18
Water Balance in Mediterranean Arthropods
mines the extent to which the water relations of Mediterranean arthropods
determine their habitat selection, thereby inducing orthokinetic migration.
Apart from summer drought, Mediterranean arthropods also face a water
surplus during the rainy season or after thunderstorms occurring chiefly in
late spring/early summer and possibly resulting in pronounced mortality
(e.g. Paris 1963). Accordingly, water relations in Mediterranean regions need
to be interpreted in relation to water surplus in winter and late spring.
2.2
Components of Water Balance
Water balance in terrestrial arthropods has been studied extensively by
Edney (1977) and Hadley (1994). Different avenues of water uptake have been
described among arthropods in general. Some species absorb water from
moist air, while others absorb water from damp particles through cuticular
"water channelling" structures (Cloudsley-Thompson 1975). For a good
number of species, food-linked water is the main source of moisture (Cloudsley-Thompson 1975).
In some resistant macroarthropods such as isopods, diplopods etc. that
lack waterproof epicuticular wax layers, most of the body water is retained in
the cuticle and tissues, while minor quantities are distributed in the digestive
and reproductive organs and in the haemolymph (e.g. Warburg 1987b). By
contrast, the haemolymph is the major water reservoir in the more tolerant
micro arthropods (e.g. Verhoef and Li 1983).
Smaller quantities of water are lost through body openings, excrements
and respiratory paths, but most of the water lost is transpired through the
cuticle (Quinlan and Hadley 1983; Hadley and Quinlan 1984; Hadley 1994).
Water loss is temperature and moisture dependent. Generally, it increases
with increasing temperature and decreasing ambient humidity.
Some arthropods restrict water loss energetically, whereas others seem to
tolerate water privation and low body water content. Animals capable of
restricting water loss may survive a loss of up to 30% of their body water.
Relevant mechanisms include the epicuticular wax layer of arthropods, conglobation in some isopods and diplopods, and fasting (Edney 1977).
Fasting is the most important mechanism employed by most arthropods,
such as the majority of collembolans, against prolonged drought. Furthermore, the connection between obligatory fasting and resistance to moisture
loss is considered to be a factor regulating population size (Poinsot-Balaguer
1988). Fasting frequently precedes or follows intensive egg deposition of
Mediterranean arthropods. Drought resistant eggs deposited early in summer are able to withstand adversity and hatch soon after the onset of the
rains. In general, feeding activity is synchronised with changes in moisture,
and intestinal content determines the survival of arthropods under stressful
Water Balance in Mediterranean Arthropods
mines the extent to which the water relations of Mediterranean arthropods
determine their habitat selection, thereby inducing orthokinetic migration.
Apart from summer drought, Mediterranean arthropods also face a water
surplus during the rainy season or after thunderstorms occurring chiefly in
late spring/early summer and possibly resulting in pronounced mortality
(e.g. Paris 1963). Accordingly, water relations in Mediterranean regions need
to be interpreted in relation to water surplus in winter and late spring.
2.2
Components of Water Balance
Water balance in terrestrial arthropods has been studied extensively by
Edney (1977) and Hadley (1994). Different avenues of water uptake have been
described among arthropods in general. Some species absorb water from
moist air, while others absorb water from damp particles through cuticular
"water channelling" structures (Cloudsley-Thompson 1975). For a good
number of species, food-linked water is the main source of moisture (Cloudsley-Thompson 1975).
In some resistant macroarthropods such as isopods, diplopods etc. that
lack waterproof epicuticular wax layers, most of the body water is retained in
the cuticle and tissues, while minor quantities are distributed in the digestive
and reproductive organs and in the haemolymph (e.g. Warburg 1987b). By
contrast, the haemolymph is the major water reservoir in the more tolerant
micro arthropods (e.g. Verhoef and Li 1983).
Smaller quantities of water are lost through body openings, excrements
and respiratory paths, but most of the water lost is transpired through the
cuticle (Quinlan and Hadley 1983; Hadley and Quinlan 1984; Hadley 1994).
Water loss is temperature and moisture dependent. Generally, it increases
with increasing temperature and decreasing ambient humidity.
Some arthropods restrict water loss energetically, whereas others seem to
tolerate water privation and low body water content. Animals capable of
restricting water loss may survive a loss of up to 30% of their body water.
Relevant mechanisms include the epicuticular wax layer of arthropods, conglobation in some isopods and diplopods, and fasting (Edney 1977).
Fasting is the most important mechanism employed by most arthropods,
such as the majority of collembolans, against prolonged drought. Furthermore, the connection between obligatory fasting and resistance to moisture
loss is considered to be a factor regulating population size (Poinsot-Balaguer
1988). Fasting frequently precedes or follows intensive egg deposition of
Mediterranean arthropods. Drought resistant eggs deposited early in summer are able to withstand adversity and hatch soon after the onset of the
rains. In general, feeding activity is synchronised with changes in moisture,
and intestinal content determines the survival of arthropods under stressful