BRADLEY M. PATTEN 451 



anterior illumination. The changes from the normal reaction in- 

 duced by elimination of the lateral eyes, however, do not show the 

 same consistency, the values being 15.9 per cent reduction of the nor- 

 mal reaction under lateral illumination, and 7 per cent reduction 

 under anterior illumination. This discrepancy is, I believe, attribut- 

 able to the anatomical location of the lateral eyes. Their position at 

 the sides of the cephalothorax, surrounded by heavily pigmented 

 chitin, is such that a very small proportion of the light in the field 

 would be effective on the retinulas of the lateral eyes when the animal 

 is directly facing the light. The lateral eyes would, therefore, be op- 

 erating below their capacity during a considerable part of the deflec- 

 tion made by animals headed into the light. On the other hand, they 

 would be operating at maximum efficiency during the greater part of 

 the deflection made by animals subjected to lateral illumination. 

 The median eyes are so placed that they receive approximately the 

 same amount of effective illumination whether the animal is sub- 

 jected to anterior or to lateral illumination. The conditions of shad- 

 ing, involved in the initial positions of the animal account, therefore, 

 for the discrepancy in the two sets of reaction measurements made on 

 animals with their lateral eyes covered. The same considerations 

 would indicate that measurements under lateral illumination express 

 more correctly than experiments under anterior illumination the rela- 

 tive effectiveness of the photoreceptors. 



Another apparent inconsistency in the data of Table VII is revealed 

 by adding the reductions in deflection produced by the separate elimi- 

 nation of each receptor. Taking as a basis the measurements made 

 under lateral illumination, the reductions in deflection caused by the 

 separate elimination of the three photoreceptors total to a value ap- 

 proximately only one-half the normal reaction; yet when the three 

 receptors are simultaneously eliminated the animals are practically 

 insensitive to light (Table V and Fig. 5). It is possible to attribute 

 these conditions to a compensatory increase in activity on the part 

 of the unblackened receptors. While this may be in part responsible, 

 I believe the greater part, at least, of the discrepancy can be other- 

 wise explained. In each of the series of measurements in which some 

 part of the photoreceptive mechanism is prevented from functioning, 

 the reduction in deflection is measured at what might be called the 



