Photographic key to the Pseudoscorpions of Canada and the adjacent USA
CJAI 10 February 3, 2010
doi:10.3752/cjai.2010.10

Christopher Buddle

 

McGill University, Department of Natural Resource Sciences, Macdonald Campus, 21,111 Lakeshore Road, Ste-Anne-de-Bellevue, Quebec, H9X 3V9, Canada. 514-398-8026. chris.buddle@mcgill.ca

 

| Abstract | Introduction | Methods | How to use this key | Key | References | PDF 62.0 MB | Cite this Article |

 

Introduction
The order Pseudoscorpiones is a small and curious group of Arachnids.  They are dorsally-ventrally flattened animals, typically less than 5 mm in length and generally light tan to reddish brown to black in colour.  Although the order is relatively species-poor (i.e., 3,385 known species, globally (Harvey 2009)), pseudoscorpions are fascinating for their natural history and behaviour (e.g., Weygoldt 1969, Zeh and Zeh 1994, Tizo-Pedroso and Del-Claro  2005), and they exhibit wonderful variability in size and form (Figure 1). Typical habitats for pseudoscorpions include leaf-litter, under rocks, within compost piles, under bark and within decaying wood, in caves, and in bird nests.  Many species are phoretic on insects or birds (e.g., Haack and Wilkinson 1987), and species such as Chelifer cancroides (L.) are cosmopolitan, typically found in buildings, barns or other human-made structures.  Key references for the pseudoscorpions in North America include Muchmore (1990), Hoff (1949), Nelson (1975), and Chamberlin (1931).  These publications, especially Hoff (1949) provide detailed information about collecting specimens and preparing them for taxonomic study.

The most recent phylogenetic treatment of the order was proposed by Harvey (1992), and there has been recent activity on the higher-level taxonomy for the group (e.g., Murienne et al. 2008).  Harvey’s (2009) on-line catalogue is a key resource for nomenclature and classification, and for its extensive bibliography. However, resources for identification of pseudoscorpions in northern North America remain scarce.  Buddle (2005) benchmarked the taxonomic status of pseudoscorpions in Canada, and suggests there are 23 valid species known from Canada, with another 20-30 species likely to occur in the country.

Taxonomic keys for pseudoscorpions in Canada are not available; the only key that is workable for North American specimens is that written by Muchmore (1990). This key is an excellent resource, but it is difficult to navigate and the drawings are difficult for those not trained or experienced in general pseudoscoropion taxonomy. Seeing the characters on actual specimens can help make the taxonomy more accessible; with this in mind, the purpose of this paper is to provide a taxonomic key to genera and some species of pseudoscorpions found in Canada and the adjacent United States of America. This key is not meant to replace other resources, but instead the goal is to provide an additional resource for workers interested in pseudoscorpions. Some information on the general anatomy of pseudoscorpions is included, with the purpose of supplementing information available elsewhere as line drawings (e.g., Hoff 1949, Muchmore 1990).  I have also included some notes on the ecology, habitat affinities, and distribution for each taxon, and as more information is gathered in the future, I hope these notes can be developed into formal species pages.

 

Methods
For each taxon, specimens were either collected in the field, were sent to me over the last several years, or were borrowed from Museums (primarily the Canadian National Collection (CNC), Ottawa, and the Florida State Collection of Arthropods (FSCA)).  Whenever possible, I used and examined specimens from northern North America for photographs.  William Muchmore also provided many Canadian specimens, and I have relied heavily on many of his slide specimens currently housed at the FSCA.  The bulk of the specimens used were slide-mounted, although some characters were better displayed with alcohol specimens and/or only wet specimens were available for study. 

Slide specimens were photographed using a Leica Infinity1 Camera housed on a Leica DM 2500 compound microscope.  For most specimens, 4-6 photographs at different depths of field were taken and subsequently stacked using Combine Zm software (available at http://hadleyweb.pwp.blueyonder.co.uk/CZM/News.htm). Wet specimens were photographed with either the Leica Infinity 1 Camera, or a Nikon Coolpix 4500 Camera, attached to a Nikon SMZ 1500 dissecting microscope. Images were then edited using Adobe Photoshop Elements (Version 4.0).

Over 150 specimens were examined, including all taxa currently represented in Canada.  About 80 of these were photographed for generalized anatomy and for diagnostic images for the taxonomic key. For some rarely encountered taxa known only from the adjacent USA, I was unable to obtain specimens to photograph and these couplets are not accompanied by images (e.g., Aspurochelifer littlefieldi).   The key was written by relying heavily on the works of Muchmore (1990), Hoff (1949), and the unpublished manuscripts by M.J. Sharkey. Distributions and ecological information is taken from Muchmore (1990), Harvey (1990, 2009), unpublished reports by M.J. Skarkey, and my personal collections and experience. Unfortunately, some taxa remain unresolved and I have taken a conservative approach when necessary.  I have adopted nomenclature proposed by Harvey (2009), unless indicated otherwise, and the key is for a total of 35 taxa (Table 1).

 

General Anatomy and Taxonomic Characters
The pseudoscorpion body is divided into an anterior cephalothorax (or prosoma) (dorsally, the carapace) and a posterior abdomen (or opisthoma), broadly connected to each other (Figure 2).   Males can be differentiated from females, but sexual determination can be difficult. The genital openings occur between the second and third opisthosomal sternites. In general the genital area of females is lightly sclerotized and appears as a bright zone behind the coxae of the legs (Figure 3).  In males, this region is relatively dark and more highly sclerotized (Figure 3).  Males are also typically smaller than females.  Some species also exhibit sexual differentiation in the palps. 

The chelicerae contain important taxonomic characters (Figures 2, 4):  they are two-segmented structures used to grasp and macerate prey items, and include a fixed and movable finger (Figure 4). Each of the chelicera bears a variety of setae, the placement of which are important for taxonomic purposes.  The rallum is a tight grouping on the medial side (Figure 4); the setae of the lateral side are named as follows: exterior seta (es), basal seta (bs), subbasal seta (sbs), interior seta (is), laminal seta (ls), and galeal seta (gs) on the movable finger. A comblike or membraneous serrula occurs along the side of the movable finger, with the spinneret or galea at the end, which carries the openings of the silk glands.  The third pair of appendages is the pair of long palps or pedipalps, with the coxae of these serving as accessory mouthparts (Figure 2). Beyond the coxa, each palp is comprised of the trochanter, femur, patella, and chela, with a fixed and movable finger.  The chela is used for defense and prey capture.  The palpal chela usually has numerous teeth along the inner margins, ending with a sharp tip (venedens), which has an opening for the venom gland.The sensory setae (trichobothria) on the movable finger are named as follows (Figure 5), divided by a lateral or exterior series and a medial or interior series: basal (b), subbasal (sb), subterminal (st) and terminal (t).  The setae for the fixed finger are named as follows: exterior basal (eb), exterior subbasal (esb), exterior subterminal (est), exterior terminal (et), interior basal (ib), interior subbasal (isb), interior subterminal (ist), and interior terminal (it).

The remaining four pairs of appendages are walking legs (Figure 2), consisting of a coxa, trochanter, basifemur, telofemur, tibia, and one or two tarsal segments.  The legs terminate with two claws and an arolium.  The abdomen is divided into 11 or 12 segments, with sclerotized tergites dorsally and sternites ventrally (Figure 2), separated by soft, flexible intersegmental and pleural membranes.  The spiracles are found at the lateral edges of the third and fourth sternites.

 

How to use this key
This dichotomous key is designed for use with both wet specimens and mounted on slides.  For most general collections of pseudoscorpions, specimens are typically preserved in 70-95% ethanol; typically slide mounts are done only for long-term storage of voucher specimens.  When working with wet specimens, it is advised to handle the specimens with small insect pins or fine forceps.  For using the key accurately, legs I and IV should be removed (including trochanter but not the coxa), and one chelicera and one chela are typically removed. These are placed separately from the whole specimens on a flat slide with a cover slip propped up with a small piece of fishing line; some structures may need to be cleared for a period of 1-3 days or more (i.e., if heavily sclerotized) with a clearing agent (e.g., lactic acid) if required.  It is possible to gently rotate the structure under the cover slip by gently moving the cover slip and, provided the fishing line is thin enough, the structure will rotate with ease.  In this manner, the correct orientation is achieved. The whole specimen can be viewed by placing in a depression slide with ethanol.

 

 

Table 1.  Summary of pseudoscorpion taxa that can be determined using the key.  Nomenclature follows Harvey (2009).

 

Family

Genus/Species

Chthoniidae

Chthonius (Chthonius) ischnocheles (Hermann)

 

Chthonius (Ephippiochthonius) tetrachelatus (Preyssler)

 

Apochthonius Chamberlin

 

Mundochthonius Chamberlin

 

Pseudotyrannochthonius Beier

Syarinidae

Syarinus Chamberlin

Neobisiidae

Americocreagris Ćurčić, Globocreagris Ćurčić, Microcreagris Balzan

 

Halobisium occidentale Beier

 

Microbisium brunneum (Hagen)

 

Microbisium parvulum (Banks)

 

Roncus lubricus L. Koch

Larcidae

Larca Chamberlin

Garypinidae

Pseudogarypinus Beier

Pseudogarypidae

Pseudogarypus Ellingsen

Cheiridiidae

Apocheiridium Chamberlin

Chernetidae

Acuminochernes Hoff

 

Americhernes Muchmore

 

Chernes Menge

 

Dendrochernes Beier

 

Dinocheirus Chamberlin

 

Epactiochernes Muchmore

 

Hesperochernes Chamberlin

 

Illinichernes Hoff

 

Mirochernes dentatus (Banks)

 

Parachernes Chamberlin

 

Pselaphochernes Beier

 

Wyochernes Hoff

Cheliferidae

Aspurochelifer littlefieldi Benedict and Malcolm

 

Chelifer cancroides (L.)

 

Dactylochelifer Beier

 

Haplochelifer philipi (Chamberlin)

 

Hysterochelifer Chamberlin

 

Idiochelifer nigripalpus (Ewing)

 

Parachelifer Chamberlin

 

Paisochelifer Hoff

 

 

Acknowledgements
Thanks to William Muchmore, GB Edwards (Florida State Collection of Arthropods), Michael Sharkey, Mark Harvey, Terry Wheeler and the Lyman Entomological Museum (McGill University), and the Arachnological Community of Canada for specimens, enthusiasm and moral support.  Funding was provided in part from a Discovery Grant (National Science and Engineering Research Council of Canada).

 

References
Buddle, C.M.  2005. A primer on pseudoscorpions and taxonomic status in Canada. Newsletter of the Biological Survey of Canada (Terrestrial Arthropods) 24: 12-16.

Chamberlin J.C. 1931. The arachnid order Chelonethida.  Stannford Univ. Publ. Univ. Serv. Biol. Sci. 7: 1-284

Haack, R.A., and R.C. Wilkinson 1987. Phoresy by Dendrochernes Pseudoscorpions on Cerambycidae (Coleoptera) and Aulacidae (Hymenoptera) in Florida.  Am. Midl. Nat. 117: 369-373.

Harvey, M.S. 1990.  Catalogue of the Pseudoscorpionida (Mahnert V, ed). Manchester Univ. Press.

Harvey, M.S. 1992. The phylogeny and classification of the Pseudoscorpionida (Chelicerata: Arachnida).  Inverteb. Taxon. 6: 1373-1435.

Harvey, M.S. (2009). Pseudoscorpions of the World, version 1.2. Western Australian Museum, Perth. http://www.museum.wa.gov.au/arachnids/pseudoscorpions/

Hoff, C.C. (1949) The pseudoscorpions of Illinois.  Bull. Illinois Nat History Survey 24:413-498.

Muchmore, W.B. 1973. Ecology of pseudoscorpions – a review.  Pp 121-127, in    Proceedings of the First Soil Microcommunites Conference (Dindall DL, ed.) Syracuse NY.

Muchmore, W.B. 1990. Pseudoscorpionida.  Pp 503-527, in Soil Biology Guide (Dindal DL, ed). John Wiley & Sons.

Murienne, J., M.S. Harvey, and G. Giribet.  2008. First molecular phylogeny of the major clades of Pseudoscorpiones (Arthropoda: Chelicerata). Molecular phylogenetics and evolution. 49: 170-184.

Nelson S, Jr. (1975) A systematic study of Michigan Pseudoscorpionida (Arachnida). Am. Midl. Nat. 93: 257-301.

Tizo-Pedroso, E., and K. Del-Claro.  2005.  Matriphage in the neotropical pseudoscorpion Paratemnoides nidificator (Balzan 1888) (Atemnidae).  Journal of Arachnology 33: 873-877.

Weygoldt, P. 1969. The biology of Pseudoscorpions.  Harvard Univ. Press, Cambridge.

Zeh, D.W., and J.A. Zeh. 1994.  When morphology misleads – interpopulation uniformity in sexual selection masks genetic-divergence in harlequin beetle-riding pseudoscorpion populations.  Evolution 18: 1168-1182.