North American camel spiders (Arachnida, Solifugae, Eremobatidae): Systematic revision and biogeography of an understudied taxon


Project Introduction

Significance of camel spiders


Members of the arachnid order Solifugae, known by such common names as camel spiders, wind scorpions, and sun spiders, are a poorly studied group of mostly nocturnal, cursorial, hunting arachnids (see photo in Fig. 1). These animals are notorious for their powerful chelicerae, voracious appetites, and tremendous speeds, which they utilize to run down their arthropod prey. The oldest known solifuge, Protosolpuga carbonaria Petrunkevitch, is from the Upper Carboniferous (Pennsylvanian)1. Solifugae comprise the sixth most diverse order of arachnids with 12 families, 141 genera, and over 1,100 described species. Although solifuges are broadly distributed throughout Africa, Eurasia, and the Americas in a variety of habitats, they are dominant predators in arid ecosystems and serve as important prey for many other desert taxa 2, 3, 4, 5.


Figure 1. Majority rule (50%) consensus tree of Eremobatidae. Nodes with strong support in Bayesian (PP > 0.95) and Maximum Likelihood (BS > 0.70) are represented by black circles; nodes supported by only one analysis by white circles.


Taxonomic status of the solifuge family Eremobatidae


The first extensive phylogenetic analysis of any family within the order Solifugae was published by our team in 2015 using a multi-locus molecular approach6 (Fig. 1). This work provided an initial phylogenetic framework for the family Eremobatidae, which is among the largest of the 12 families of Solifugae, comprising 188 described species currently placed in two subfamilies, eight genera and 18 species groups (Table 1). The family is primarily distributed throughout xeric regions of western North America (including northern Mexico), particularly in desert ecosystems. Although Eremobatidae and the subfamily Eremobatinae are monophyletic, the subfamily Therobatinae and its three genera (Chanbria, Eremochelis and Hemerotrecha) were demonstrated to be poly- or paraphyletic suggesting that the current morphology-based classification is in disarray and in dire need of taxonomic revision (the different genera are color-coded in Fig. 1). Cushing & Brookhart (2016)7 began the revisionary process with a taxonomic analysis of the Eremobates scaber group. They used a morphological approach to describe nine new species in the group, eight of which appear to be endemic to the Chihuahuan desert of northern Mexico. The E. scaber group is well defined by morphological synapomorphies7. The same authors are currently recently revised the genus Eremocosta in an article in Zootaxa41. Although Cushing and Brookhart have made progress revising the taxonomy of the Eremobatidae using the molecular phylogeny as a guide, they began with relatively small and well supported clades; larger groups, such as the genera Hemerotrecha and Eremochelis and large species groups such as the Eremobates palpisetulosus group, are still in urgent need of revision. Taxonomic revision of the larger paraphyletic and polyphyletic groups will require extensive field work and innovative molecular and analytical approaches.


Subfamily Eremobatinae Kraepelin 1901
  Genus Eremobates Banks 1900
    angustus species group (3 species)
    aztecus species group (1 species)
    lapazi species group (1 species)
    **pallipes species group (17 species)
    **palpisetulosus species group (>40 species)
    **scaber species group (20 species)
    vallis species group (1 species)
  Genus Eremocosta Roewer 1934 (9 species)
  Genus Eremorhax Roewer 1934 (10 species)
  Genus Eremothera Muma 1951 (2 species)
  Genus Horribates Muma 1962 (3 species)
Subfamily Therobatinae Muma 1951
  Genus Chanbria Muma 1951 (4 species, several undescribed)
  Genus Eremochelis Roewer 1934
    andreasana species group (2 species)
    bilobatus species group (16 species)
    branchi species group (14 species)
    imperialis species group (5 species)
    striodorsalis species group (1 species)
  Genus Hemerotrecha Banks 1903
    *banksi species group (9 species)
    branchi species group (9 species)
    denticulata species group (6 species)
    serrata species group (1 species)
    simplex species group (7 species)
    texana species group (1 species)
Table 1. Taxonomy of Solifugae family Eremobatidae (after 6, 8, 9).  *Monophyletic taxa; ** Well-supported (but paraphyletic) clades or    geographically cohesive clades6.


Taxonomic impediments


Despite the high species diversity of solifuges and their importance in desert ecosystems, fewer than five researchers worldwide are presently focused on solifuge biology. The most active lab is Cushing’s where she collaborates most closely with solifuge expert Jack Brookhart, who has been publishing on this group since the 1960s. Brookhart is long retired and there is a desperate need to train more specialists in this diverse, interesting, and important group. One of the goals of this grant will be to train new generations of scientists in the taxonomy and biology of camel spiders. We plan to include in this training high school students (2-4 each year of the grant), undergraduates (4-8 each year of the grant), graduate students (one MS and one PhD), and one postdoctoral scholar. The high school students will be recruited through the DMNS Teen Science Scholars (TSS) program ( jobs-and-internships/internships/teen-science-scholars/) – a program that targets students from groups underrepresented in STEM fields, particularly women, African Americans, Hispanics, and Native Americans. The undergraduates will be trained in Cushing’s lab as REU students (one per year) and in Graham’s lab at Eastern Connecticut State University. Graham will also mentor the postdoc and will serve on the graduate committees and Cushing will be the primary advisor for the MS and PhD students. All students (except the high school students) will receive training in both PI’s labs. Brookhart, a former high school teacher, has extensive experience mentoring teens and will help Cushing provide meaningful research experiences to the TSS students. Savary has extensive knowledge of this order of arachnids and will participate in taxonomic revisions.


Using camel spiders to understand the evolution of desert biotas


Many desert adapted arthropods, including camel spiders, are endemic to arid habitats10, 11, 12, 13. Compared to widespread species, endemics are frequently at risk of extirpation from habitat alterations because of their limited distributions, lower population numbers, specific habitat requirements, and often limited dispersal capabilities14, 15, 16. This risk is particularly prevalent in some deserts where endemics are threatened by habitat destruction, agricultural development, urbanization, reduction of groundwater reserves, and other anthropogenic activities17, 18, 19, 20. In addition, desert endemics that have limited dispersal capabilities, such as solifuges, are often unable to respond to changing environmental conditions fast enough to keep up with the frightening pace of ongoing and future climate change21.


A recent time-calibrated phylogeny dated the most recent common ancestor of one family of camel spiders (Eremobatidae) to between the late Eocene to early Miocene, with a mean estimate in the late Oligocene (32.2 Ma)6. Given these results, the diversification of Eremobatidae appears to be associated with the complex geologic and climatic histories of western North America and resultant changes in aridity that gave rise to the region’s modern deserts. More recent diversification in Eremobatidae seems be tied to changes in climate during the Pleistocene. Therefore, camel spiders are an ideal, but underexplored, system to study the evolution of North American desert biotas. One of the goals of this project will be to assess the extent to which Neogene tectonics and Pleistocene climates influenced camel spiders by conducting a thorough biogeographic appraisal of the Eremobates palpisetulosus group (including Eremorhax and two Eremochelis spp. that clearly belong to this group). Results from our study can be directly compared and contrasted with ongoing initiatives focused on understanding how desert organisms respond to climate change22, 23, 24, 25.