Vergara - 1976 - Physiological and morphological adaptability of ri

354 CLIMATE AND RICE
constant-factor relationship. Since that time a considerable amount of twiork has
been done by many workers attempting t0 define as precisely as possible the
differences that occur in the developmental. reproductive, and survival responses
of insects to variable versus constant temperatures.
Three approaches to the design of variable temperature experiments have been
devised. In the first, insects were exposed to tvro different temperature levels, the
change from one to the other being made only once during the exposure period.
The second approach involved exposures to two constant temperatures in altematiiig
periods. usually 12 hours at each thermal level. thus in rough approximation
to the diurnal cycle of natural temperature patterns. The third
technique W215 to expose insects to smoothly fluctuating, cyclical temperatures
closely simulating the actual diurnal patterns of temperature occurring in the
field. The first two approaches required nothing new in the way of equipment.
except perhaps the addition of a time clock. The third approach required an
altogether different. and much more complex piece of equipment. involving
complicated temperature and humidity control instruments. refrigeration systems,
humidification systems and sensing devices. and so on,
The differences in development of insects in fluctuating or alternating temperatures.
as compared to constant conditions. seem to show for many insects
a general pattern. first enunciated by Ludwig and Cable (1933) who used alternating
temperatures to examine pupal development in the vinegar fly. Drosophila
melanogaster- Meigen. This general relationship states that at the lower temperature
levels, fluctuating temperatures accelerate development relative to what
would be expected under constant temperatures. while in the mid-range of
temperatures no difference or very little difference occurs. and in the upper
range development is decelerated. The author. in an effort to verify this relationship.
found it to occur for the tropical fruit flies Dacus dorsalis Ilendel.
D. cucrrrbitae (Coquillet). and C. crrprtata, when different stages of these species
were exposed to cyclical temperature patterns in bioclimatie chambers (Messengcr
and Flitters. 1958. 1959). It was later shown to apply in the ease of the
alfalfa aphid Therioapirrs trrfolii (Monell) (Messenger, 1964a) and the hymenopterous
parasitic Wflfip Praon exsoleium Nees (Messenger. 1969).
Another aspect of constant temperature relations and insect development that
has been brought into question by work with fluctuating conditions is the matter
of the threshold for development. 1t is possible to expose insects to fluctuating
temperature patterns such that some of the daily exposure period involves subthreshold
values. The constant temperature threshold (and temperature summing)
concept requires that during each of these exposure periods to sub-threshold
temperatures. development should be considered to cease. IIou-‘ever. studies
by the author with the tropical fruit llies (vide supra) and the aphid, T. rrrfolii.
suggests that this may not be realistic. For example. when froung aphids are
exposed to fluctuating temperatures in bioclimatic chambers such that the mean
temperature falls exactly on the constant temperature threshold, 101°C.
development proceeds at a rate more than one-third faster than that predicted