166 



DISPERSION OF POLLEN BY ANIMALS. 



For flowers to serve as refuges and nocturnal haunts for insects they need 

 not necessarily be fashioned into hollow receptacles, pouches, bells, or anything 

 of the kind, as is proved by the following observation. In my garden the flowers 

 of plants of Phlox paniculata, indigenous to North America, and of the Canadian 

 Golden-rod (Solidago Ganadensis), which bloom simultaneously in the autumn, 

 were visited by numberless flies particularly by the large bee-like Eristalis arbus- 



Fig. 242. Aristolochia ringens. (After Baillon.) 



torum which feasted on as much of the pollen as was accessible to them. By 

 day they stayed as readily on the Phlox-flowers as on the Golden-rod ; but as 

 night approached they one and all migrated to the Golden-rod. Not a single fly 

 remained on the Phlox, whereas the great bunches of Golden-rod capitula were 

 covered with hundreds of flies. On the following night, which was still and cold, 

 I examined the flowers by the light of a lantern and found that less dew was 

 deposited on the flowers of the Golden-rod than on those of the Phlox, and this 

 led me to conjecture that the temperature of the former flowers had risen in the 

 night above that of the surrounding atmosphere. And this turned out to be the 

 case. A thermometer inserted in the middle of the inflorescence of the Golden-rod, 



