himidities. 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 9^ J at lower relative humidities it takes up 

 water. The retention of water at hvimidities 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 desicca- 

 tion. Near the point of equilibrium the loss or 

 gain of water over a wide range of temperature is 

 determined by the relative hxunidity. 



'"The uptake of water from httmid air occiirs 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 cpmes to retain water com- 

 pletely. 



'"Both unfed and engorged ticks possess the abil- 

 ity 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. Evapora- 

 tion 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 epicuticiilar 

 lipoid takes place. 



"'The nine species of ticks examined differ con- 

 siderably in their powers of limiting evaporation. 

 This may reflect specific differences in the nature 



- 152 - 



