COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 385 



viously been thrown so that lamp / is lighted, and the animal, being pos- 

 itive to light and protected from lights Lg and Sm by the sides of the 

 runway, starts crawling straight ahead toward /. As soon as its head 

 emerges from the runway it comes under the influence of the lights at 

 the sides, and the lamp /, which was used only to bring it out in proper 

 orientation, is immediately switched off. If the animal now turns 

 toward either of the side lights, 



1-4 



Nc 



So. 



the record is taken, Lg or Sm, 

 according to the way it turns, the 

 record not being taken, however, 

 until it has crawled far enough 

 to cross the outer (15 cm.) circle. 

 Should it go straight ahead and 

 enter the opposite runway, the 

 record is indifferent, or 0. The 

 next trial is made from the 

 other runway. No, and so on 

 alternately, the specimen being 

 beaded first in one direction and 

 then in the other, in order to 

 avoid the possible establishment 

 of a habit of turning in the same 

 direction. The likelihood of this 

 occurring without proper precau- 

 tion is indicated by Holmes's 

 work (: 05*, p. 336). Still further 

 variation in operation was ob- 

 tained by occasionally running 

 two successive trials in the same 

 direction, thus shifting the order 

 of alternation. 



A varying number of trials 

 were made with 12 different in- 

 dividuals, the results of which 

 are summarized in Table VII. 



The results of the trials with the single lights, which were made from 

 time to time, are included for comparison with those to both lights, 

 since they can with considerable accuracy be taken as a measure of the 

 positive phototropism of the animal during the period it was being 

 experimented with. A test in each of the two orientations was usually 

 made with each of the lights separately at the beginning of the work 

 with any individual. The insect was then tested with both lights for 



Figure 14. Apparatus employed for 

 orienting Ranatra ; I, I', 2 candle power 

 incandescent lamps ; No, runway on 

 nortli side of working position ; Sm, side 

 directed toward small light ; So, runway 

 on south side of working position. 



VOL. XLII. 



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