PALMEX'S ORGAN IX HEPTAGENIA AND ECDYURUS. 26l 



inverted in the water, or when the specimens are dropped on a 

 stone in the water in a dark-room they remain on the upper 

 surface or on the sides of the rock for a considerably longer time 

 than do the normal individuals By the removal of the organ 

 the nymphs have no doubt lost, to some extent, their usual 

 keen sense of orientation, for under such conditions they would 

 !'< seen on the top, sides or any part of the rock for hours, days, 

 and even weeks after the operation had been performed. The 

 same was true of every lot experimented with. It was also 

 noticed, with several lots of operated specimen-, that the tend- 

 ency to remain on almost any purl of the stone was gradually 

 dimini-hed and that after -everal week- and in some rases- about 

 two months there were comparatively few individuals l( , he seen 

 on the upper surface, regardless of the fact that in some -pedal 

 experiments the stone would be in\ cried at e\ cry >b-er\ ation 

 with the view of bringing more specimen- to the upper surface 

 \\ith little disturbance. This growing partiality to the lower 

 -nit". ice of the stone does not lessen the significance oi their tornu-r 

 behavior, for, from my studies on the po\\er of the tormation 

 of a--odations in the nymphs of II. inter (nnntata \ \\ '< 1-edalek, 

 *I2) it was found that they gradually formed -e\eral i\\n>< \ 

 a ' iaiions. The associations formed in the- rimeiu- were 



in connection with their thi^motactic inclination-, which were 

 in ^ivat part responsible for the ^radual de. i ! the number 



i on lop, and the gradual diminishing of ihe time the various 

 indi\ iduals required to retreat to tiie l..\\er surfa 



In another paper (\Vodsedalek, 'u) on the natural history 

 and general behavior uf these insects I ha\e di-cu ed their 

 thigmotaxis in considerable detail. It was le.irned from a -imple 

 experiment that their thigmotactic ])ropen-ities are. be-,t -ati-lied 

 when their dorsal as well as their \entral -urface- are in contact 

 \\ith some object. "XYhen several specimen- are placed in an 

 aquarium they mass together into clusters where they remain for 

 hours, and if recently collected, even days. As soon as a rock or 

 any other object is placed in the water the loose forms swim toward 

 it, while con-iderable time often elapses before the masses are 

 broken up. Two long bricks were placed one over the other in a 

 basin of water and between them were placed small pebbles 



