416 C. H. TURNER 



that inhabit a pond of about 300 square feet, Weiss (98) ob- 

 served interesting cases of what he calls positive hydrotropism. 

 When the wingless beetles Gerris marginatus were removed one 

 to nine yards from the pond they immediately returned to it. 

 When removed ten yards from the water they had some trouble 

 in getting started in the right direction; but finally reached 

 the pond. Thirty yards from the water they seemed to be 

 lost. The case of Dineutes assimilis, a winged beetle, is even 

 more interesting. When removed nine or ten feet from the 

 water, it tried to walk to the pond, then arose and flew directly 

 to it. When removed to a distance of seventy-five feet, it walked 

 about in all directions, then arose, on its wings, to a height 

 of twenty feet and flew directly to the water. When removed 

 half a mile from the pond, it soared in a widening sub-spiral to 

 a height of seventy-five feet and flew off in the direction of the 

 water. He does not know whether they reached the water or not. 

 3. Phototropism. — Beutel-Reepen (6) does not think swarming 

 honey bees are positively heliotropic. 



OLIGOTROPISM 



Robertson (75) does not accept the opinion that " Therefore 

 the entomophilous flora of a region, as a whole, is not better 

 pollinated because a part of the bees are oligotropic than it 

 would be if they were all poly tropic." He writes: ' My view 

 is that the bee fauna is all that the flora will support, that there 

 is a constant competition between bees, and that natural selec- 

 tion favors those which are the most diversified, i.e., the least 

 competitive in food habits." He believes that short flight is a 

 result of oligotropy. To show the reasonableness of his con- 

 tention he insists that if a bee limits itself to a given species of 

 flowers it gains the immediate advantage of being able to antici- 

 pate others in their visits to the chosen plant. By locating near 

 the flowers, it may augment this advantage, and, by concen- 

 trating its attention on that flower, learn to manipulate its 

 pollen to greater advantage and even develop special structures 

 which will increase this advantage. In support of this last 

 statement, he cites the following examples: — (1) Bees that collect 

 large pollen have loosely plumose scopae, while others which 

 collect from the compositae have densely plumose scopae. (2) 



