families is largely based on secondary sexual characters of adult males (flagellum, cheliceral dentition) (e.g., Kraepelin 1899; Roewer 1932, 1933, 1934, 1941; Muma 1951; Lawrence 1955; Wharton 1981), rendering females and immatures difficult or impossible to identify below genus. Attempts at using sexual characters of adult females have shown limited success in species delimitation, at least within the family Eremobatidae (Garcia et al. In press). Extensive field collecting, new morphological character systems, and molecular approaches for associating different sexes and life stages are needed to advance solifuge systematics.
The current taxonomy of the order Solifugae can be accessed via the World Solifugae Catalog at https://wac.nmbe.ch/order/solifugae/6.
An illustrated field guide to the Camel Spiders of North America can be found at https://ecdysis.org/checklists/checklist.php?clid=12176&pid=0. This online resource is being continually updated and a key to the Solifugae of North America will be added to this site. We suggest that this online resource with illustrated diagnostic characters might be used as a model for guides to Solifugae families elsewhere in the world.