VI. DESERT ARTHROPODS
359
colors of many desert beetles in terms of heat load, and in a long series
of comparative measurements under a variety of conditions, Stower and
Griffiths (1966) found no significant difference (P > 0.2) between the
body temperatures of the extreme green (light) and red (dark) forms of
desert locust hoppers (Schistocerca). W. J. Hamilton (personal communication) believes that the black coloration of so many desert beetles serves
to raise their temperature early in the morning by absorbing most of the
incident radiation. Cloudsley-Thompson (1964a,b) believes that while pale
colors in desert insects are probably cryptic, the very common black color
is an example of Müllerian mimicry: black beetles taste unpleasant and
it is an advantage to all such bad-tasting species if this fact can be learned
quickly by potential predators.
In some earlier work, Bolwig (1957) found somewhat higher temperatures in the subelytral space of the black beetle, Onymacris multistriata,
than in the white, O. bicolor, except above 38°C when the black beetle
was cooler, perhaps because of ventilation. Hadley (1970b) examined the
problem indirectly in Eleodes armata by painting the black elytral surface
white and measuring the resulting subelytral and body temperatures. He
found a fairly consistent depression of subelytral cavity temperature in the
"white" beetle (about 4°C) but the "white" beetle's body temperature was
not consistently lower than that of the normal black beetle, at least during
15 minutes exposures.
Two species of tenebrionids in the Namib Desert provide a good opportunity for looking at this question: One, Onymacris rugatipennis, is black
all over, while another, O. brincki, which is fairly similar in size, has white
elytra, (Fig. 19). In an extended series of measurements of temperatures
in the thorax and on the surface of the abdomen below the elytra of these
beetles, I reached the conclusion that in sunshine (about 0.9 cal cm
-2
min
-1
)
a white surface may depress the temperature of the abdomen by some
4°-5°C compared with that of the abdomen of a beetle with black elytra
(Edney, 1971b).
The abdominal temperature of living O. brincki was lower than that
of its own thorax (which is black) and also lower than the abdominal
temperature of O. rugatipennis (Fig. 20). The same was. true of dead insects, so that the higher thoracic temperature of O. rugatipennis was not
ascribable to muscular activity, and the effect was abolished by painting
the white elytra with carbon black.
Attitude to the sun's rays affects the temperature of different parts of
a beetle very significantly, and it is not always easy to be sure in field
experiments that this variable is controlled. However, in laboratory experiments where the beetles were exposed to the sun so that the angle of incidence of solar radiation could be varied from directly in front to directly
359
colors of many desert beetles in terms of heat load, and in a long series
of comparative measurements under a variety of conditions, Stower and
Griffiths (1966) found no significant difference (P > 0.2) between the
body temperatures of the extreme green (light) and red (dark) forms of
desert locust hoppers (Schistocerca). W. J. Hamilton (personal communication) believes that the black coloration of so many desert beetles serves
to raise their temperature early in the morning by absorbing most of the
incident radiation. Cloudsley-Thompson (1964a,b) believes that while pale
colors in desert insects are probably cryptic, the very common black color
is an example of Müllerian mimicry: black beetles taste unpleasant and
it is an advantage to all such bad-tasting species if this fact can be learned
quickly by potential predators.
In some earlier work, Bolwig (1957) found somewhat higher temperatures in the subelytral space of the black beetle, Onymacris multistriata,
than in the white, O. bicolor, except above 38°C when the black beetle
was cooler, perhaps because of ventilation. Hadley (1970b) examined the
problem indirectly in Eleodes armata by painting the black elytral surface
white and measuring the resulting subelytral and body temperatures. He
found a fairly consistent depression of subelytral cavity temperature in the
"white" beetle (about 4°C) but the "white" beetle's body temperature was
not consistently lower than that of the normal black beetle, at least during
15 minutes exposures.
Two species of tenebrionids in the Namib Desert provide a good opportunity for looking at this question: One, Onymacris rugatipennis, is black
all over, while another, O. brincki, which is fairly similar in size, has white
elytra, (Fig. 19). In an extended series of measurements of temperatures
in the thorax and on the surface of the abdomen below the elytra of these
beetles, I reached the conclusion that in sunshine (about 0.9 cal cm
-2
min
-1
)
a white surface may depress the temperature of the abdomen by some
4°-5°C compared with that of the abdomen of a beetle with black elytra
(Edney, 1971b).
The abdominal temperature of living O. brincki was lower than that
of its own thorax (which is black) and also lower than the abdominal
temperature of O. rugatipennis (Fig. 20). The same was. true of dead insects, so that the higher thoracic temperature of O. rugatipennis was not
ascribable to muscular activity, and the effect was abolished by painting
the white elytra with carbon black.
Attitude to the sun's rays affects the temperature of different parts of
a beetle very significantly, and it is not always easy to be sure in field
experiments that this variable is controlled. However, in laboratory experiments where the beetles were exposed to the sun so that the angle of incidence of solar radiation could be varied from directly in front to directly