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246 look like minute moving grains of sand. They are characterized by having only three pairs of legs, no stigmata, and no sexual orifice. A suitable opportunity presenting, the larva attaches itself to its vertebrate host. After a period of growth it goes through a first moult (ecdysis), and emerges from its larval skin as a nympha, provided with eight legs instead of six, and with a pair of large stigmata placed, one cm. each side of the body, behind the fourth pair of legs. After a second period of growth and a second moult it becomes sexually mature. In some species, as in the case of Margaropus bovis, the metamorphosis from larva to nymph, and from nymph to imago, takes place upon the same host, the parasite remaining attached during the process. In other species, as in the case of Hœmaphysails leachi, the tick, before each moult, drops off as soon as it ceases feeding, and in consequence has to find a host three times during its life instead of once. Having reached maturity, the sexes unite. After fertilization the male dies, but the female proceeds to engorge herself with blood for the development of her ova. On account of the difficulty of finding an appropriate host, ticks at all stages are endowed with a phenomenal capacity for fasting. Megnin found Argas persicus alive after a fast of four years' duration.

Ticks are referable to two families, the Ixodidœ and Argasidœ, differing both in structure and in life habits. Their characteristics and generic classification are indicated sufficiently for practical purposes in the synoptical Table on p. 248.

The species which are of particular interest to the human pathologist are Ornithodoros moubata, O. savignyi, Argas persicus, and Dermatocentor venustus— the first two being the transmitters of tbe spirochæte of African relapsing fever; the third, of the spirochæte of the miana disease (Balfour). It is quite likely that other species will in the future ba found to be concerned in the transmission of disease germs to man. Important animal diseases due to Spirochœtœ and Babesiœ are also known to have tick transmitters. Spirochœtœ gallinarum of fowls in Brazil is transmitted by Argas miniatus. Babesia bigemina (bovis) is conveyed in various countries by different species of Margaropns, such as M. annulatus (Plate VI., 1), M. australis, M. decoloratus; Theileria parva by Rhipicephalus appendiculatus and R. simus; Babesia ovis by R. bursa; Nuttallia equi by R.evertsi, and Babesia canis by Hœmaphysalis leachi and Dermacentor reticulatus. Ornithodoros moubata (Murray) (O. savignyivar. cœca, Neumann) is extensively distributed throughout tropical Africa (Plate VI., 3). Its body is rotund and is oval in outline. Its colour, when alive, is greenish brown. The integument is hard, leathery, covered with close-set shining granules or tubercles, and marked both above and below with symmetrically arranged grooves. The females may attain about 8 mm, in length by 6 to 7 mm. in breadth, and moult