G. S. Graham-Smith 
497 
flies, since these insects are seldom active before 8 a.m. and do not 
appear to be influenced by the early hours of sunshine. Nor are they 
usually active after 6 p.m. In this country, therefore, only the influence 
of bright sunshine occurring between 8 a.m. and 6 p.m. need be con¬ 
sidered. 
Undoubtedly flies tend to be most active in bright sunshine, though 
on bright days with cold winds they often only emerge from their 
retreats to sun themselves in sheltered situations. They also seem to 
avoid very hot sunshine, for during the hottest hours of exceptionally 
hot days very few flies are caught in traps or enter rooms, and many 
of those confined in exposed traps die. 
Many days occur in the spring and autumn and a few in summer 
on which flies are active only during brief intervals of bright sunshine. 
Cloudy days. In May, October and November flies are usually very 
inactive on dull cloudy days, but during the rest of the season they are 
moderately active on such days, if the temperature is not too low. 
Wind. Flies appear to be extremely sensitive to wind, and on 
very windy days, whether hot or cold, seldom venture into the open, 
though they may be moderately active and abundant in sheltered 
situations. Days with very little wind and otherwise suitable are the 
most favourable. 
Rain. Flies seldom venture out of their hiding places in rain, fog, 
or mist, but are often very abundant between heavy showers on warm 
days. 
Temperature. The activities of flies are more influenced by tempera¬ 
ture than by any of the weather conditions yet mentioned. When the 
temperature is low they are very inactive, and seldom make any attempt 
to move, and on the other hand when the temperature is very high they 
seem to suffer from the heat. 
A number of charts were constructed on which the daily numbers 
of flies caught in traps, the hours of bright sunshine and of rainfall 
between 8 a.m. and 6 p.m., the mean velocity of the wind, the maximum 
temperatures recorded by thermometers exposed to the sun and in the 
screen, the minimum temperatures recorded by thermometers in the 
screen and on the grass, and the mean temperature in the screen were 
plotted. These charts showed that the curve for flies caught in traps 
corresponded most closely to the curve for the maximum temperature 
recorded by a thermometer exposed to the sun (Chart 5). 
On a few occasions the flies caught in the traps were counted hourly, 
and the results obtained on June 12, a very fine day, are illustrated in 
