TROPISMS AND RELATED PHENOMENA 155 



reaction which is quite common among Infusorians, and to which 

 Jennings has given a special name, motor reflex or motor reaction.* 

 Garrey found that the phenomenon described above can be produced 



by many inorganic acids, provided their concentration is n or 



1500 



above. AlkaUs bring about the same effect at a somewhat higher 



concentration; namely, — n. Salt solutions require a still higher 



concentration, e.g. NaCl, and LiCl require a minimum concentration 



of about — , and KCl about — . MgCL„ CaCL,, SrCL,, and BaCL, acted 



at a lower concentration ; namely, to . ZnSO,, ZnCL, CuSO^, 



100 200 * ^ 



and AgNO, were effective in a concentration of to . 



1000 2000 



The immense biological r61e of these reactions is known to every one 

 who has worked with insects. The finding of food, the depositing of 

 eggs, and the meeting of the two sexes for the process of pairing are 

 determined to a large extent by diffusing substances. I may relate 

 the following observation f which certainly has been made often enough 

 before. A female butterfly was put into a small, closed wooden box 

 which was suspended from the middle of the ceiling of a room whose 

 windows were open. At first no other butterfly of the species to which 

 the female belonged was visible, but during the next half hour three 

 male butterflies of the same kind approached the house, stopped at the 

 window, then flew into the room, and settled on the wooden box through 

 the openings of which they tried to enter. This effect could have been 

 produced only by an emanation from the female butterfly. As an 

 example of how emanations direct the motions of females that are ready 

 to deposit their eggs, the fact may be cited that certain volatile sub- 

 stances emanating from meat "attract" the female fly. If fat and 

 meat of the same animal are put side by side on the window sill, the 

 female fly will light on the meat, where she deposits her egg, but not 

 on the fat. This tropismhke reaction guarantees the perpetuation 

 of the race, inasmuch as the larvae feed and develop on meat, but not 

 on fat. 



5. Stereotropism (Thigmotropism) 



Certain animals are compelled to put their bodies as much as pos- 

 sible into contact with soKd bodies, while other organisms show the 



♦ Jennings, Am. Jour. Physiology, Vol. 2, p. 374, 1899 ; and numerous subsequent papers 

 by the same author. 



t Loeb, Animal Heliotropism and its Identity with the Heliotropism of Plants, Wurz- 

 burg, 1889. 



