1966 ] Nutting — Paraneotermes 141 
when the relative humidity was as high as 80 or 90%. It is hardly 
necessary to point out that the relative humidity at the time of flight, 
generally well below 60%, is closely correlated with the high tem- 
peratures during afternoon and evening (fig. 4). 
Evaporation and Saturation Deficit. — Since evaporation was 
measured 2.5 miles from the study area, the following figures are 
included only as a general indication of the evaporative power of the 
atmosphere in a region occupied by Paraneotermes. Figure 5 shows 
that flights occur during the season with the highest evaporation rate 
of the entire year. The mean daily evaporation rate for 29 flight 
days in 1962 and 11 in 1963 was 0.36" (range, 0.10 — 0.5 1) in 
1962, and 0.30" (range, 0.10 — 0.56) in 1963 (U. S. Department 
cf Commerce, ’62; ’63). The overriding influence of high tempera- 
ture at this time of year insures a high level of evaporation in spite of 
the moderating effect of the rains which are, at best, sporadic and 
only infrequently heavy. 
The saturation deficit at the study site provides another measure 
of the dryness of the atmosphere which the alates encounter during 
their brief period of actual flight. Also closely dependent on tem- 
perature, this factor varies about a very high level during the flight 
season. The weekly mean saturation deficit for the approximate 
time of flight (sunset plus 15 minutes) is plotted in figure 5. In 
1962 the 29 daily flights began at a mean saturation deficit of 12.8 
mm. Hg (range, 2.1 — 20.7 mm. Hg). The typically wide daily 
variation in saturation deficit is shown in figure 4. Although this 
information may be of little value at present, laboratory experiments 
might be designed to determine whether the alates respond to differ- 
ences in relative humidity or in evaporation (or saturation deficit), 
and thus provide a means for evaluating the effect of these factors 
on flight behavior. For example, perhaps the contrast between a 
relatively dry atmosphere and the higher moisture levels in micro- 
climates at the level of the soil provides gradients which are used by 
the alates, first in leaving the colony and later in seeking out suitable 
nesting sites. 
Light 
The rather consistent appearance of alates at lights in the early 
evening for several seasons suggested that light intensity might be a 
cue for the initiation of daily flight periods. The times of “first 
sightings” of alates during 1962 are shown in relation to time of 
sunset for Tucson in figure 7. Starting in 1963, measurements of 
light intensity at the zenith have been made with a photographic light 
