REACTIONS OF WHIP-TAIL SCORPION TO LIGHT 257 



remained comparatively passive, while the posterior legs acted 

 in a manner just the reverse of the anterior pair. 



During the prehminary experiments various methods of 

 handling the animals were tried. It was found that picking 

 them up with forceps lowered vertically down upon the ani- 

 mals between the walking legs in such a way as to seize them 

 across the thorax gave least disturbance. The matter of hand- 

 ling was of great importance because in animals over-stimu- 

 lated mechanically the 'fighting reaction' obliterated all other 

 responses. Particularly was this true of the males. The 

 vibration of the tail, however, served as a warning of over- 

 stimulation, so that with care it was possible to ehminate this 

 complicating factor and obtain consistent reactions to light. 

 The preliminary experiments served to demonstrate that Mas- 

 tigoproctus giganteus is a typical negatively phototactic animal, 

 and that quantitative experiments could be planned on that 

 basis. 



As has been already stated, the experiments on normal animals 

 were designed primarily to serve as a basis of comparison for 

 subsequent experiments directed toward the determination of 

 the relative effectiveness of the various photoreceptors. To 

 ascertain the role played by each part of the receptive mechan- 

 ism, the method of progressive elimination appeared to be most 

 practicable. Under this method three general types of interfer- 

 ence with the photosensitive organs would be possible: uni- 

 lateral, bilateral, and a combination of unilateral on one set of 

 organs with bilateral on another. With this scheme of pro- 

 cedure in view, it was attempted to select for measurement the 

 normal reactions which would be most readily, and most 

 recognizably disturbed by the proposed elimination experiments. 



We know that a negatively phototactic animal will turn and 

 move away from the source of light, when placed heading across 

 a horizontal beam, or when placed heading into the beam. 

 These responses are ^initiated in the photoreceptive mechanism, 

 and the amount of deflection, other factors being constant, de- 

 pends on the sensitiveness of the mechanism. Consequently, if 

 these two reactions are measured for normal animals, and then 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 23, NO. 2 



