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E. B. EDNEY
the tissues as dehydration proceeded, but I was not able to see this in
Arenivaga until dehydration was far advanced.
Djajakusumah and Miles (1966) found a similar effect to that in
Arenivaga in the Australian locust, Chortoicetes terminijera, where the
hemolymph osmotic pressure (OP) remained nearly constant in the face
of volume changes of 25% or more after dehydration and rehydration.
In these insects there were concomitant changes of amino acids into soluble
proteins and vice versa; changes which would contribute to, but not account fully for, the osmoregulation observed.
Earlier work on Periplaneta by Munson and Yeager (1949), by Wharton et al. (1965), and by Yeager and Muson (1950) suggests that both
water and ions may be moved between tissues and hemolymph under water
stress, and Wall (1970) not only confirmed and extended my own observations on Periplaneta, finding a similar osmoregulatory ability, but also
calculated the extent of solute movement between hemolymph and tissues.
However, the mechanism of this process, and indeed the advantage of
maintaining the hemolymph OP constant at the expense of transferring
salts to the tissues (if this is what happens) are still somewhat obscure.
L. THE ENERGETICS OF WATER VAPOR ABSORPTION
The ability to absorb water vapor is indeed remarkable, particularly in
view of the steep osmotic gradient against which it occurs. If we express
the concentrations concerned in terms of osmotic pressures, the difference
between the water activity of the sand roach hemolymph and that of air
at 82.5% relative humidity is 258 atm. We can calculate the amount
of energy involved in moving water against this gradient from the relation
AG = —nRT In (α 2 /α Ύ ), where AG is the Gibbs' free energy change, n
the moles of water involved, R the gas constant, T the absolute temperature, and a x and a 2 the activities in the initial and final states, respectively.
The most rapid rate of absorption thus far found in a sand roach was
6 mg in a day from air at 90.0% relative humidity by a nymph weighing
100 mg. Concentration of water through the required gradient takes about
3.10 cal/gm, so the insect expended 0.0186 cal in a day in absorbing 6
mg of water. Now such an insect's 0 2 uptake is about 22 /J/hour, which
is equivalent to 2.6 cal/day, so that even if we allow for thermodynamic
inefficiency in the system, the extra energy involved in the absorption of
water vapor is very small compared with the energy resources of the insect.
Lees (1948), Kanungo (1965), and Ramsay (1964) came to essentially
similar conclusions for the ticks, mites, and mealworm beetles, respectively,
with which they worked.
Wharton and Devine (1968), using tritiated water, measured the rate
E. B. EDNEY
the tissues as dehydration proceeded, but I was not able to see this in
Arenivaga until dehydration was far advanced.
Djajakusumah and Miles (1966) found a similar effect to that in
Arenivaga in the Australian locust, Chortoicetes terminijera, where the
hemolymph osmotic pressure (OP) remained nearly constant in the face
of volume changes of 25% or more after dehydration and rehydration.
In these insects there were concomitant changes of amino acids into soluble
proteins and vice versa; changes which would contribute to, but not account fully for, the osmoregulation observed.
Earlier work on Periplaneta by Munson and Yeager (1949), by Wharton et al. (1965), and by Yeager and Muson (1950) suggests that both
water and ions may be moved between tissues and hemolymph under water
stress, and Wall (1970) not only confirmed and extended my own observations on Periplaneta, finding a similar osmoregulatory ability, but also
calculated the extent of solute movement between hemolymph and tissues.
However, the mechanism of this process, and indeed the advantage of
maintaining the hemolymph OP constant at the expense of transferring
salts to the tissues (if this is what happens) are still somewhat obscure.
L. THE ENERGETICS OF WATER VAPOR ABSORPTION
The ability to absorb water vapor is indeed remarkable, particularly in
view of the steep osmotic gradient against which it occurs. If we express
the concentrations concerned in terms of osmotic pressures, the difference
between the water activity of the sand roach hemolymph and that of air
at 82.5% relative humidity is 258 atm. We can calculate the amount
of energy involved in moving water against this gradient from the relation
AG = —nRT In (α 2 /α Ύ ), where AG is the Gibbs' free energy change, n
the moles of water involved, R the gas constant, T the absolute temperature, and a x and a 2 the activities in the initial and final states, respectively.
The most rapid rate of absorption thus far found in a sand roach was
6 mg in a day from air at 90.0% relative humidity by a nymph weighing
100 mg. Concentration of water through the required gradient takes about
3.10 cal/gm, so the insect expended 0.0186 cal in a day in absorbing 6
mg of water. Now such an insect's 0 2 uptake is about 22 /J/hour, which
is equivalent to 2.6 cal/day, so that even if we allow for thermodynamic
inefficiency in the system, the extra energy involved in the absorption of
water vapor is very small compared with the energy resources of the insect.
Lees (1948), Kanungo (1965), and Ramsay (1964) came to essentially
similar conclusions for the ticks, mites, and mealworm beetles, respectively,
with which they worked.
Wharton and Devine (1968), using tritiated water, measured the rate