VI. DESERT ARTHROPODS
343
at least, uptake was inhibited by blocking the anus. This is particularly
intriguing in view of the suggestion of Grimstone et al. (see above, p.
335) showing that in Tenebrio larvae the posterior part of the rectum
contains feces from which water is absorbed by the rectal cells until their
water activity is the same as that of air at 90% RH. However, Okasha
(1971, 1972), continuing the work on Thermobia, believes that rehydration is a response to decreased volume of the insect, rather than to
decreased proportional water content, and that anal blockage may act
indirectly by interfering with the sensory nervous pathways involved rather
than directly by preventing the entrance of water vapor into the rectum.
Maddrell (1971), basing his calculations on the measured rate of vapor
uptake in Thermobia of 5 ^g min
-1
, found that the rectum of that insect
would have to fill with air and empty 80 times a second—a highly unlikely
process. However, he also showed that since the rectum is short and near
the surface, water vapor could enter by diffusion alone at the required
rate. In some preliminary experiments in my laboratory with Arenivaga,
rectal blockage prevented uptake, but so did blockage of the mouth. The
theoretical possibility of rectal uptake exists, but further experimental inquiry is needed.
Okasha's work also showed that Thermobia continued to take water up
beyond the amount necessary to restore the normal proportion of water
to dry matter, until its original wet weight was reached, and this may well
be true of most arthropods that show the property of water vapor uptake.
Whether or not the mechanism that mediates sorption above a critical
relative humidity also helps to reduce the rate of loss when the external
humidity is below that level is an interesting question. Noble-Nesbitt
(1969) has evidence suggesting that in Thermobia the mechanism does
not restrict water loss in these conditions, since rates of transpiration indicate that the cuticle has a water activity similar to that of the hemolymph
rather than one equivalent to the critical relative humidity (about 5 0 % ) .
The problem has also been approached by Knülle and Devine (1972) who
used tritiated water to measure net exchange of water in the tick, Dermacentor variabilis, which takes up water from air down to an activity of
0.85 (85% relative humidity). Their results point to a net active absorption rate of 0.044 μg/houτ at 92.5% RH and 25°C, and further, that active
absorption occurs only at or above the equilibrium humidity—it is not
a component of total water exchange below this humidity.
In Arenivaga almost certainly the process is adaptive. If the sand
roaches are exposed to dry sand at high temperatures while feeding and
this results in their losing water, they have only to move downwards to
a region where the relative humidity is above 80% or so to become
rehydrated.
343
at least, uptake was inhibited by blocking the anus. This is particularly
intriguing in view of the suggestion of Grimstone et al. (see above, p.
335) showing that in Tenebrio larvae the posterior part of the rectum
contains feces from which water is absorbed by the rectal cells until their
water activity is the same as that of air at 90% RH. However, Okasha
(1971, 1972), continuing the work on Thermobia, believes that rehydration is a response to decreased volume of the insect, rather than to
decreased proportional water content, and that anal blockage may act
indirectly by interfering with the sensory nervous pathways involved rather
than directly by preventing the entrance of water vapor into the rectum.
Maddrell (1971), basing his calculations on the measured rate of vapor
uptake in Thermobia of 5 ^g min
-1
, found that the rectum of that insect
would have to fill with air and empty 80 times a second—a highly unlikely
process. However, he also showed that since the rectum is short and near
the surface, water vapor could enter by diffusion alone at the required
rate. In some preliminary experiments in my laboratory with Arenivaga,
rectal blockage prevented uptake, but so did blockage of the mouth. The
theoretical possibility of rectal uptake exists, but further experimental inquiry is needed.
Okasha's work also showed that Thermobia continued to take water up
beyond the amount necessary to restore the normal proportion of water
to dry matter, until its original wet weight was reached, and this may well
be true of most arthropods that show the property of water vapor uptake.
Whether or not the mechanism that mediates sorption above a critical
relative humidity also helps to reduce the rate of loss when the external
humidity is below that level is an interesting question. Noble-Nesbitt
(1969) has evidence suggesting that in Thermobia the mechanism does
not restrict water loss in these conditions, since rates of transpiration indicate that the cuticle has a water activity similar to that of the hemolymph
rather than one equivalent to the critical relative humidity (about 5 0 % ) .
The problem has also been approached by Knülle and Devine (1972) who
used tritiated water to measure net exchange of water in the tick, Dermacentor variabilis, which takes up water from air down to an activity of
0.85 (85% relative humidity). Their results point to a net active absorption rate of 0.044 μg/houτ at 92.5% RH and 25°C, and further, that active
absorption occurs only at or above the equilibrium humidity—it is not
a component of total water exchange below this humidity.
In Arenivaga almost certainly the process is adaptive. If the sand
roaches are exposed to dry sand at high temperatures while feeding and
this results in their losing water, they have only to move downwards to
a region where the relative humidity is above 80% or so to become
rehydrated.