THYSANOPTERA AND ORTHOPTERA. 199 



TABLE IV. — Showing number of thrips taken in spikes of Pickerel weed {Fonte- 

 den'a cordata) in various parts of Ilush Lake. 



Location of pickerel weed Number of thrips taken 



examined. on 16 spikes. 



20 ni. from northwest shore 2 



200 m. from west end 1 



200 m. from west end (J 



500 m. from west end 



Middle of lake 2 



Middle of lake 1 



100 m. from southeast shore 1 



50 m. from southeast shore 1 



150 m. from south shore 



30 m. from east shore 2 



These figures go to show that there are no more thrips in one part of 

 the lake than in another, and hence to oppose the view that they have 

 drifted to the plants on the water. It would be expected, furthermore, 

 if this drifting occurred, that many specimens would drift across the 

 lake without meeting any plants, and could be found on the leeward 

 side. Two hours careful towing at the surface on the southeast side 

 of the lake with a muslin net after a long-continued northwest wind, 

 however, failed to secure a single specimen. 



I have concluded, therefore, that the thrips did not cbift to the pic- 

 kerel weed, but flew to it. This would involve flights of at least 100 

 m., and I know of no reason why single flights should not be much long- 

 er. It is barety possible that the thrips have been carried to the plants, 

 for there are a few private boats on the lake, but the traffic is slight, 

 and the boats usually carry only passengers, which does not present a 

 very likely method of transporting thrips for considerable distances. 

 Moreover, if this were the method of dissemination, one would expect 

 to find more thrips on the Pontederia near the boat-landings than else- 

 where, which was not the case. If it is true that thrips fly long dis- 

 tances, they do not remain near roadsides because the\' are incapable 

 of sufficiently long flight. 



(3) Aimlessness of flight. This factor can not, I believe, be opera- 

 tive in the cases examined. Except where positive phototaxis may ex- 

 plain the direction of their flight, I believe their migrations to be prac- 

 tically aimless. For thrips do not apparently see obstacles or enemies 

 several centimeters away; and in the experiment described above in 

 which a fresh and an old thistle head were placed 3 cm. apart, only 15 

 of the probable 75 thrips on the old head migrated to the fresh one. 

 Supposing their flight to be aimless, if the distribution of the food 

 plants were irregular, then many individuals would probal)ly be lost by 

 flying to locations where there were no plants. But in the cases ex- 



