368
E. B. EDNEY
that the insects obtained water from plant food. Their insects, like ours,
showed a daily subsurface activity pattern, commencing after dark, but
Hawke and Farley were unable to come to a firm conclusion as to the
controlling mechanisms involved.
During a recent visit to the Namib Desert, I was able to collaborate
with Erik Holm in obtaining some information about the daily cycles of
activity in relation to microclimatic conditions (Holm and Edney, 1973).
For example, in the dry Kuiseb River bed Onymacris rugatipennis and
Physosterna cribripes were both very plentiful in summer and winter. Figures 24 and 25 show the activity distribution and microclimatic data for
O. rugatipennis. In the summer, maximum beetle activity occurred between
9 and 10 A.M. when the sand surface temperature was 50°C in the sun
and 37°C in the shade. Activity was very low between noon and 3 P.M.
during which time the surface temperature rose to 66°C. A second lesser
peak of activity occurred from 4 to 6 P.M., when the surface was 43°C
in sunlight and 34°C in the shade (Fig. 24). At night, and in the heat
of the day, the insects retired into Eragrostis bushes or into the sand. In
the winter, however, beetle activity was unimodal, with a broad activity
peak centered at 1 P.M. when the surface temperature in the sun varied
between 39° and 51°C on different days (Fig. 25).
In the sand dune areas, species of beetles tend to replace one another
in activity cycles throughout the day and night. For example, during the
summer, O. laeviceps was most active from 7 to 8 A.M. and again from
6 to 8 P.M., while O. plana was active from 9 A.M. to noon and from
4 to 6 P.M. Both species were bimodal in activity but the modes occurred
at different times.
Ahearn and Hadley (1969) similarly found that of the two tenebrionids,
Eleodes armata and Cryptoglossa verrucosa, both from the Sonoran Desert
in Arizona, the latter lost water less rapidly than the former, and they
were able to correlate this with the phenology of the species—thus C. verrucosa alone was active during the hottest summer months, while E.
armata became more abundant on the surface in late summer and fall.
Scorpions also exhibit daily activity cycles, as Hadley and Williams
(1968) observed in the Sonoran Desert of Mexico. By using ultraviolet
light and counting the numbers of scorpions seen in standard rounds, they
found that two species, Vejovis confusus and V. mesaensis, were active
early in the night and became progressively less active later, while Centruroides sculpturatus was equally active throughout the night. The activity
pattern of any one individual was apparently very variable (e.g., they did
not all come out every night), and the authors were unable to identify any
one environmental factor as being responsible for determining the times
of activity, although activity did not occur in two winter months, so that
E. B. EDNEY
that the insects obtained water from plant food. Their insects, like ours,
showed a daily subsurface activity pattern, commencing after dark, but
Hawke and Farley were unable to come to a firm conclusion as to the
controlling mechanisms involved.
During a recent visit to the Namib Desert, I was able to collaborate
with Erik Holm in obtaining some information about the daily cycles of
activity in relation to microclimatic conditions (Holm and Edney, 1973).
For example, in the dry Kuiseb River bed Onymacris rugatipennis and
Physosterna cribripes were both very plentiful in summer and winter. Figures 24 and 25 show the activity distribution and microclimatic data for
O. rugatipennis. In the summer, maximum beetle activity occurred between
9 and 10 A.M. when the sand surface temperature was 50°C in the sun
and 37°C in the shade. Activity was very low between noon and 3 P.M.
during which time the surface temperature rose to 66°C. A second lesser
peak of activity occurred from 4 to 6 P.M., when the surface was 43°C
in sunlight and 34°C in the shade (Fig. 24). At night, and in the heat
of the day, the insects retired into Eragrostis bushes or into the sand. In
the winter, however, beetle activity was unimodal, with a broad activity
peak centered at 1 P.M. when the surface temperature in the sun varied
between 39° and 51°C on different days (Fig. 25).
In the sand dune areas, species of beetles tend to replace one another
in activity cycles throughout the day and night. For example, during the
summer, O. laeviceps was most active from 7 to 8 A.M. and again from
6 to 8 P.M., while O. plana was active from 9 A.M. to noon and from
4 to 6 P.M. Both species were bimodal in activity but the modes occurred
at different times.
Ahearn and Hadley (1969) similarly found that of the two tenebrionids,
Eleodes armata and Cryptoglossa verrucosa, both from the Sonoran Desert
in Arizona, the latter lost water less rapidly than the former, and they
were able to correlate this with the phenology of the species—thus C. verrucosa alone was active during the hottest summer months, while E.
armata became more abundant on the surface in late summer and fall.
Scorpions also exhibit daily activity cycles, as Hadley and Williams
(1968) observed in the Sonoran Desert of Mexico. By using ultraviolet
light and counting the numbers of scorpions seen in standard rounds, they
found that two species, Vejovis confusus and V. mesaensis, were active
early in the night and became progressively less active later, while Centruroides sculpturatus was equally active throughout the night. The activity
pattern of any one individual was apparently very variable (e.g., they did
not all come out every night), and the authors were unable to identify any
one environmental factor as being responsible for determining the times
of activity, although activity did not occur in two winter months, so that