Effects of Water Stress and Relative Humidity on Ventilation in the Tick Dermacentor andersoni (Acari: Ixodidae)

We examined the effect of water stress on the ventilation of 4-mo-old unfed adult female Dermacentor andersoni using a CO₂ flow-through respirometry system. Whether defined in terms of hydrational status or the ambient relative humidity of the air, water stress significantly affected the cyclic pattern of gas exchange. Hydrated animals exposed to humid air (95% relative humidity) ventilated discontinuously with long interburst periods in which no measurable CO₂ emission occurred. Exposure to dry air (0% relative humidity) resulted in more frequent but smaller CO₂ bursts and was accompanied by greater rates of water loss. Dehydrated animals, particularly those in humid air, displayed much higher standard metabolic rates than hydrated animals. This increase in CO₂ emission was accommodated by an increase in cycling frequency and interburst gaseous exchange with an eventual breakdown of the discontinuous ventilation cycle. Higher standard metabolic rate in dehydrated animals (0% relative humidity) was not correlated with locomotary activity but rather reflected a decrease in water content since metabolic rate expressed on a dry-weight basis did not differ significantly from that of hydrated animals. A further increase in the rate of CO₂ emission accompanied by very rapid cycling and water uptake occurred when dehydrated ticks were exposed to humid air. This increase in metabolic expenditure in dehydrated animals at 95% relative humidity includes the energy cost of active water vapor uptake.