The order Pseudoscorpiones belongs to the class Arachnida. It is a fairly large order with about 3064 described species from 434 genera and 24 families, throughout the world. Pseudoscorpions constitute an abundant and diverse group of arachnids. As predators they are of importance in regulating the number of potentially harmful insects and mites.
COMMON NAMES .
Pseudoscorpions are also known as book scorpions or false scorpions.
Like other arachnids, pseudoscorpions have 8 legs. In addition, they have enormous pedipalps (pincers) on the front of the body that gives them a strong resemblance to the true scorpions. However, unlike scorpions, the hind part of the abdomen is not narrow and they have no caudal sting or telson.
These animals are common but not easily seen because of their small size - usually less than 5 mm long, although the giants such as Garypus titanius Beier can attain a body length of 12 mm. The bodies are flattened and oval shaped.
The cephalothorax is often smooth but may be crossed by one or two transverse furrows. The portion of the carapace lying in front of the eyes is referred to as the "cucullus". On the ventral side of the cephalothorax the coxae of the appendages ordinarily meet on the middle line so that there are almost no traces of a true sternum.
Most pseudoscorpions possess 2 or 4 eyes, although some species are blind and lack eyes altogether. When eyes are present, they are situated near the front of the carapace on each lateral margin (anteriorly on the prosoma).
The chelicerae consist of two segments and are pincer-like (chelate) in shape with the distal segment bearing a spinneret that produces silk. They are essentially horizontal in position, the moveable finger forming the lateral member of the pincers and are provided with a more or less specialized armature of teeth.
The chelicerae are complexly developed structures, which serve as
- grasping organs (chelicerae are employed in holding the prey while transporting or feeding upon it. It is also utilized in transporting the grains of sand or bits of material used in nest building.)
- spinning organs (chelicerae have a more or less complex spinneret (galea) developed on the tip of the moveable finger.),
- cleansing organs (As cleansing organs chelicerae are continually employed in life in grooming the chelae of the pedipalps, which are the seat of the most important sensory structures possessed by these animals)
- sensory organs. (chelicerae are the seat of certain important sensory functions, as witnessed by the development of the peculiar flagellum and the well-developed and ordinally constant lyrifissures).
The spinneret is situated on the outer side of the movable finger near the tip. Two types are present, either simply a slight eminence or tubercle or it is prolonged into a slender more or less branched appendage (galea).
The pedipalpi are large and consist of six segments, namely the coxa, trochanter, femur, platella, chela (modified tibia) and moveable finger (modified tarsus). The last two segments form a chelate appendage, the chela (resembling those of scorpions). Members of most families possess venom glands in one or both chelal fingers. A venom apparatus occurring in both chelal fingers would appear to be the primitive state. The pedipalpi serve as prehensile organs to capture and kill their prey and are at the same time the seat of important sensory organs.
All of the four pairs of legs are fitted for walking. The legs of a pseudoscorpion consist of: coxa, trochanter, femur, platella, tibia, metatarsus (or bastitarsus) and tarsus (or telotarsus). The number of tarsal joints is of great systematic importance in the order.
The respiratory organs are tubular tracheae, which open by two pairs of long slit-like spiracles on the second and third abdominal segments, on the ventral side.
The opening of the reproductive organs is on the middle line between the second and third abdominal sterna. The female carries the eggs about, attached to the abdomen; they are on the outer surface of a sac, which is attached to the wall of the opening of the reproductive organs. The sac is filled with food material and the young when hatched are furnished with a long sucking beak by means of which they obtain the food. In the males lateral genital sacs are present under the genital operculum, close to the genital aperture. In some species they are capable of being extruded.
A generalized life-cycle is as follows: the protonymph hatches from the brood-sac, becomes free-living for usually one year, constructs a silken chamber and moults into a deutonymph. The silk is spun into an igloo or disc shaped chamber that is often embedded with particles of wood, sand or discarded insect remains. The deutonymph then abandons the chamber and becomes free-living until it constructs a silken chamber and moults into a tritonymph. This process is repeated once again and it becomes an adult. Adults do not moult. Adults and nymphs spin small, disk shaped whitish cocoon like structures (nests) in which they lay eggs, overwinter, or shed their skin (moulting).
- Pseudoscorpions usually go unnoticed because of their small size. They are only noticed when they accidentally invade homes and wander into sight.
- Pseudoscorpions are predaceous and therefore beneficial. They feed on other arthropods, particularly insects and arachnids such as springtails, thrips, mites, beetle larvae, psocids, flies and other minute insects. They use the pedipalpi and chelicerae to subdue their prey; some species also use venom secreted by the chelae.
- Natural habitats for pseudoscorpions are under leaf litter and mulch, in moss, under stones and beneath tree bark or other debris on the soil surface. The flattened bodies enable these creatures to live in narrow spaces.
- A few species are arboreal and commonly found on trees in South Africa, e.g. Lophochernes mucronatus (Tullgren, 1907).
- Several species have a phagophilous relationship with other animals. They are frequently collected from guano or mammals and bird retreats where they possibly feed on organisms like mites associated with the animals. In South Africa, species of Nudochernes and Plesiochernes are associated with small mammals, while Beierius walliskewi (Ellingsen, 1912) are found in caves in the guano.
- A few species are synanthropic and found in houses, living in boards of buildings, in books, crevices in furniture and in chicken coops.
- Some pseudoscorpion species have long been known to be phoretic, "a nonparasitic association of one kind of animal, with another that results in transportation of the smaller by the larger". Phoresy appears to have the effect of dispersing pseudoscorpions to preferred habitats and they can be carried around by insects such as bees, flies and beetles.
Pseudoscorpions are predaceous and therefore beneficial. They are not household pests because they cannot sting or bite and they do not attack the house structure, furniture or occupants. They may occasionally be a nuisance, usually during the spring and summer as they become occasional "accidental invaders".