COLE. —IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 389 



comes to rest with the head directed away from the light, but Carpenter 

 does not consider this an indication of negative phototropism. His 

 explanation is that " The fatigued insects remain quiet in this position 

 because it is the one in which the least light enters the eyes, and iu 

 which, as a consequence, the kinetic stimulus is least" (p. 170). The 

 desirability in the present instance of having animals which were ac- 

 tively phototropic and in which the character of the response, whether 

 positive or negative, remained constant, has already been mentioned. 

 The pomace fly seemed to fulfil these conditions remarkably well, 

 since it is always, when in motion, positive to light, and can usually 

 be put into motion by slight mechanical agitation whenever it shows a 

 tendency to come to rest. 



All this works out very well with a single light, but in using both 

 lights the difficulty arises of getting the flies properly oriented at the 

 beginning of the trial. If they chance at the beginning to be headed 

 toward either light, they appear to continue crawling in that direction, 

 heedless of the other light and indifferent as to whether the one toward 

 which they are headed is the larger or the smaller. The plan was first 

 tried of liberating a number of individuals in a box through a hole 

 in its bottom, the box being provided with glass sides, one directed 

 toward either of the lights. When once they alighted on either glass, 

 they crawled upward, in consequence of their natural negative geotro- 

 pism, and so passed into a trap device at the top, where they could 

 be counted at leisure. It was expected that when they were placed in 

 a small dark box beneath the hole in the bottom of the one with 

 the glass sides, they would at once fly upward from the dark into 

 the light of the upper box, and so would become suddenly exposed 

 (as they flew upward, and oriented at random) to the influence of the 

 large and small illuminated areas. It was found that on the contrary 

 few of the insects flew out ; the most of them crawled up over the edges 

 of the opening and proceeded to crawl on in the direction in which 

 they chanced to be oriented. Furthermore, a large proportion of them 

 evinced a decided tendency to settle down and remain quiet soon after 

 getting out into the light, coming to rest here and there all over the 

 bottom, top, ends, and the glass sides of the box, and it took more than 

 moderate jarring of the box to get them started to crawling again. 



Next, the plan was tried of placing backwardly projecting strips of 

 paper around the opening, making it more difficult for the flies to crawl 

 out at the edge of the opening. At the same time there was placed 



which for the time inliibit or overcome the natural phototropic reaction, and is 

 not to be considered an active negative reaction to lit^ht. 



