Previous studies on variation in size in the immature stages of ixodid ticks have been based on the assumption that the progeny of single females constitute a homogeneous population.

From the present work on the immature stages of Hyalomma anatolicum anatolicum, and by following larvae and nymphs to the adult stage, it is clear that there are two size groups of larvae and nymphs which reflect their potential sexuality. Larvae and nymphs which give rise to males are smaller in size than those which produce females. This size difference of the immature stages has other implications, for larvae and nymphs destined to become males imbibe, on average, about two-thirds the quantity of nutrients of those which yield females. Because of these apparent morphological and behavioural feeding mechanisms, which appear to be genetically determined, differences in physiological patterns of activity in the immature stages of the two sexes cannot be overlooked and may prove useful in elucidating the transmission of pathogenic organisms by larvae and nymphs.

We are grateful to Dr R. P. Chaudhuri for providing us with the original ticks from which our culture was started, and one of us (K.S.) to the Science Research Council for the provision of a research grant.