No. 497] 



TROPISMS OF INSECTS 



301 



as it passes from flower to flower, often traversing a con- 

 siderable space in a nearly straight line toward a particu- 

 lar plant whose location may be known to it through past 

 experience. Thus it continues along, sometimes rising 

 slightly or flying lower, but exhibiting no movements 

 whatever which could be attributed to geotropism or 

 phototropism. 



Let it be caught in the collector's net, however, and it 

 immediately develops the negative geotropism, flying 

 wildly about and seeking to ascend. If the net be kept 

 inverted it will not escape until it accidentally drops out 

 as a result of flying at the cloth of the net or of losing its 

 foothold in crawling upward. Once out, it soars upward 

 perhaps a short distance, and then resumes its former 

 occupation. 



Do these actions of the fly and the bee when confined, 

 which are characteristic of other insects as well, represent 

 their normal tropisms! 



It has been usually assumed that they do, and several 

 ingenious explanations have been suggested which en- 

 deavor to show why phototropism and negative geo- 

 tropism become inactive in nature after certain periods, 

 since the logical result of their continued action never 

 presents itself to observation. 



From the behavior of species in nature, these are most 

 certainly not normal and are evidently caused by the con- 

 ditions of the experiment. The most probable explana- 

 tion of their appearance is that they are the expression of 

 an instinct to seek the open whenever disturbed. In 

 nature this freedom can always be obtained by flying up- 

 ward and toward the light, that is to say, by phototropic 

 and negatively geotropic movement which carries them 

 away from all obstacles. Such a reflex in response to 

 disturbances is a very valuable one and is no doubt main- 

 tained by natural selection, since it automatically offers an 

 avenue of escape from disturbing conditions or danger. 



In some species this reflex is in another direction, and 

 these exceptions are most instructive in support of this 



