16 NORMAL AND EXPERIMENTAL POLLINATION. 



Bombus bifarius lands on the sepal platform with its head above the group 

 of stamens and pointing toward the base of the nectaries. It then moves 

 up to such a position that its thorax is above the stamens and its head is 

 at the base of the nectaries, the hind legs resting upon the two front sepals. 

 As it sucks nectar, its body moves back and forth, thus rubbing the lower 

 side of the thorax and abdomen against the anthers and stigmatic surfaces. 



None of the North American species of Aconitum has previously been 

 studied with respect to its pollination. In Europe, A. napellus and 

 lycoctonum have received the most attention, and Kronfeld has pointed out 

 that the species of this genus are almost exclusively bumble-bee flowers, 

 the areal limits of Aconitum and Bombus coinciding in a remarkable way. 

 In addition to Bombus, Knuth cites only visits by Macroglossa and Lycaena 

 (1908:50); in the Pike's Peak region the sole visitors are bumble-bees. 



EXPERIMENTS. 



CHANGE OF POSITION. 

 Horizontal racemes. — Racemes were placed in a horizontal position 

 and attached to the normal ones by thread. Since the flowers of Aconitum 

 are fastened at various points around the stem, the tip of the hood on 

 some pointed sidewise and in others it pointed up or down. Both Bombus 

 juxtus and bifarius easily went into flowers with the hood pointing down- 

 ward, using the side petals instead of the lower sepals as a landing plat- 

 form. They then secured nectar without taking an uncomfortable position. 

 B. juxtus hovered over the hood in flowers where it pointed sidewise. 

 Finally, it went to the open end where the stamens were exposed, pushed 

 its proboscis about as if exploring, and then found the nectary. The next 

 flower had the tip of the hood pointing up, and B. juxtus landed at once 

 on the side petals and took nectar without any inconvenience. In another 

 case where the hood pointed sidewise, the bee hovered over the spur first, 

 apparently trying to find a place to land. It then went to the side of the 

 flower and attempted to land, but slipped off the edge of the side petal, 

 succeeding only on the second attempt. The next flower was horizontal 

 with the hood directed upward. The bee hovered at the spur, went to 

 the other end, and landed easily. It did not learn by one experience that 

 it could find the nectar at this end when the flower was in the horizontal 

 position. In each case it hovered where it would normally expect to alight, 

 before going to the place where landing was possible. 



Racemes inverted. — Racemes were cut off and tied to the plant in 

 an inverted position and a piece of moist absorbent cotton was placed 

 around the cut end to prevent wilting. Some individuals of both Bombus 

 juxtus and bifarius found the nectar readily, while in other cases they were 

 frightened away or gave up too soon. The former passed over some flowers 

 without attempting to land, and merely hovered near others. A few 

 individuals started to alight and then flew away as if bothered by the 

 change. Some bees landed at the lower sepals, which now pointed up and 

 occupied the position usually taken by the hood, turned around, and 

 quickly walked into the flower upside down. The next flower was normal 

 and B. juxtus went to this as usual. The third was inverted and the bee 



