In the course of our studies on species of Rhipicephalus, we have noted the great variability in size and structure observable in the genus. This variability has been a cause of much confusion in classification, as noted by Warburton (Parasitology, 1912, v. 1). It soon occurred to me, in seeking for an explanation of this variability, that it might well be due to nutrition, it being probable that, under natural conditions, a certain number of ticks are removed from the host before they have fed to repletion. When a host is infested with ticks it will brush or rub off a certain number prematurely and the ticks which are imperfectly fed would, doubtless, give rise to adults of small size. As stated, the great variability observed in Rhipicephalus is very striking. In the genus Ixodes, on the other hand, the size of the adults is fairly constant. In the latter case, we have, as a rule, to deal with small ticks provided in the immature stages with long hypostomes, both the small size and hypostomal structure rendering it most difficult for the tick to be removed prematurely from the host. It is difficult to remove Ixodes from the host without breaking the hypostome and using considerable force. In Haemaphysalis, as in Rhipicephalus, the immature stages are provided with short hypostomes and the ticks are more readily removed. The variation in size noted in adult Haemaphysalis, whilst considerable, is not as great as in Rhipicephalus, which would appear to be attributable to the smaller size of the tick, being in a measure a protection against its being prematurely removed from the host.