Ixodes ricinusin Relation to its Physical Environment

In a previous paper (MacLeod, 1934) the different physical factors in the environmental complex of the sheep tick were discussed in their relation to the development of the gorged tick, and an experimental analysis was made of the effects produced, under laboratory conditions, by their action and interaction. The present paper deals with the active unfed ticks. These present rather a different problem from the gorged ticks, for, whereas in their case the climatic optimum resolved itself into that combination of conditions under which development proceeded favourably and with a low attendant mortality, the unfed tick, on the other hand, is faced with two desiderata, survival and hostparasitisation, the physical optima for which may not be coincident. It is necessary, therefore, to consider not only the climatic optimum for survival and activity, but also the factor or factors which influence parasitisation, and those conditions under which host-parasitisation is facilitated.