[Illustration: FIG. 38.—_Ornithodoros moubata._ (Murray from Doflein.)] A relapsing fever of Persia is transmitted by a tick of the genus _Ornithodoros_, which is also true for the relapsing fever of Panama. There is great variation in the description of the different spirochaetes, and frequently measurements are given for short forms and long forms. They also vary from wave-like lines to corkscrew spirals. Again, different species have different types and different activities of movement. As a rule they are about 20 × 0.4 microns. The spirochaetes of the relapsing fever of Panama varied in length from 4.6µ to 22µ. Of these the most common were those measuring 13µ. Transverse division of the longest forms was seen in dark-field preparations. The monkey is susceptible to all species of relapsing fever spirochaetes. White mice and white rats are readily infected by _S. duttoni_ and the Panama spirochaete but are refractory to _S. recurrentis_ except after passage through monkeys. _S. carteri_ causes only a transient infection in these small rodents. =Epidemiology.=—With tick fever the epidemiology rests upon the life history of the tick _O. moubata_. This tick infests the rest houses along the route of travel, hiding in the crevices of floors and walls during the day and coming out at night to bite the sleeping inmates. The feeding occupies a long time, more than an hour. Both sexes bite man. The female lays about 100 eggs, from which nymphs emerge in about twenty days. The larval stage takes place in the egg. Shortly after emerging the nymphs suck blood. An important fact is that the female transmits the spirochaete to its ova, so that the ticks from such ova may transmit the disease. [Illustration: FIG. 39.—Siphunculata and Rhynchota. 1. _Pediculus capitis._ 2. _Pediculus vestimenti._ 2_a_. Protruded rostrum of _Pediculus_. 3. _Phthirius pubis._ 4. _Acanthia lectularia._ 5. _A. rotundata._ 6. _Conorhinus megistus._] Natives seem to suffer severely from tick fever in childhood but in adult life possess a sufficient degree of immunity so that the disease shows itself in a very mild form in those harboring spirochaetes.