VI. DESERT ARTHROPODS
363
and head movements, but his evidence needs confirmation. CloudsleyThompson (1964a) was unable to measure any differences in temperature
or humidity between the air inside and outside the subelytral cavity of
Pimelia grandis, although he found that removal of the elytra, thus exposing the tergites of the abdomen, led to an increase in transpiration which
he ascribed to air movement in the vicinity of the spiracles. However, it
would seem that if air movement lowers the water activity outside an open
spiracle it would also raise the oxygen activity, so that the spiracles could
be more nearly closed.
There is no correlation between strongly convex elytral surfaces and
diurnally active species, both flat and curved elytra being found in these
as well as in nocturnally active beetles (Koch, 1961). Furthermore the
extent of the underlying cavity varies greatly with the degree of nourishment and particularly with the state of hydration of individual insects.
The question is not whether the elytra by their presence reduce water
loss. The work of Dizer (1955), Cloudsley-Thompson (1964a), Ahearn
and Hadley (1969), and Hadley (1970b) shows clearly that this is so.
The question is whether an air cavity between the elytra and the cuticle
of the abdominal tergites reduces water loss—and this is not proven. For
the present, the conclusion, as Cloudsley-Thompson suggests, must be that
the size of subelytral cavities of desert tenebrionids has net been shown to
be adaptive for heat or water balance.
VII. Phenology and Behavior
In this final section we come to a consideration of the parts played by
phenology and behavioral mechanisms in adapting arthropods to deserts,
and there is good evidence that they are of the greatest significance. Timing
of life cycles, leading to appearance and feeding at the right time, is very
general. The cricket, Macrobaenetes, occurs in very large numbers on a
few nights in spring in the sand dunes of the Coachella Valley in southern
California. Lawrence (1959) refers to the sand dunes of the Namib Desert
in southwest Africa as being "virtually covered" with a flat, ivory-colored
beetle, Lepidochora, at certain seasons. These sudden flushes of very large
numbers of individuals of a species have often been reported (Edney,
1966b).
There is reason to believe that many desert forms are slow developers
(Bodenheimer, 1953; Hafez and Makky, 1959; Tinkham, 1965), and I suspect that Arenivaga takes at least 1 year to reach maturity in the field.
In the laboratory it takes up to 18 months. This is all the more striking
in view of the high temperatures encountered. Linked with slow develop-
363
and head movements, but his evidence needs confirmation. CloudsleyThompson (1964a) was unable to measure any differences in temperature
or humidity between the air inside and outside the subelytral cavity of
Pimelia grandis, although he found that removal of the elytra, thus exposing the tergites of the abdomen, led to an increase in transpiration which
he ascribed to air movement in the vicinity of the spiracles. However, it
would seem that if air movement lowers the water activity outside an open
spiracle it would also raise the oxygen activity, so that the spiracles could
be more nearly closed.
There is no correlation between strongly convex elytral surfaces and
diurnally active species, both flat and curved elytra being found in these
as well as in nocturnally active beetles (Koch, 1961). Furthermore the
extent of the underlying cavity varies greatly with the degree of nourishment and particularly with the state of hydration of individual insects.
The question is not whether the elytra by their presence reduce water
loss. The work of Dizer (1955), Cloudsley-Thompson (1964a), Ahearn
and Hadley (1969), and Hadley (1970b) shows clearly that this is so.
The question is whether an air cavity between the elytra and the cuticle
of the abdominal tergites reduces water loss—and this is not proven. For
the present, the conclusion, as Cloudsley-Thompson suggests, must be that
the size of subelytral cavities of desert tenebrionids has net been shown to
be adaptive for heat or water balance.
VII. Phenology and Behavior
In this final section we come to a consideration of the parts played by
phenology and behavioral mechanisms in adapting arthropods to deserts,
and there is good evidence that they are of the greatest significance. Timing
of life cycles, leading to appearance and feeding at the right time, is very
general. The cricket, Macrobaenetes, occurs in very large numbers on a
few nights in spring in the sand dunes of the Coachella Valley in southern
California. Lawrence (1959) refers to the sand dunes of the Namib Desert
in southwest Africa as being "virtually covered" with a flat, ivory-colored
beetle, Lepidochora, at certain seasons. These sudden flushes of very large
numbers of individuals of a species have often been reported (Edney,
1966b).
There is reason to believe that many desert forms are slow developers
(Bodenheimer, 1953; Hafez and Makky, 1959; Tinkham, 1965), and I suspect that Arenivaga takes at least 1 year to reach maturity in the field.
In the laboratory it takes up to 18 months. This is all the more striking
in view of the high temperatures encountered. Linked with slow develop-