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Chelonethida (Pseudoscorpionida)
Chthonioidea

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Small animals, mostly 1-2 mm in length, characterized by being heterotarsate. That is, the tarsi of legs I and II consist of a single segment each, and those of legs III and IV have two segments each. For this reason they have been classified as Heterosphyronida by many taxonomists. Members of this group also have large chelicera, sometimes as much as two-thirds the length of the carapace. In addition to the typical 12 long trichobothria on the palpal chela, there is a pair of smaller trichobothria near the tip of the fixed finger. Usually the two internal basal (ib) trichobothria are located on the dorsum of the chelal hand; usually there are modified setae called coxal spines on the coxae of legs I, II, or III; and usually there are four corneate eyes.
Members of this group are found in all the faunal regions of the world. They live mainly in soil, litter, and debris, and occasionally under the bark of trees. Many are found in caves, especially in Europe and America. The cave dwellers are usually strongly modified; in comparison with other members of the superfamily, they are larger, their appendages are more attenuated, and their eyes and coloration are reduced.
The 2 families in this superfamily are: Tridenchthoniidae (=Dithidae) and Chthoniidae.

TRIDENCHTHONIIDAE

There are distinct sclerotic plates around the spiracles; these plates are oriented obliquely to the long axis of the abdomen. In addition, the genital area of the female is strongly sclerotized. There are usually 50 or more setae on the carapace. A cheliceral galea is lacking in the adult, but the tritonymph (at least in some species) has a triple galea. Coxal spines are present on pedal coxae I and II: the intercoxal tubercle, when present, bears a single seta; the internal basal (ib) and internal subbasal (is) trichobothria are placed transversely on the dorsum of the chelal hand. On the movable chelal finger the subbasal (s) trichobothrium is much closer to the subterminal (st) than to the basal (b). The abdomen is often little longer than the carapace.
The family is mainly tropical, although a few species are found in temperate areas, for example, Verrucaditha spinosa, which ranges as far north in the United States as Ohio. These forms are small and cryptic and live in litter and under bark. They have not been well collected or studied.
About 20 genera have been placed in the family (=Dithidae), among them, Tridenchthonius of South and Central America, Ditha of southeast Asia, Verrucaditha of eastern North America, Compsaditha of Africa, Anaulacodithella of Australia, and Chelignathus, known only from Baltic amber.

CHTHONIIDAE

There are indistinct plates around the spiracles; these are placed transversely to the long axis of the abdomen. In addition, the genital area of the female is only weakly sclerotized. There are fewer than 30 setae on the carapace. Both nymphs and adults often have a cheliceral galea in the form of a single sclerotic knob or crest. Coxal spines are usually present on some combination of coxae I, II and III. The intercoxal tubercle, when present, bears two small setae. The internal basal (ib) and internal subbasal (is) trichobothria are usually placed transversely on the dorsum of the chelal hand, but may be located more distally at the base of the fixed finger (Pseudotyrannochthonius), or may be joined by the external basal (eb) and external subbasal (es) trichobothria (Lechytia), or may be arranged in tandem (Mexichthonius). On the movable chelal finger the subbasal (sb) trichobothrium may be close to the subterminal (st), as in the Tridenchthoniidae, or it may be close to the basal (b), near the base of the finger. The abdomen is usually distinctly longer than the carapace.
This large family comprises about 30 genera. Members are widely distributed in both tropical and temperate areas and on all continents. They are mainly small and cryptic, usually living in litter and soil, and occasionally under the bark of trees. Many have gone into caves, where some have become highly modified troglobitic forms: the most successful cavernicolous genera are Chthonius and Troglochthonius in Europe; and Aphrastochthonius, Kleptochthonius, and Tyrannochthonius in North and Central America; and Pseudotyrannochthonius in Australia.