GENERAL FACTORS CONDITIONING AGGREGATIONS 77 



ings. This gave evidence sufficient to prove that the same bees re- 

 turn to their chosen spot regularly .... all were males." 



Here Boyer and Buchsbaum took up the problem, using the soli- 

 tary digger bee, Melissodes agilis or aurigensia, which Professor 

 Cockerell in a personal communication says are variants of the same 

 species. They found Melissodes active in the field only on warm 

 sunny days, with the temperature 16° C. or above, depending on the 

 light. When it is cloudy or cool, the bees remain inactive on the sun- 

 flowers Helianthus annuus and petiolaris, which they frequent. The 

 male bees usually arrange themselves so that two are in contact when 

 two or more become inactive on the same blossom. Several groups 

 of two's have been found on the same flower, isolated from each other. 



The bees are invariably inactive between twilight and sunrise. 

 The beginning of activity depends on the amount of light and on the 

 temperature. Controlled laboratory experiments showed that in 

 bright sunlight activity started under stimulation at about 7° C, 

 while in dim light the first activity came at 9° C. Similarly, spon- 

 taneous activity began at 18° C. in the sunlight and at 21° in dim 

 hght. 



Aggregations of males at night were recorded as follows for one 

 particular group of sunflowers: 



Year 



Singly 



Pairs 



Groups of 3 Groups of 4 Groups of 5 



Groups of 6 



1927. 

 1928. 



100 

 39 



31 



5 



Boyer and Buchsbaum marked some of these bees with paints of 

 different colors so that they could follow individual reactions. There 

 were some 500 flowers in this particular group. Each of these was 

 plotted and followed night after night for its bee population. In all, 

 201 bees were observed in 1927. Thirty-four of these were success- 

 fully painted. Of the 20 painted bees which were seen again, 10 bees 

 returned to the flower they occupied when painted. Eight others 

 returned to the same plant but to a different flower. Fourteen re- 

 turned at least twice to the same plant but to a different flower from 

 that on which they had been painted. In all, 37 returns were noted 

 to the same flower or to a flower within 10 feet of the original one. 



