QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 267 



Now by construction hx is the projection of DN on AB and Iry 

 the projection of MD on AB, and by construction hy = 2hx 



This fulfils all the conditions of construction 



The equal lines MD and DN represent equal bilateral sensitive 

 areas inclined to each other at such an angle, MDN, that the 

 surface represented by MD intercepts an area of light twice as 

 great as the surface represented by DN, its projection on the 

 perpendicular to the light rays being twice as great {hy = 2hx). 

 But the light falling on DN is of twice the intensity of the light 

 falling on DM, so that the total amount of light received by each 

 of the equal areas is the same. 



By this method of construction, the average angle of sensitive- 

 ness was computed for four intensity differences, using as a basis the 

 angular deflection of the larvae as determined by experiment. The 

 angles are shown in figure 24. The magnitude of the angles is 

 almost identical in all four cases. This angle, I would emphasize 

 again, probably does not represent the actual position of the sen- 

 sitive surfaces in the larva. ^ There are too many modifying 

 factors intervening between the direction of the rays of light in the 

 field and the angle of incidence of the light on the sensitive areas 

 to pemiit of basing any conclusions directly on the magnitude of 

 the computed angle, for the structural peculiarities of the dioptric 

 apparatus, or of the tissue overlying the sensitive area would 

 change its value to a large extent. However, the computed angle 

 would be constant in a given animal, and the angle of the sensi- 

 tive surfaces would be equal to K times the computed angle 

 where K may be defined as the 'structural constant' of the animal. 



An interesting problem in connection with theories of orienta- 

 tion is whether light operates as '^a constant directive stimulus" 

 or whether changes in the intensity are the main cause of stimu- 

 lation. Without entering into a detailed discussion, some ex- 

 perimental evidence bearing on this question may be presented. 



' The organs concerned with light reception in the blowfly larva have not as 

 yet been identified. Pouchet ('72) concluded that the two pairs of cones on the 

 maxillal segment were not the light sensitive organs, and suggests that the imag- 

 inal discs of the adult eyes may function in the larva. Certain unpublished ex- 

 periments of my own confirm his exclusion of the anterior cones. I have not 

 been able, however, to obtain any positive evidence as to the organs concerned. 



