22 BULLETIN 1328, IT. S. DEPARTMENT OF AGRICULTURE 



2. On dull, overcast days the bees usually do not begin their flight 

 until the temperature has risen at least 2° C. (3.6° F.) higher than 

 the usual temperature for beginning flight for that period. This 

 was very marked in April, May, and the early part of June, but 

 was not so obvious during the latter part of June and the month of 



3. The internal condition of the colony is important in determin- 

 ing the temperature at which flight will begin. Colonies in the 

 apiary that were stronger than the one under experimentation al- 

 ways commenced their flight at somewhat lower temperatures. The 

 lowest temperature at which the flights from this colony began was 

 on two days in April, when the temperature was as low as 10° C. 

 (50° F.). 



4. No marked difference in flight temperature between honey flow 

 and dearth conditions in the same period can be found, and no con- 

 clusive evidence has been observed from these data that a heavy 

 honey flow induces the bees to go out in large numbers at a tempera- 

 ture lower than they would if no nectar were available. 



5. If from a lower temperature the thermometer rises rapidly to a 

 condition suitable for flight, the temperature at which flight com- 

 mences is usually 2° C. or more higher than if the temperature had 

 risen slowly. In the first case, the flight curve usually shows a more 

 gradual ascent to its peak ; in the latter, a more rapid rise is observed, 

 indicating, as might be expected, that the hive as a whole must ab- 

 sorb a certain amount of heat before the bees become active enough 

 for a general flight. 



6. A comparison of the temperature at which the flight begins its 

 upward trend and that at which it begins its descent in the after- 

 noon shows that, without exception, the temperature at which the 

 bees begin to slacken their flight activities in the afternoon is higher 

 than that at which they become active in the morning, the difference 

 ranging from 1° to 9 C C. This indicates that it is the waning light 

 which accompanies the approach of sunset, rather than a fall in 

 temperature at that time, that causes the decrease in flight. 



7. A study of the seasonal flight curve (fig. 2) shows that a low 

 morning temperature, by postponing the time at which flight com- 

 mences, may reduce the total possible daily exits by as much as 50 

 or even 75 per cent. Days which appear to be similar in every other 

 respect, but on which a variation in flight of from 10 to 25 per cent 

 is noted, furnish records which show that this difference may be 

 traced to lower temperatures in the early part of the day. 



8. On some excessively hot days the flight curves remained low. 

 Whether this behavior is due to the high temperature or to the 

 dearth existing it is impossible to say. 



THE EFFECT OF LIGHT 



Since temperature and light intensity are so closely correlated, it 

 is exceedingly difficult to differentiate the behavior produced by 

 variations in either of these factors. In the study of the effect of 

 light, a record was made of the degree of cloudiness each quarter 

 of an hour while flight was occurring. Because of variations in the 

 density, altitude, and position of the clouds, to say nothing of the 



