The unfed tick gains water from humid air or from water in contact with the cuticle, and loses water by evaporation. Whilst attached to the host the tick is gaining water from the ingested blood and losing water in the excrement. The engorged tick usually lacks the ability to take up water from humid air.

The exchange of water takes place mainly through the cuticle. Regulation of the water balance is therefore brought about by the activity of the epidermal cells.

The cuticle comprises two principal layers, the epicuticle and endocuticle. The epicuticle is overlaid by a lipoid possessing important waterproofing properties. The pore canals, which traverse the endocuticle, are occupied by cytoplasm, and may in consequence play an important role in the active -transfer of water through the cuticle: they do not penetrate the epicuticle.

Water loss from the unfed tick is not closely related to saturation deficiency, particularly at high humidities. This departure is due to a physiological cause, namely, to the ability to secrete water. The effects of this activity are such that a state of equilibrium is attained at a relative humidity of about 92%: at lower relative humidities the tick loses water by evaporation, while at higher humidities it takes up water. The retention of water at humidities below the point of equilibrium is due not only to the physical properties of the epicuticle but also to this secretory activity, for water loss increases when the tick is temporarily asphyxiated, poisoned with cyanide or injured through excessive desiccation. Near the point of equilibrium the loss or gain of water over a wide range of temperature is determined by the relative humidity.

The uptake of water from humid air occurs when the tick is in a desiccated condition but ceases as the normal water content is restored. After previous exposure to saturated air the adapted tick at first loses water at relative humidities above the point of equilibrium, but later comes to retain water completely.

Both unfed and engorged ticks possess the ability to prevent or to limit temporarily the entry of water in contact with the cuticle.

The engorging female, originally weighing about 2 mg., ingests about 600 mg. of blood. About 300 mg. or two-thirds of the contained water are usually eliminated before the end of engorgement. Evaporation from the cuticle may account for a considerable fraction of this, for the temperature to which the attached tick is exposed (about 37°C.) is, in Ixodes ricinus, above that temperature at which a marked increase in the permeability of the epicuticular lipoid takes place.

The nine species of ticks examined differ considerably in their powers of limiting evaporation. This may reflect specific differences in the nature of the epicuticular lipoid. The order of their resistance is as follows: Ornithodorus moubata; Dermacentor andersoni; D. reticulatus; Rhipicephalus sanguineus; Amblyomma cajennense and A. maculatum; Ixodes canisuga; I. hexagonus; I. ricinus. In dry air water loss throuǵh the cuticle is 10–15 times more rapid in Ixodes ricinus than in Dermacentor andersoni. The more resistant species also take up water through the cuticle after desiccation; indeed, the rate of uptake over a unit area of cuticle is approximately the same in all species of Ixodidae. Uptake thus appears to be limited by the ability of the epidermal cells to secrete water.

Stocks of Dermacentor andersoni, Rhipicephalus and Amblyomma spp. were kindly supplied by Dr R. A. Cooley, Director of Entomology, Rocky Mountain Laboratory, U.S.A., through the good offices of Prof. P. A. Buxton, F.R.S. I am also indebted to Dr H. H. Green of the Veterinary Laboratory, Weybridge, for providing useful facilities, and to Dr V. B. Wigglesworth, F.R.S., for his generous help throughout the various stages of this work.