24
Water Balance in Mediterranean Arthropods
other. Shifts in the proportion of water in the gut and gut contents as well as
in the haemolymph, cuticle and tissues have been recorded only in populations of desert diplopods; but an analogous osmoregulatory mechanism has
also been reported by Warburg (1987b) in certain isopod species from xeric
Mediterranean environments.
The comparatively higher transpiration rate recorded in Mediterranean
macroarthropods enables them to quickly excrete their water surplus. In addition, changes in the epicuticular lipid layers and membranes of microarthropods affect the permeability of the cuticle and change the transpiration
rate. Higher transpiration rates due either to osmoregulation or higher permeability of the cuticle allow arthropods in general to withstand high summer
temperatures by thermoregulation via rapid water evaporation through the
cuticle. Thus, due to higher transpiration rates, evaporative thermoregulation
should be of greater importance in semi-arid arthropods than in desert ones
(e.g. Cloudsley-Thompson 1975). Higher transpiration rates may also be
beneficial to arthropods living in habitats that suffer from periodic flooding.
In summary, Mediterranean arthropods appear to be moderately resistant
to desiccation. Their capability for osmoregulation and the translocation of
water among their body parts enable them to overcome summer drought.
The higher cuticle permeability recorded in some of these arthropods allows
for the excretion of excessive amounts of body water after temporary flooding in winter, late spring and early summer (e.g. Cloudsley-Thompson 1965).
Moreover, water relations within ontogenetic stages influence the orthokinetic behaviour of animals, i.e. the distribution of a population among microsites. Thus, mid-instar rise in haemolymph osmolality and transpiration rate
as well as ecdysial increase in cuticle permeability can be considered correlates of microhabitat selection.
2.S
Responses of Arthropods to Environmental Extremes
2.5.1
Behavioural Responses
In general, mesic species are less tolerable of environmental extremes such as
dessication and cold than species from adverse habitats. However, cautious
interpretations should be made under specific circumstances since other factors may equally be involved. For example, Cloudsley-Thompson and Constantinou (1983) compared water relations in spiders from mesic and xeric
environments and suggested that body size may be a determining factor.
However, among species of the same size, water budgets appear to be habitat
and habit dependent. Thus, burrowing mesic mygalomorphs are more vulnerable than desert or arboreal species.
Water Balance in Mediterranean Arthropods
other. Shifts in the proportion of water in the gut and gut contents as well as
in the haemolymph, cuticle and tissues have been recorded only in populations of desert diplopods; but an analogous osmoregulatory mechanism has
also been reported by Warburg (1987b) in certain isopod species from xeric
Mediterranean environments.
The comparatively higher transpiration rate recorded in Mediterranean
macroarthropods enables them to quickly excrete their water surplus. In addition, changes in the epicuticular lipid layers and membranes of microarthropods affect the permeability of the cuticle and change the transpiration
rate. Higher transpiration rates due either to osmoregulation or higher permeability of the cuticle allow arthropods in general to withstand high summer
temperatures by thermoregulation via rapid water evaporation through the
cuticle. Thus, due to higher transpiration rates, evaporative thermoregulation
should be of greater importance in semi-arid arthropods than in desert ones
(e.g. Cloudsley-Thompson 1975). Higher transpiration rates may also be
beneficial to arthropods living in habitats that suffer from periodic flooding.
In summary, Mediterranean arthropods appear to be moderately resistant
to desiccation. Their capability for osmoregulation and the translocation of
water among their body parts enable them to overcome summer drought.
The higher cuticle permeability recorded in some of these arthropods allows
for the excretion of excessive amounts of body water after temporary flooding in winter, late spring and early summer (e.g. Cloudsley-Thompson 1965).
Moreover, water relations within ontogenetic stages influence the orthokinetic behaviour of animals, i.e. the distribution of a population among microsites. Thus, mid-instar rise in haemolymph osmolality and transpiration rate
as well as ecdysial increase in cuticle permeability can be considered correlates of microhabitat selection.
2.S
Responses of Arthropods to Environmental Extremes
2.5.1
Behavioural Responses
In general, mesic species are less tolerable of environmental extremes such as
dessication and cold than species from adverse habitats. However, cautious
interpretations should be made under specific circumstances since other factors may equally be involved. For example, Cloudsley-Thompson and Constantinou (1983) compared water relations in spiders from mesic and xeric
environments and suggested that body size may be a determining factor.
However, among species of the same size, water budgets appear to be habitat
and habit dependent. Thus, burrowing mesic mygalomorphs are more vulnerable than desert or arboreal species.