374 PROCEEDINGS OF THE AMERICAN ACADEMY, 



tions were away from the light in excess of those toward the light, 

 whereas in Tenebrio the excess in this direction ranged from 66 per 

 cent to 92 per cent of the total number of trials. The results of the 

 experiments on Oniscus are represented graphically in Figure 12. 



In the trials with both lights the records on the two sides are nearly 

 balanced, there being an excess of only 7 per cent of the reactions away 

 from the larger light. This excess again lies in the direction we should 

 expect if the eyes enable the animal to discriminate between the two 

 luminous areas, but, as in the case of Tenebrio, it is too small to be of 

 great significance. It will be noticed by comparing the figures ob- 

 tained with the large and small lights used separately that the result 

 is the opposite of that obtained with the two lights acting simul- 

 taneously, and also opposite to the results in the case of Tenebrio, for 

 in Oniscus a larger percentage turned away from the small light than 

 from the large one. Since the experiments on Tenebrio and on Oniscus 

 were performed at different times, it is possible that a difference in the 

 (absolute, not relative) intensities of the lights at the two times may 

 be responsible for the want of harmony in the results. When both 

 lights were used at the same time, they were adjusted so that their 

 intensities were equal, and therefore no errors could have arisen from 

 this source. 



It may be concluded from these experiments that, although Oniscus 

 has apparently much better developed eyes than the larva of Tenebrio, 

 its responses to light are of a considerably less definite character. 

 This means that with unilateral illumination Tenebrio turns toward the 

 light much oftener than Oniscus, and although in a frequency polygon 

 representing the reactions to light the modo for each of the animals 

 falls upon the side away from the light, in the case of Oniscus it 

 falls much nearer to the median (indifferent) line than it does in 

 Tenebrio. This may be seen by a comparison of Figures 11 and 12. 

 The greater delicacy of the adjustment to light conditions in Tenebrio 

 is also shown in the reactions to the two lights used simultaneously. 

 There the responses of this animal are concentrated about the indiffer- 

 ent or zero class (Figure 1 1, C), whereas in Oniscus under similar con- 

 ditions (Figure 12, C) they are scattered much more evenly through 

 the nineteen classes. This should not be taken to mean that Tenebrio 

 probably has better vision than Oniscus, but merely that in nature its 

 phototropism is more strongly negative, a state which may be inde- 

 pendent of the acuteness of vision, but is commonly related to the 

 conditions of existence. Whatever evidence is afforded by these 

 experiments as to the comparative ability of the eyes of Tenebrio 

 larvae and of Oniscus to form images lies in the fact of the apparently 



