1. Pleomorphic, bacterium-like, Gram-negative, intracellular micro-organisms, which stained much less intensely with ordinary dyes than most bacteria, were found in fifteen ticks comprising examples of both the Argasidae and the Leodidae and belonging to the following species: Amblyomma americana, Argas persicus, Boophilus decoloratus, Dermacentor variabilis, Dermacentor venustus, Haemophysalis leachi, Hyalomma aegyptium, Margaropus annulatus, Margaropus annulatus australis, Ornithodorus megnini, Ornithodorus turicata, Rhipicephalus appendiculatus, Rhipicephalus evertsi, Rhipicephalus sanguineus, and Rhipicephalus simus.
2. No morphological or tinctorial evidence could be adduced of injury to the tissues of their arachnid hosts other than physical distension o£ the cells to accommodate them in large numbers. Since microscopic examination of favourable preparations of the species which contained them revealed an incidence of 100 per cent. not only in ticks collected in South Africa, but also in others from Jamaica, Trinidad, Honolulu, and several parts of the United States, it is probable that the micro-organisms were in no sense harmful to their hosts.
3. In consideration of the detection of the micro-organisms in the egg of ten species, in the unfed larvae of seven species, and at very close stages throughout the life-cycle of two others, the conclusion was reached that they were transmitted hereditarily.
4. The micro-organisms in several respects resembled Rickettsia, but differed from them in being of larger size. They also resembled the symbionts of certain lice and blood-feeding flies, but never gave rise to definite organ-like structures comparable with the mycetomes, and were restricted in their distribution to the Malpighian tubules and the egg-cells, as contrasted with the digestive tract to which the symbionts of these insects are confined.
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