PHOTOREACTIONS OF PARTIALLY BLINDED WHIP-TAIL 



SCORPIONS. 



By BRADLEY M. PATTEN. 



(From the Laboratory of Histology and Embryology, School of Medicine, Western 

 Reserve University, Cleveland) 



(Received for publication, December 11, 1918.) 

 INTRODUCTION. 



The experiments dealt with in this paper were devised to ascertain 

 (1) the relative effectiveness as photoreceptors of the whip-tail scor- 

 pion's median eyes, lateral eye groups, and cutaneous sensitive areas, 

 and (2) the effect on orientation produced by symmetrical and by 

 asymmetrical interference with the photoreceptive mechanism. 



Most of the animals familiar in the Hterature on phototropism have 

 light-sensitive mechanisms which consist of a single pair of receptors, 

 or mechanisms in which one of the receptors overshadows the others 

 in effectiveness. The whip-tail scorpion has three pairs of photore- 

 ceptors, each of which, acting alone, is capable of bringing about orien- 

 tation. It offers, therefore, unusually varied possibihties for experi- 

 ments altering the normal, symmetrical condition of the photorecep- 

 tive mechanism. 



As a basis for working on partially blinded scorpions, the responses 

 of normal animals were quantitatively determined. The results of 

 this prehminary work have already been published (1), but certain of 

 the more important points may be summarized here. The species of 

 whip-tail used (Mastigoproctus giganteus Lucas) was found to be 

 negatively phofotropic, and very consistent in its precision of orienta- 

 tion. The method of measuring the normal reactions and the ex- 

 perimental conditions under which the measurements were carried out, 

 were chosen with a view to making even slight changes in reaction 

 ■clearly recognizable. Responses to known intensities of illumination 



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