CHAPTER 2
Water Balance in Mediterranean Arthropods
2.1
Water Relations
Water is essential for life. It is a key element in the function and regulation of
terrestrial organisms. The body water content of arthropods usually ranges
from 65 to 75% (Hadley 1994). Terrestrial arthropods are highly sensitive to
humidity on account of their large surface to volume ratio. High atmospheric
humidity favouring absorption of water into the body and reducing transpiration rates, tends to result in higher water balance. The opposite occurs with
low atmospheric humidity (Block 1996). Consequently, water conditions
induce moisture uptake as well as watersaving adaptations such as reduced
cuticular transpiration (Cloudsley-Thompson and Constantinou 1983).
Environmental moisture fluctuates seasonally in Mediterranean-type ecosystems, and water conditions govern the abundance, spatial distribution and
activity of terrestrial arthropods to a large extent. When coupled with high
temperatures, low atmospheric moisture during the summer results in high
saturation deficiencies. In addition, the scarcity of significant rainfall strongly affects soil moisture conditions. Mediterranean surface and soil arthropods evidently undergo seasonal variations in moisture conditions ranging
from subaquatic to xeric (Poinsot-Balaguer 1988). Vannier (1978) suggested
that taxon specific moisture thresholds determine the survival of arthropods
and reported relevant logarithmic water potential values of 4.2 for collembolans and 5 for oribatids. Accordingly, water relationships of Mediterranean
arthropods have to be discussed with respect to severe water shortage in
summer.
Vannier (1978) described a three-step progressive process of soil desiccation. During the rainy period, wet soil particles result in saturated air in soil
crevices, pores and animal burrows. At the beginning of the drought period,
water begins to migrate downwards and surface-dwelling animals face water
depletion. Moreover, dehydration of the substrate gives rise to patchy microenvironments and creates microgradients from saturated to water-deficient
sites. During the third stage, liquid water disappears completely and soil animals have to take up water retained in soil particles. This phenomenon deter-
Water Balance in Mediterranean Arthropods
2.1
Water Relations
Water is essential for life. It is a key element in the function and regulation of
terrestrial organisms. The body water content of arthropods usually ranges
from 65 to 75% (Hadley 1994). Terrestrial arthropods are highly sensitive to
humidity on account of their large surface to volume ratio. High atmospheric
humidity favouring absorption of water into the body and reducing transpiration rates, tends to result in higher water balance. The opposite occurs with
low atmospheric humidity (Block 1996). Consequently, water conditions
induce moisture uptake as well as watersaving adaptations such as reduced
cuticular transpiration (Cloudsley-Thompson and Constantinou 1983).
Environmental moisture fluctuates seasonally in Mediterranean-type ecosystems, and water conditions govern the abundance, spatial distribution and
activity of terrestrial arthropods to a large extent. When coupled with high
temperatures, low atmospheric moisture during the summer results in high
saturation deficiencies. In addition, the scarcity of significant rainfall strongly affects soil moisture conditions. Mediterranean surface and soil arthropods evidently undergo seasonal variations in moisture conditions ranging
from subaquatic to xeric (Poinsot-Balaguer 1988). Vannier (1978) suggested
that taxon specific moisture thresholds determine the survival of arthropods
and reported relevant logarithmic water potential values of 4.2 for collembolans and 5 for oribatids. Accordingly, water relationships of Mediterranean
arthropods have to be discussed with respect to severe water shortage in
summer.
Vannier (1978) described a three-step progressive process of soil desiccation. During the rainy period, wet soil particles result in saturated air in soil
crevices, pores and animal burrows. At the beginning of the drought period,
water begins to migrate downwards and surface-dwelling animals face water
depletion. Moreover, dehydration of the substrate gives rise to patchy microenvironments and creates microgradients from saturated to water-deficient
sites. During the third stage, liquid water disappears completely and soil animals have to take up water retained in soil particles. This phenomenon deter-