Nearly a hundred species of Scorpionidae have been described, distributed among fifteen genera. The following sub-families are recognised: Diplocentrinae, Urodacinae, Scorpioninae, Hemiscorpioninae, and Ischnurinae. The DIPLOCENTRINAE have a spur under the aculeus. They form a small group of only eight species. The principal genus, _Diplocentrus_, is entirely Neotropical, but _Nebo_ has a single Old World representative in Syria. The URODACINAE, with the single genus _Urodacus_, are Australian scorpions. As in the next sub-family, there are rounded lobes on the tarsi, but there is only a single keel on the “tail,” and the lateral eyes are two in number. Six good and three doubtful species are recognised. The SCORPIONINAE are Asiatic and African forms, and are recognised by the tarsi having a large lobe on each side, by the convex upper surface of the “hand,” by the presence of two median keels on the “tail,” and by the possession of three lateral eyes. _Palamnaeus_ (_Heterometrus_) has sixteen species in the Indian region. There are about thirty species of _Opisthophthalmus_, all natives of South Africa. _Pandinus_ includes about ten species, but there are only two species of the type genus _Scorpio_, _S. maurus_ and _S. boehmei_. The sub-family HEMISCORPIONINAE was formed for the reception of the single Arabian species _Hemiscorpion lepturus_. Its most striking characteristic is the cylindrical vesicle of the tail in the male. The ISCHNURINAE differ from the Scorpioninae chiefly in the absence of the tarsal lobes, the presence of a well-marked finger-keel, and the generally more depressed form of the body and hand. In the opinion of some authors they should be separated from the Scorpionidae as a distinct family, the Ischnuridae. There are more than twenty species, divided among six genera. The type genus _Ischnurus_ has only the single species _I. ochropus_. There are eight species of _Opisthacanthus_, which has representatives in Africa and America. =Fam. 3. Chaerilidae.=—_Sternum pentagonal with median depression or “sulcus” rounded posteriorly. Two pedal spurs. Stigmata circular. Two lateral eyes with a yellow spot behind the second. Pectines very short._ This small family has the single genus _Chaerilus_ with but seven species, natives of the Oriental region. =Fam. 4. Chactidae.=—_Two pedal spurs. Two lateral eyes (or, rarely, no eyes) but without yellow spot. Characteristic dentition on movable finger of “hand.”_ There are three sub-families, Megacorminae, Euscorpiinae, and Chactinae. The MEGACORMINAE include but a single Mexican form, _Megacormus granosus_. There is a single toothed keel under the “tail,” and all the under surface is spiny. There is a row of long bristles under the tarsus.