Pseudoscorpions

Above: a 3D computer model of a Pseudoscorpion, dorsal view. The body is divided into two parts: the incompletely segmented (i.e. with segments fused) prosoma (cephalothorax) is the frontal section, followed by the segmented opisthosoma (abdomen). The prosoma bear 0, 1 or 2 pairs of eyes (2 pairs in this model). Cave dwelling (troglomorphic) forms, for example, often have no eyes. Pseudoscorpions are no larger than 8 mm in length and most are only 2 to 3 mm long. There are more than 3000 described species of pseudoscorpion, with more still being discovered on a regular basis, and they clearly play an important role in the ecosystem as predators of tiny invertebrates.

Pseudoscorpions are common, found beneath the bark of trees, in leaf litter, soil, beneath stones, in moss, among seaweeds and drift in the intertidal zone of the shore, in caves and in homes but their small size means they are often overlooked. Additionally, if leaf litter is disturbed they will initially freeze and only begin to move when other litter dwellers have fled and so observing them requires patience. Thus, although common they are rarely seen by most. Once seen, however, they are easily remembered due to their striking form! The pseudoscorpions of the British Isles were the first to be quite thoroughly characterized (although this fauna is likely to still be changing as climate changes) and this fauna is probably still the most completely characterized.

Pseudoscorpions are chelicerates, meaning they have a set of modified appendages or jaws, also called chelicerae, consisting of a larger immovable finger (or hand) and a smaller movable finger articulating primarily in the horizontal plane. In most forms the chelicerae are very small, but in types like Chthonius they are large, as in our model. The large pedipalps (modified legs) consist of the following segments from base to tip: coxa, trochanter, femur, patella and the chela which is divided into a fixed and movable finger. The chela may be curved and pincer-like or straighter and more forceps like (as in our model). The palps or pedipalps can be very long in some species and are held outwards and forwards in the horizontal plane when the pseudoscorpion advances, probing the substrate. they are equipped with sensory hairs (trichobothria) which can detect touch, vibrations and sounds. The larger immovable finger (or hand) of the chela is dorsal, the movable finger ventral such that the chela pincer operates primarily in the vertical plane.

The jaws or chelicerae are equipped, in many species, with a silk gland or spinneret (galea), essentially a modified venom gland. This consists of one or more tiny openings which, for example in Chthonius tuberculatus is borne on the end of a small protuberance near the tip of the movable finger, but may be on one or both fingers of the jaws. The chelicerae are equipped with teeth and hairs or setae. Some of these setae form the serrula (of which there are often two on each jaw) which is a comblike or membranous structure used for grooming. The fixed finger (also called the fixed ramus) bears the serrula interior, the free finger (free ramus) the serrula exterior. These take the form of setae, plates, teeth or laminar papillae. Each chelicera may also bear a rallum (sometimes called a flagellum though this term seems misleading) - a tight grouping of setae on the medial side which may form a cheliceral blade. (The term 'blade' is apt when rows of setae partially fuse to give a saw-blade like structure). Other isolated setae are present in a more-or-less precise arrangement: the arrangement of setae and trichobothria on pseudoscorpions is of taxonomic value, being largely determined according to species.

The pedipalps also have small teeth (palpal teeth) along their biting edges resembling saw blades and a sharp tip (venedens) which bears the opening of the venom gland. The coxae of the pedipalps are often described as possessing manducatory processes that are assumed to play a role in feeding.

Above: psuedoscorpion computer model in ventral view (made in Pov-Ray)

The 4 pairs of walking legs each consist of the following segments from base to tip: coxa, trochanter, basifemur, telofemur, tibia and one or two tarsal segments (alternatively given as: coxa, trochanter, femur, tibia, metatarsus, one or tarsal segments). In particular, the femur may subdivide into two segments on the two hindmost pairs of legs. The terminal tarsus ends in two tarsal claws, for gripping a rough surface, and a central pad or arolium for gripping smoother surfaces. (Note: when taxonomic descriptions refer to the first coxae, it is often unclear whether this is referring to the coxae of the pedipalps or the first pair of walking legs).

The abdomen consists of 11 or 12 segments. The last segment is reduced and consists of the retractable anal cone which bears the anus opening at its tip. The sclerite and tergite of segment 11 may join to form a sclerotized ring into which the anal cone retracts. Each of the abdominal segments consists of a series of exoskeletal plates: the dorsal tergite, ventral sternite and the flexible pleural membranes (along the flanks) and the flexible intersegmental membranes.

The tergite and sternite are particularly well sclerotized, meaning they are biochemically hardened and darkened to give them more mechanical rigidity for the operation of muscles and better protection from desiccation. the degree of sclerotization depends on species, for example troglomorphic forms have much paler exoskeletons, whilst other forms may be yellow-orange, brown or black. The legs, pedipalps and jaws are also heavily sclerotized and the fused dorsal plate of the prosoma, and hence darker in color.

Also visible in the ventral view are the following anatomical features:

1. Coxal spines (groups of setae) are visible on the coxae of some of the walking legs: typically either the first pair or the second and third pairs, depending on species. It is not clear what their function is, but they could be proprioceptive. They occur on the first two pairs of walking-leg coxae in Chthonius cephalotes and Chthonius hungaricus, for example.
2. Tracheal openings - generally 2 pairs at the posterior sides of the sternites on the third and fourth abdominal segments (those on S4 can be seen above, the other pair being obscured by the legs). These openings take in air which enters the respiratory tracheal system.
3. Genital structures - the genital aperture is bordered anteriorly by the genital operculum (sternite 2) and posteriorly by the post-genital operculum (sternite 3) which are heavily sclerotized (especially in the male). Setae on the opercula guard the genital opening.
4. An intercoxal tubercle is often present as a small protuberance between the coxae of the 3rd and 4th pairs of walking legs (as seen on our model) and this may bear setae.

Above: the mouthparts of a pseudoscorpion (based on: Dunlop 2000).

Nutrition

The mouthparts need to be considered in more detail. The mouth (opening) is situated to the back of the preoral cavity. This cavity is formed by i) extensions of the pedipalp coxae (the ganthocoxae) an anterior process of which (the endite) forms the floor of the preoral cavity; ii) extensions of the ganthocoxal endites form the lateral lips (divided into inferior laminae and superior laminae); iii) a frontal process called the epistomo-labral plate (EPL, or intermaxilliary jugum) forms the roof or upper lip. (The main mouthpart features have been incorporated into our model).

The mouth opens into the muscular pharynx which joins to a long esophagus (oesophagus) that leads to the midgut. The midgut fills most of the abdomen and its cells store food reserves in the form of glycogen and lipids (enough to last several months without food). The midgut opens into the short hindgut that opens to the outside via the anus on the end of the anal cone. The midgut gives off pouches called digestive caecae.

Feeding

1. Pseudoscorpions feed on small arthropods such as mites and collembolans (springtails).
2. The pedipalps usually have a poison gland in one or both fingers of the chela. These are used to paralyze or kill prey.
3. The chelicerae break the exoskeleton of the prey and digestive fluids (from the midgut) are injected; the preoral cavity acts like a syringe as digestive fluids are injected into the prey.
4. Hairs (setae) in the prebuccal chamber strain out solids whilst the muscular pharynx sucks in fluids.
5. The comblike serrulae on the chelicerae fingers clean the mouthparts.

As a variant on this theme, in Novobisium and Chthonius, the well-developed chelicerae masticate food into a pulp to which midgut digestive fluids are added and then the resultant fluid sucked up.

Respiration

The two pairs of spiracles open into the branched tubes of a tracheal system that delivers air to the respiring cells.

Excretion

Granules (of guanine?) formed in the midgut caecae are eliminated through the anus. guanine is a nitrogen-rich compound and in this way nitrogenous waste is removed. One pair of coxal glands also open via nephridiopores between the coxae of the third and fourth pair of legs.

Circulation

The circulatory system is open with blood (haemolymph) circulated within the haemocoel cavity with the help of the heart or short dorsal vessel located in abdominal segments 1 to 4, above the midgut. The heart takes in haemolymph through 1, 2 or 4 pairs of ostia (up to one in each abdominal segment) and pumps it from one end of the haemocoel to the other. (In arthropods the dorsal vessel generally pumps blood forwards, but can undergo reversals).

Nervous System

the pseudoscorpion nervous system follows the basic arthropod bauplan. The brain is relatively large and consists of the protocerebrum and tritocerebrum, together forming the supra-esophageal ganglion which occupies the dorsal half of the cephalothorax. This is connected to the sub-esophageal ganglion, beneath the gut, by a circum-esophageal nerve ring.

Sensory setae (hairs) are situated at strategic positions on the exoskeleton. The trichobothria can detect airborne vibrations and are often particularly elongated on the pedipalps and on the rear of the abdomen. Other sensory systems are poorly studied. The simple eyes are said to be sensitive only to changes in light intensity.

Behavior

Pseudoscorpions have quite a large repertoire of hard-wired behavioral patterns. Most pseudoscorpions are solitary, but some are social and hunt prey cooperatively. Their relatively large brains can generate some 95 distinctly different behavioral patterns in one well observed species, up to 16 of which are associated with parental care.

When disturbed, for example if leaf litter is placed on a sheet, pseudoscorpions will exhibit a freeze response (whilst other arthropods run off the sheet) and only begin to move after some time. Whilst in this immotile startled state the pedipalps are pulled back over the carapace. They can also run backwards at considerable speed if prodded from the front.

Most pseudoscorpions appear to be strictly nocturnal, which is another reason why they are so seldom seen.

Phoresy

Phoresy is the association of one animal with another for transportation. Pseudoscorpions may use their pedipalps to hitch a ride on flies and other flying insects, such as moths and beetles and also highly mobile non-fliers such as harvestmen. Phoresy is common in certain pseudoscorpion groups, particularly the families Chernetidae and Cheliferidae.

Parental care

All species carry their eggs in a silk brood sac attached to the female's genital opening. Additionally, the female may construct a silk nest (e.g. beneath tree bark) in which she will hide with her brood sac. The female feeds the embryos (with a nutritive milk secreted by her ovary) and grooms the embryos. She will care for them either until they become adults or until she forces them from the nest (as in Paratemnoides nidificator). In Paratemnoides nidificator if food is scarce then the female will allow herself to be eaten by her offspring (a phenomenon called matriphagy, Tizo-Pedroso & Del-Claro 2005). The nymphs may work together to build molt chambers in which they can pass through the vulnerable stage of molting in increased safety.

Reproductive behavior is quite variable and occurs by one of the following modes, depending on species.

1. In many species there is no contact between the males and females: the males simply deposit stalked spermatophores (sperm packets) on the substrate, waiting for the female to encounter them. A chemoattractant in the spermatophore helps guide the female to them and then takes the sperm up directly in through her genital opening. (A secretion from the female atrium - the chamber inside the female opening - causes the spermatophore to swell and this assists uptake). This approach is quite wasteful as spermatophores may not be found.
2. In others the male encounters the female and then deposits silk signal threads to guide the female to the deposited spermatophore.
3. In more sophisticated forms, the male directs the female to the spermatophore by direct physical contact. The male grasps the female with his pedipalps and positions her over the spermatophore.
4. In some species the male evaginates two tubular organs and attracts the female (are these organs releasing pheromones?). The male then attaches the spermatophore to the substrate and seizes the female by her pedipalp femurs and helps position her over the spermatophore and then pushes her down with his forelegs to assist take-up, for example in Chelifer cancrodies.

Methods 3 and 4 are not without risk: the male must leave quickly once the task is complete or the female may mistake him for prey.

The female builds a nest from plant debris and lines it with silk whilst depositing her eggs in a membranous sac attached to her genital opening.

Reproduction

The sexes are separate. There is a single gonad, a median tube.

Male system:

1. The testis transfers sperm along with nutritive fluid along the vas deferntia ducts to the seminal vesicle where sperm are temporarily stored.
2. A posterior dorsal gland secretes the spermatophore stalk which is attached to the substrate.
3. The ejaculatory canal delivers sperm to the genital atrium where they are encapsulated in secretion from the anterior galnds, to form the spermatophore, and deposited, via the genital opening, on to the spermatophore stalk. The genital atrium is varied and complex since this is where the spermatophore is packaged with the help of various accessory glands, and cuticular thickenings with attached muscles to mold the spermatophore.
4. In the Chthonidae subgroup, lateral glands deposit a pheromone droplet on the spermatophore stalk to attract the female.

Simpler spermatophores may consist simply of a sperm droplet on the spermatophore stalk, but in more sophisticated types the sperm drop is encased in a protective membrane to protect the sperm against desiccation.

Female system:

The female system is anatomically less complex than the male system. The genital opening leads into an atrium or invagination, the median diverticulum, which connects to 2 lateral and a posterior diverticulum. The median diverticulum receives two oviducts via a common opening. median and lateral accessory glands open into median and lateral diverticula, respectively, via cribriform (porous) plates.

Sexual dimorphism

Females are usually larger and have a more pale genital region. there may also be differences in the chelicerae and pedipalps.

Life-cycle

The young undergo their first molt inside the egg and then complete their second molt during hatching and leave the brood sac as third stage instars. Between 2 and 50 young are brooded at the same time.

The hatchlings (protonymphs) molt into deuteronymphs which molt into tritonymphs which molt into adults. (There seems to be some confusion in the literature here: do hatchlings molt 2 or 3 times to become adults?). The nymphs are smaller and paler than the adults and lack genital openings (and also have different arrangements of setae and trichobothria). Molting takes place in a nest of silk.

Pseudoscorpions may overwinter in a silk nest and some species may live up to 5 years (they generally live 2 to 5 years though some temperate species may undergo several generations in a year).

Ecology

Pseudoscorpions are important predators of tiny arthropods, such as  mites and springtails, which are abundant in leaf litter, seaweeds and among organic material washed ashore. Chelifer cancroides (House Scorpion) is a cosmopolitan species found in houses. If you have old books then you may well have booklice feeding on the glue used in the binding, then you may well have pseudoscorpions predating them and keeping their numbers down. They will also feed on carpet beetle larvae and clothes moth larvae.

Some species of pseudoscorpion are adapted to life in caves, for example, species of Pseudotyrannochthonius (see review by Harris & Harvey, 2013). Such forms may lack eyes and appear pale due to lack of sclerotization.

Taxonomy

Pseudoscorpions are an order of arachnids. The Arachnida is a class of arthropods (phylum Arthropoda). The arachnids, spiders and their allies, may have a bad reputation due to the creepy and usually unwanted house spiders, but actually it is a remarkably diverse group containing some fascinating and beautiful animals. (Search internet images for 'spider eyes' to see just how beautiful some spiders can be, let alone the other remarkable groups of arachnids).

The arachnids may be divided into the following orders:

1. Acari - mites and ticks, e.g. Demodex
2. Scorpiones - scorpions
3. Pseudoscorpiones - pseudoscorpions
4. Solifugae - Sun Spiders, Wind Scorpions
5. Opiliones - Harvestmen
6. Palpigradi - Microwhip Scorpions
7. Schizomida - Shorttailed Whipscorpions
8. Uropygi - Whip Scorpions
9. Areneae - Spiders
10. Amblypygi - Whip Spiders
11. Ricinuleie - Tick Spiders

All arachnids belong to the sub-phylum Chelicerata, the chelicerates, on account of their jaws. The chelicerates include the arachnids, the pycnogonids (Sea Spiders) and the Xiphosura (Horseshoe Crabs) - a very curious group of fascinating organisms!

The distribution of setae (cheatae) and trichobothria on pseudoscorpions is important in identifying certain species. This is known as chaetotaxy.

References and Further Reading

Dunlop, J.A. 2000. The epistomo-labral plate and lateral lips in solifuges, pseudoscorpions and mites. Ekologia (Bratislava) 19(S3): 67-78.

Harris, D. and Harvey, M.S. 2013. Review of the cave-dwelling species of Pseudotyrannochthonius Beier (Arachnida: Pseudoscorpiones: Pseudotyrannochthoniidae) from mainland Australia, with description of two troglob ... . Australian J. Entomol. 52: 129-143.

Legg, G. and Farr-Cox, F. Illustrated key to the British False scorpions (Pseudoscorpions). FSC, www.field-studies-council.org

Tizo-Pedroso, E. and Del-Claro, K. 2005. Matriphagy in the neotropical pseudoscorpion Paratemnoides nidificator (Balzan 1888) (Artemidae). J. Arachnology 33: 873-877.