There has been much discussion concerning the relationship Of the spirochætes. Formerly, they were regarded as bacteria closely related to the forms grouped in the genus _Spirillum_. The results of the detailed study which the spirochætes have received in recent years, have led most of the workers to consider them as belonging to the protozoa. The merits of the discussion we are not concerned with here, but rather with the fact that a number of diseases caused by spirochætes are arthropod-borne. The better known of these we shall discuss. AFRICAN RELAPSING FEVER OF MAN--It has long been known to the natives of Africa and to travelers in that country, that the bite of a certain tick, _Ornithodoros moubata_, may be followed by severe or even fatal fever of the relapsing type. Until recent years, it was supposed that the effect was due to some special virulence of the tick, just as nagana of cattle was attributed to the direct effect of the bite of the tsetse-fly. The disease is commonly known as "tick-fever" or by the various native names of the tick. In 1904, Ross and Milne, in Uganda, and Dutton and Todd on the Congo, discovered that the cause of the disease is a spirochæte which is transmitted by the tick. This organism has been designated by Novy and Knapp as _Spirochæta duttoni_. _Ornithodoros moubata_ (fig. 142), the carrier of African relapsing fever, or "tick-fever," is widely distributed in tropical Africa, and occurs in great numbers in the huts of natives, in the dust, cracks and crevices of the dirt floors, or the walls. It feeds voraciously on man as well as upon birds and mammals. Like others of the _Argasidæ_, it resembles the bed-bug in its habit of feeding primarily at night. Dutton and Todd observed that the larval stage is undergone in the egg and that the first free stage is that of the octopod nymph. [Illustration: 142. Ornithodoros moubata. (_a_) Anterior part of venter; (_b_) second stage nymph; (_c_) capitulum; (_d_) dorsal and (_e_) ventral aspect of female; (_f_) ventral aspect of nymph; (_g_) capitulum of nymph. After Nuttall and Warburton.] The evidence that the fever is transmitted by this tick is conclusive. Koch found that from five per cent to fifteen per cent, and in some places, fifty per cent of the ticks captured, harbored the spirochæte. The disease is readily transmitted to monkeys, rats, mice and other animals and the earlier experiments along these lines have been confirmed by many workers.