Chelonethida: an order of Arachnida giving the general impression of tiny scorpions but lacking the long, narrow tail and sting. Most are quite small, ranging from less than 1 mm to "giants" of 7 mm in length.
The body is divided into an anterior prosoma or cephalothorax, and a posterior opisthosoma or abdomen, broadly connected to one another (check out these unusual family Feaelloidea and the genus Menthidae). The cephalothorax is covered dorsally by a single carapace, whereas ventrally it shows distinct segmentation. The abdomen is clearly divided both dorsally and ventrally into 11 or 12 segments. The carapace may bear two pairs of eyes laterally near the front, or it may have only one pair or none. The cephalothorax bears six pairs of appendages. Most anterior, and extending straight forward, are the short chelicerae, each of which is two segmented and chelate; the chelicerae are generally used for grasping and macerating food. The chelicera bears several setae some of which are grouped together into a flagellum; two serrulae along the inner margin of the fingers; and usually a galea, or spinneret, on the movable finger. The galea is used to spin silken cocoons for protection during molting, hibernation or the brooding of the eggs. The second appendages are the long, chelate palpi, or pedipalpi, which are used for food gathering and defense. Usually, the palpal chela contains venom glands which open at the sharp tip of one or both fingers, the venom being used to subdue prey. The remaining four pairs of appendages are walking legs; usually the anterior two pairs are distinctly different from the posterior two pairs in details of structure and movement. The coxae of the palpi and legs form the ventral sclerites of the cephalothorax, there being no true sternites present. The second and third abdominal sternites are more or less modified as genital opercula and are sexually dimorphic. Tracheae open at the lateral edges of the third and fourth abdominal sternites. The twelfth tergite and sternite are very small and surround the anus. Both body and appendages are covered with setae, many of which are specialized sensory organs; specially outstanding are the long, thin trichobothria on the palpal chela, usually eight on the fixed finger and four on the movable finger of an adult.
These animals are mostly dioecious, and often a courtship ritual precedes mating. The male deposits a spermatophore which is picked up by the female for fertilization. Eggs are held by the female in a special pouch attached to the genital opercula, and are nourished by special secretions until they are well developed. There are three nymphal stages before the adult stage. Brooding and molting often occur within silken chambers, fashioned with the chelicerae.
Pseudoscorpions are usually cryptic, living in soil and litter, in animal nests, and under logs, rocks, and the bark of fallen and standing trees. No members of the order are truly aquatic, but a few can live in sheltered places between tidemarks and the seashore. All are predacious, feeding on the body juices of other arthropods and small invertebrates, after immobilizing the prey with the palpal chelae and chelicerae.
Quite a few pseudoscorpions engage in the behavior called phoresy in which a pseudoscorpion grasps a larger arthropod, usually an insect, by a leg and is carried from place to place. As the pseudoscorpions involved are usually adult females, this behavior may serve to disperse the species.
The order (=Chelonethida) is usually considered to comprise 3 suborders; Heterosphyronida (=Chthoniinea), Diplosphyronida (=Neobisiinea), and Monosphyronida (=Cheliferinea). Recent studies, however, have cast some doubt on the usefulness of this classification. Therefore, herein the order is divided directly into 6 superfamilies: Chthonioidea, Neobisioidea, Garypoidea, Cheiridioidea, Feaelloidea, and Cheliferoidea. There are probably 2000 or more species. They are found all over the world, except in the Arctic and Antarctic regions, but reach their greatest numbers in the tropics.