326
E. B. EDNEY
loss was little more than in normal air, whereas when ventilation occurred,
the rate more than doubled. Furthermore, locusts preconditioned to dry
air, lost far less water through the spiracles than locusts preconditioned
to 96% relative humidity, when both were exposed to the same conditions—3.2 and 5.3 mg/gm/hours, respectively. At the same time the "dry"
locusts ventilated less frequently, and this suggest that reduction of the
extent of ventilatory movements may conserve water.
In this case and perhaps in others as well, it may be worth wondering
whether the regulatory system works both ways. Perhaps the onset of
vigorous respiratory movements is triggered not by the need for oxygen
—indeed this seems very unlikely since the "dry" locusts did not show such
movements—but rather by the need to eliminate water.
Be that as it may, some very effective mechanisms for regulating water
loss are at work in insects such as tsetse flies and locusts, whose habitats
are fairly xeric. On the other hand, the mealworm larva, Tenebrio, is said
to lose water at a rate independent of its own water content (Mellanby,
1958) so that conservation mechanisms are not invariably found where
they might be expected. Further information about other insects is given
byBursell (1964a, 1970).
Evidence has been obtained in recent years that some insects (usually
in the pupal or other inactive stage) do not release C 0 2 continuously but
release it in widely separated bursts (Buck and Keister, 1955; Buck, 1958,
1962; Schneiderman and Williams, 1955). The subject has been reviewed
by Miller (1964). During such cyclical C 0 2 release, oxygen uptake is continuous, and it seems that during interburst periods the spiracles are left
very narrowly open. This permits an inward movement of air to compensate for the reduced internal pressure which itself results from uptake
of 0 2 from the tracheoles. Such inward movement reduces the outward
diffusion of C0 2 and forces the latter temporarily into solution in the
tissues.
The importance of this phenomenon from out point of view is that outward diffusion of water vapor is also reduced, and Buck believes that the
process is indeed an adaptation which enhances water conservation. The
fact that it has been observed mostly in inactive insects, and that it does
not occur in pupae kept in high humidity, are consistent with this interpretation, but further experimental evidence is very desirable. Perhaps
Loveridge's locusts reduce water loss in a somewhat similar way. Certainly
the incidence of pauses between breathing movements increased when his
insects were exposed to dry air. Ahearn (1970a) believes that unidirectional airflow and discontinuous ventilation may decrease water loss in the
desert beetle, Eleodes armata, but as he implies, the evidence is
inconclusive.
E. B. EDNEY
loss was little more than in normal air, whereas when ventilation occurred,
the rate more than doubled. Furthermore, locusts preconditioned to dry
air, lost far less water through the spiracles than locusts preconditioned
to 96% relative humidity, when both were exposed to the same conditions—3.2 and 5.3 mg/gm/hours, respectively. At the same time the "dry"
locusts ventilated less frequently, and this suggest that reduction of the
extent of ventilatory movements may conserve water.
In this case and perhaps in others as well, it may be worth wondering
whether the regulatory system works both ways. Perhaps the onset of
vigorous respiratory movements is triggered not by the need for oxygen
—indeed this seems very unlikely since the "dry" locusts did not show such
movements—but rather by the need to eliminate water.
Be that as it may, some very effective mechanisms for regulating water
loss are at work in insects such as tsetse flies and locusts, whose habitats
are fairly xeric. On the other hand, the mealworm larva, Tenebrio, is said
to lose water at a rate independent of its own water content (Mellanby,
1958) so that conservation mechanisms are not invariably found where
they might be expected. Further information about other insects is given
byBursell (1964a, 1970).
Evidence has been obtained in recent years that some insects (usually
in the pupal or other inactive stage) do not release C 0 2 continuously but
release it in widely separated bursts (Buck and Keister, 1955; Buck, 1958,
1962; Schneiderman and Williams, 1955). The subject has been reviewed
by Miller (1964). During such cyclical C 0 2 release, oxygen uptake is continuous, and it seems that during interburst periods the spiracles are left
very narrowly open. This permits an inward movement of air to compensate for the reduced internal pressure which itself results from uptake
of 0 2 from the tracheoles. Such inward movement reduces the outward
diffusion of C0 2 and forces the latter temporarily into solution in the
tissues.
The importance of this phenomenon from out point of view is that outward diffusion of water vapor is also reduced, and Buck believes that the
process is indeed an adaptation which enhances water conservation. The
fact that it has been observed mostly in inactive insects, and that it does
not occur in pupae kept in high humidity, are consistent with this interpretation, but further experimental evidence is very desirable. Perhaps
Loveridge's locusts reduce water loss in a somewhat similar way. Certainly
the incidence of pauses between breathing movements increased when his
insects were exposed to dry air. Ahearn (1970a) believes that unidirectional airflow and discontinuous ventilation may decrease water loss in the
desert beetle, Eleodes armata, but as he implies, the evidence is
inconclusive.