22
Water Balance in Mediterranean Arthropods
Dynamics of water balance may regulate the spatial distribution of collembolans among microsites. Verhoef (1981) stated that the mid-instar rise of
blood osmolality pressure in O. cincta might be a stimulus forcing animals to
seek wet micro sites after feeding. Thus, during the feeding period, animals
can exploit wider areas. Both the approach to food and its diversity are
apparently regulated by haemolymph osmolality, while ecdysis occurs in
water saturated micro sites.
2.4
Adaptation to Moisture Variations
Water relations are a key element of habitat selection by arthropods. Differences in habitat selection can therefore be partly explained in terms of water
budgets. For example, less tolerable lithobiomorph centipedes inhabit rather
damp environments, sheltering among less insulated microsites such as the
undersides surface stones, fallen litter and bark, whereas more tolerable
scolopedromorph centipedes live in more xeric habitats, taking shelter in
deep burrows and cracks (Cloudsley-Thompson and Crawford 1970).
Transpiration rate, as well as the resistances of populations of the same
species to desiccation appear to be habitat specific. Within the same species,
populations from xeric habitats may display lower transpiration rates than
populations from temperate ones (Cloudsley-Thompson 1969, 1975). Moreover, considerable differences in transpiration rate may be recorded among
different populations of the same species due either to acclimation to moisture conditions within the lifetimes of individuals or to selection or even both
(Cloudsley-Thompson 1969).
According to Poinsot-Balaguer and Barra (1991), ancestral adaptations
enable Mediterranean arthropods to withstand rapid changes in their moisture regimes. Comparative studies may elucidate the roles of the different
components involved in the water balance of arthropods adapting to seasonally varying water regimes. Crawford et al. (1986) conducted a comparative
study of the components of water balance in populations of the diplopod
Archispirostreptus tumuliporus judaicus (=syriacus) from both mesic and
xeric Mediterranean habitats of Israel and found that the populations' different attributes of water balance depended on the habitat. Thus, desert spirostreptids were more resistant to desiccation than populations of A. t. judaicus
from mesic habitats. Total water loss, amounting to about 17%, was recorded
in the latter after 75-76 h exposure to dry air «10% relative humidity;
Fig. 2.5). In both mesic and xeric populations, total body water content was
significantly lower and haemolymph osmolality significantly higher in summer, while in summer more water was retained in the cuticle and tissues than
in the haemolymph and gut. These seasonal differences were partly attributed to the lower water content of foods in summer. Baker (1980) found sea-
Water Balance in Mediterranean Arthropods
Dynamics of water balance may regulate the spatial distribution of collembolans among microsites. Verhoef (1981) stated that the mid-instar rise of
blood osmolality pressure in O. cincta might be a stimulus forcing animals to
seek wet micro sites after feeding. Thus, during the feeding period, animals
can exploit wider areas. Both the approach to food and its diversity are
apparently regulated by haemolymph osmolality, while ecdysis occurs in
water saturated micro sites.
2.4
Adaptation to Moisture Variations
Water relations are a key element of habitat selection by arthropods. Differences in habitat selection can therefore be partly explained in terms of water
budgets. For example, less tolerable lithobiomorph centipedes inhabit rather
damp environments, sheltering among less insulated microsites such as the
undersides surface stones, fallen litter and bark, whereas more tolerable
scolopedromorph centipedes live in more xeric habitats, taking shelter in
deep burrows and cracks (Cloudsley-Thompson and Crawford 1970).
Transpiration rate, as well as the resistances of populations of the same
species to desiccation appear to be habitat specific. Within the same species,
populations from xeric habitats may display lower transpiration rates than
populations from temperate ones (Cloudsley-Thompson 1969, 1975). Moreover, considerable differences in transpiration rate may be recorded among
different populations of the same species due either to acclimation to moisture conditions within the lifetimes of individuals or to selection or even both
(Cloudsley-Thompson 1969).
According to Poinsot-Balaguer and Barra (1991), ancestral adaptations
enable Mediterranean arthropods to withstand rapid changes in their moisture regimes. Comparative studies may elucidate the roles of the different
components involved in the water balance of arthropods adapting to seasonally varying water regimes. Crawford et al. (1986) conducted a comparative
study of the components of water balance in populations of the diplopod
Archispirostreptus tumuliporus judaicus (=syriacus) from both mesic and
xeric Mediterranean habitats of Israel and found that the populations' different attributes of water balance depended on the habitat. Thus, desert spirostreptids were more resistant to desiccation than populations of A. t. judaicus
from mesic habitats. Total water loss, amounting to about 17%, was recorded
in the latter after 75-76 h exposure to dry air «10% relative humidity;
Fig. 2.5). In both mesic and xeric populations, total body water content was
significantly lower and haemolymph osmolality significantly higher in summer, while in summer more water was retained in the cuticle and tissues than
in the haemolymph and gut. These seasonal differences were partly attributed to the lower water content of foods in summer. Baker (1980) found sea-