344
E. B. EDNEY
It would be interesting to know whether other desert arthropods possess
this facility. We have information about a few: Arenivaga floridensis behaves in a similar way, and the giant red velvet mite, Dinothrombium,
also absorbs water vapor although extremely slowly [our results differed
from those of Cloudsley-Thompson (1962b) in this respect]. The cockroaches, Periplaneta and Blatta, with which we experimented at the same
time as the sand roach, do not; neither do the desert tenebrionid, Eleodes
armata, the cricket, Macrobaenetes sp., nor the noctuid larva, Copablepharon (E. B. Edney, unpublished).
K. OSMOREGULATION IN THE SAND ROACH Arenivaga
Having found that the sand roach gains weight in high humidities, I
wanted to make sure that this was due to the absorption of water, and
to find what effect, if any, it had on the hemolymph concentration. Fortunately Arenivaga is a fairly large insect—adult females weigh up to 800
mg—so that sampling and analyzing the blood of single individuals is possible. Most of the other arthropods which are known to absorb water vapor
are so small that experimentation on individual animals is technically
unattractive.
A population of nymphs was divided into three similar groups, so that
total water content, dry matter, and hemolymph concentrations could be
measured at three points in a dehydration-rehydration cycle. The overall
picture is shown in Fig. 14. Before dehydration the "normal" insects had
a mean water content of 67.2% of their total wet weight and a hemolymph
osmotic pressure of 433 milliosmoles/liter (which is equivalent to a sodium chloride solution of about 1.3%). Four days later, after dehydration
in dry air at 25°C, the second group had lost 26.2% of their original
weight, of which 4.5% was accounted for by fecal pellets (the third group
behaved very similarly). Their absolute water content had fallen to 48.2%
of the original wet weight but, since the dry weight had also decreased,
their water content worked out at 65.1% of their own wet weight.
At this stage hemolymph osmotic pressure was 452 milliosmoles/liter—
a rise, but nothing like the rise (to 616 milliosmoles/liter) which would
be expected from the amount of water lost, so that regulation was evident,
perhaps by the removal of solutes from the hemolymph or by the addition
of water to it from the tissues. The third group of nymphs, after precisely
similar dehydration, was transferred to 95% relative humidity for an additional 7 days, during which time they increased in weight from 73.8%
of original to 91.9%, while their dry material had decreased by an additional 2.3%. The difference was accounted for by an absolute increase
in water content of 15.7%. The mean hemolymph concentration had fallen
E. B. EDNEY
It would be interesting to know whether other desert arthropods possess
this facility. We have information about a few: Arenivaga floridensis behaves in a similar way, and the giant red velvet mite, Dinothrombium,
also absorbs water vapor although extremely slowly [our results differed
from those of Cloudsley-Thompson (1962b) in this respect]. The cockroaches, Periplaneta and Blatta, with which we experimented at the same
time as the sand roach, do not; neither do the desert tenebrionid, Eleodes
armata, the cricket, Macrobaenetes sp., nor the noctuid larva, Copablepharon (E. B. Edney, unpublished).
K. OSMOREGULATION IN THE SAND ROACH Arenivaga
Having found that the sand roach gains weight in high humidities, I
wanted to make sure that this was due to the absorption of water, and
to find what effect, if any, it had on the hemolymph concentration. Fortunately Arenivaga is a fairly large insect—adult females weigh up to 800
mg—so that sampling and analyzing the blood of single individuals is possible. Most of the other arthropods which are known to absorb water vapor
are so small that experimentation on individual animals is technically
unattractive.
A population of nymphs was divided into three similar groups, so that
total water content, dry matter, and hemolymph concentrations could be
measured at three points in a dehydration-rehydration cycle. The overall
picture is shown in Fig. 14. Before dehydration the "normal" insects had
a mean water content of 67.2% of their total wet weight and a hemolymph
osmotic pressure of 433 milliosmoles/liter (which is equivalent to a sodium chloride solution of about 1.3%). Four days later, after dehydration
in dry air at 25°C, the second group had lost 26.2% of their original
weight, of which 4.5% was accounted for by fecal pellets (the third group
behaved very similarly). Their absolute water content had fallen to 48.2%
of the original wet weight but, since the dry weight had also decreased,
their water content worked out at 65.1% of their own wet weight.
At this stage hemolymph osmotic pressure was 452 milliosmoles/liter—
a rise, but nothing like the rise (to 616 milliosmoles/liter) which would
be expected from the amount of water lost, so that regulation was evident,
perhaps by the removal of solutes from the hemolymph or by the addition
of water to it from the tissues. The third group of nymphs, after precisely
similar dehydration, was transferred to 95% relative humidity for an additional 7 days, during which time they increased in weight from 73.8%
of original to 91.9%, while their dry material had decreased by an additional 2.3%. The difference was accounted for by an absolute increase
in water content of 15.7%. The mean hemolymph concentration had fallen