The caudal organs, or white spots which, as already mentioned, are usually found on the last of the three post-abdominal segments of _Thelyphonus_, are of doubtful function. They have been variously explained as the stink-gland orifices, and as organs sensitive to light (“ommatoids”). Laurie[253] was unable to find any pore in this region, nor was there any of the pigment so characteristic of organs of sight. The histological structure indicated a sense-organ rather than a gland, but the use of these organs is entirely conjectural. =Classification.=—The order Pedipalpi is divided into three families— Thelyphonidae, Schizonotidae and Tarantulidae. The first two are considered by some authors to form a sub-order, UROPYGI, or tailed Pedipalpi, while the Tarantulidae constitute the remaining sub-order AMBLYPYGI, the members of which are tailless. =Fam. 1. Thelyphonidae.=[254]—This family comprises nine or more genera, differing chiefly in the position of the eyes, the structure of the genital operculum, the armature of the pedipalps, and the presence or absence of “ommatoids” in the anal segment. The three following genera are among those most likely to be met with. Two ommatoids are present in each. _Thelyphonus_ has a spine on the second ventral plate, and a deep median impression on the male genital operculum, which is, however, absent from that of the female. There are about fifteen known species of this genus, inhabiting Southern Asia and the East Indies. _Typopeltis_ has ridges running forward from the lateral eyes. The middle third of the female operculum is raised and deeply impressed in the middle. This genus is represented in China and Japan. _Mastigoproctus_ has a short and stout coxal apophysis of the pedipalp, without a tooth on its inner side. It is found in Mexico, Brazil, and the West Indies. Other genera are _Thelyphonellus_ (Demerara), _Labochirus_ (Ceylon), _Hypoctonus_ (Burma), _Mimoscorpius_ (Philippines), _Uroproctus_ (Assam), _Abalius_ (New Guinea), without ommatoids, and _Tetrabalius_ (Borneo), with two pairs of ommatoids. =Fam. 2. Schizonotidae (= Tartaridae).=—This family contains only two genera, _Schizonotus_ (= _Nyctalops_, Pickard-Cambridge, nom. preocc. Aves) and Trithyreus[255] (= _Tripeltis_, Thorell, nom. preocc. Reptilia). They are very small, pale-coloured forms (about 5 mm. in length), found in Burma and Ceylon. =Fam. 3. Tarantulidae=, better known as =Phrynidae=. Pocock has shown that Fabricius established the genus _Tarantula_ from the species _T. reniformis_ in 1793, while there is no earlier record of Olivier’s _Phrynus_, established for the same species, than Lamarck’s citation of it in 1801. The family is divided into three sub-families, Tarantulinae, Phrynichinae, and Charontinae. (i.) The TARANTULINAE are new-world forms, represented by three genera, _Tarantula_, _Acanthophrynus_ (_Phrynopsis_), and _Admetus_ (_Heterophrynus_), in Central and South America and the West Indies.