352
E. B. EDNEY
tained information about the temperatures experienced by the scorpions.
During the day, when soil surface temperatures were as high as 65°C,
a scorpion at —20 cm below the soil was at 34°C, and conversely, during
the night, subterranean burrows provided a warm shelter. One scorpion,
coming to the surface from —20 cm below, at 2 P.M., experienced a drop
in body temperature from 33° to 21 °C.
On one day, Eleodes beetles had a body temperatue of 42°C while the
air was at 32°C and the ground was 42°C. Assuming a value of 63%
for surface reflectivity of the beetle, Hadley calculated a heat balance
which read as follows, in cal cm
-2
min
-1
: radiation (0.141) + metabolism (0.003) -f conduction (?) = convection (0.134) + evaporation
(0.008).
Clearly the roles of evaporative heat loss and of metabolism were minor
in these circumstances, and this seems to be the general rule for many
insects and other arthropods. In flying locusts, for example, the thoracic
temperature in dry air is but 1 °C below that in moist air where transpiration is prevented (Church, 1960), and a further instance is supplied by
my own measurements on Onymacris spp. in the Namib Desert (Edney,
1971b) referred to below in connection with the effects of surface color.
On the other hand, not all arthropods have highly impermeable integuments, and in these evaporative cooling may be of importance.
It has been known for a long time that many insects go through a period
of warming up by shivering before flight, and hawk moths provide good
examples. Heinrich and Bartholomew (1971) showed that in Manduca
sexta, which lives in the Mojave Desert of California and feeds on Datura
stramonium (Jimson weed), the thorax is warmed to 37°-39°C before
flight by muscular activity, the rate of increase being linear but depending
on ambient temperature. Interestingly enough, the temperature of the abdomen during this warm-up period remains near the ambient temperature,
probably as a result of the cessation of blood flow from the thorax. During
free flight, Manduca maintains its thoracic temperature within 1° of 42°C
in ambient temperatures from 17°-32°C (Heinrich, 1970, 1971). Metabolic heat generated by the flight muscles tends to overheat the thorax,
but the excess is carried away by circulating blood which is pumped rather
vigorously by the dorsal tubular heart into the hemocoele of the thorax
from where it flows back to the cooler abdomen.
C. EVAPORATIVE COOLING IN ISOPODS
Some years ago, I investigated the situation in isopods and the sea slater
Ligia, 2i littoral isopod about the size of Blatta orientalis. (These are by
no means desert animals, but the information about them may serve to
E. B. EDNEY
tained information about the temperatures experienced by the scorpions.
During the day, when soil surface temperatures were as high as 65°C,
a scorpion at —20 cm below the soil was at 34°C, and conversely, during
the night, subterranean burrows provided a warm shelter. One scorpion,
coming to the surface from —20 cm below, at 2 P.M., experienced a drop
in body temperature from 33° to 21 °C.
On one day, Eleodes beetles had a body temperatue of 42°C while the
air was at 32°C and the ground was 42°C. Assuming a value of 63%
for surface reflectivity of the beetle, Hadley calculated a heat balance
which read as follows, in cal cm
-2
min
-1
: radiation (0.141) + metabolism (0.003) -f conduction (?) = convection (0.134) + evaporation
(0.008).
Clearly the roles of evaporative heat loss and of metabolism were minor
in these circumstances, and this seems to be the general rule for many
insects and other arthropods. In flying locusts, for example, the thoracic
temperature in dry air is but 1 °C below that in moist air where transpiration is prevented (Church, 1960), and a further instance is supplied by
my own measurements on Onymacris spp. in the Namib Desert (Edney,
1971b) referred to below in connection with the effects of surface color.
On the other hand, not all arthropods have highly impermeable integuments, and in these evaporative cooling may be of importance.
It has been known for a long time that many insects go through a period
of warming up by shivering before flight, and hawk moths provide good
examples. Heinrich and Bartholomew (1971) showed that in Manduca
sexta, which lives in the Mojave Desert of California and feeds on Datura
stramonium (Jimson weed), the thorax is warmed to 37°-39°C before
flight by muscular activity, the rate of increase being linear but depending
on ambient temperature. Interestingly enough, the temperature of the abdomen during this warm-up period remains near the ambient temperature,
probably as a result of the cessation of blood flow from the thorax. During
free flight, Manduca maintains its thoracic temperature within 1° of 42°C
in ambient temperatures from 17°-32°C (Heinrich, 1970, 1971). Metabolic heat generated by the flight muscles tends to overheat the thorax,
but the excess is carried away by circulating blood which is pumped rather
vigorously by the dorsal tubular heart into the hemocoele of the thorax
from where it flows back to the cooler abdomen.
C. EVAPORATIVE COOLING IN ISOPODS
Some years ago, I investigated the situation in isopods and the sea slater
Ligia, 2i littoral isopod about the size of Blatta orientalis. (These are by
no means desert animals, but the information about them may serve to