Spatially Determined Reactions 189 



of righting itself: a quick "flop" executed with the tail, 

 and a slow and laborious raising of itself on one side and 

 tipping over (179). 



Many Crustacea show marked responses to gravity: 

 for example, Parker found decided negative geotropism in 

 the females of the marine copepods whose depth migrations 

 he studied. It seems to be needed to counteract the ten- 

 dency of the animals to fall to the bottom by their own 

 weight (534). In certain copepods, light was observed to 

 change the sense of the response to gravity, not by taking 

 its place as a directive stimulus, but apparently by pro- 

 ducing some physiological change in the animals. Their 

 normal geotropism was positive, that is, they had a ten- 

 dency to move downwards. In darkness, however, their 

 geotropism became negative. They were also negatively 

 phototropic to strong light. If, when in the negatively 

 geotropic phase, they were illuminated from below by in- 

 tense light, from which they would ordinarily have moved 

 away, the change from negative to positive geotropism 

 induced by the light was of sufficient influence to make them 

 move downward toward it (210). Other facts regarding 

 the relation of geotropism and phototropism are mentioned 

 on pp. 209 ff . 



55. Orientation to Gravity: Spiders and Insects 



Spiders and insects have no statolith organs. Bethe 

 thinks that equilibrium is maintained in their case as a 

 natural result of the position of the centre of gravity and the 

 distribution of air in the body. He supports this view by 

 experiments in which dead insects, allowed to fall through 

 the air, assume the normal position, and is inclined to think 

 that all animals without special static organs maintain their 

 balance in this way (48). Negative geotropism in certain 



