Photographic key to the Pseudoscorpions of Canada and the adjacent USA

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.

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. 

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.