This subject will be discussed, therefore, under two general categories--( 1) that 

 which directly relates to the physiology of the organ of sight or the physiological develop- 

 ment of the insect and (2) that which is more or less indirectly related. 



Phototropic response of the codling moth and the physiology of the compound eye . 

 One of the most significant studies to relate the physiology of the compound eye to 

 photosensitivity was conducted on the codling moth (3,4, 5). 2 In ordinary behavior the 

 moths are at rest during the day. As evening approaches they fly around the upper and 

 outer leaves, engage in mating flights, and in oviposition. At darkness their activities 

 cease more or less abruptly. Activity starts at about 30 foot-candles, increases to about 

 1 foot-candle, and stops at zero. They remain inactive throughout the night even in the 

 brightest of moonlight. Activity is resumed again at dusk at the same light intensities. 



The two periods of inactivity were found to be due to the adaptation of the compound 

 eyes. During the day they are said to be in the light- adapted condition; during the night, 

 in the dark-adapted condition. Moths that were exposed to varying degrees of illumination 

 from daylight to darkness were killed and fixed at known intervals and histological studies 

 were made of pigment distribution in the compound eyes. The iris pigment migration was 

 found chiefly responsible for moths being in these two states. In the light-adapted condi- 

 tion the iris pigment is withdrawn from the area of the crystalline cones and migrates 

 towards the apices of the retinulae cells. The retinulae pigment is distributed throughout 

 their cells. Pigment distribution thus prevents sufficient light from reaching the retina 

 to cause a phototropic response. In the dark-adapted condition there is a dense mantle of 

 iris pigment around each crystalline cone and withdrawal of the retinulae pigment towards 

 the basement membrane. 



Under natural light, migrations of pigment from the light to the dark- adapted condi- 

 tion or vice versa required about an hour. Natural field populations of moths were 

 killed and fixed at intervals from 5 p. m. to 5 a. m. Dark adaptation did not begin until 

 about 15 minutes before sunset and was completed 30 to 50 minutes after sunset. Migra- 

 tion towards light adaptation started about 30 minutes before sunrise and was completed 

 about 30 minutes after sunrise. Light traps with mercury vapor and tungsten lamps were 

 exposed to moths in the orchard. In the presence of artificial light, the moths ceased 

 normal activity as usual, but after 5 to 30 minutes resumed activity in response to the 

 stimulus of artificial light. 



Moths became phototropic in the transitional stage at three-quarter s to seven-eighths 

 dark- adapted condition and continued during the first hour of complete dark adaptation. 

 Pigment migration proceeded more rapidly under the influence of ultraviolet light than 

 under "white" light. 



The following conclusions were reached: The vital activities of the moths are carried 

 on almost exclusively during periods of changing light intensity. Iris-pigment migration 

 adapts the eye to fluctuations in the light environment. The moth's reaction to either 

 constant or changing light varies according to the position of the iris pigment. Iris pig- 

 ment migrations are thus a prominent factor in determining behavior of moths. Of two 

 light sources of the same continuous spectrum, the most billiant source elicits more 

 rapid pigment migration and is the more attractive. Of two light sources of unequal 

 spectral range, the one that evokes the more rapid pigment migration even though in- 

 tensity and relative energy is less is the more attractive. 



Adaptation to light sensitivity . Roeder (19) states that under constant stimulation 

 the eye becomes adapted so the animal may no longer respond to illumination. An in- 

 crease in intensity is required to cause recurrence of a response. Light adaptation may 

 be described as a loss in sensitivity. It is a reversible process and sensitivity can be 

 induced by subjecting the eye to darkness long enough to bring about dark adaptation. 

 This adaptation was studied in the honey bee (24) by using a postural reaction of the 

 antennae in response to moving stripes of light as the indicator. The logarithm of 

 threshold intensity in millilamberts from to -3 was plotted against time in minutes 

 that bees were held in darkness. The curve showed that the sensitivity of the light-adapted 



2 Figures in parentheses refer to literature cited at end of this paper. 



39 



