BIOLOGY OF LIGHT PRODUCTION — MALUF 381 



The air was full of extraordinary fireflies. About every fifteen minutes these 

 flies separated into two armies, one settling on the trees growing on the left 

 bank of the river and the other on the right. Then, when I had decided that the 

 fireflies had gone to bed for the night, the whole army on the left bank gave one 

 big flash in perfect unison, which was immediately answered by one big flash on 

 the right. How these flies managed to keep time absolutely beats me, but they 

 did so, though there must have been thousands of them stretching along the 

 river-banks for a hundred yards or more. The illumination was so strong that 

 the branches of the trees could be seen quite distinctly. 



This is only one exemplary case out of many. Several other animals 

 exhibit rhythmic synchronous behavior of various types, e. g., the 

 rhythmic synchronous chirping of certain crickets and locusts (Allard, 

 1918). 



2. Diurnal rhythm. — Fireflies generally flash only at night, i. e., 

 exhibit a 24-hour (diurnal) periodicity. In a paper read before the 

 American Society of Zoologists, Buck (1935 and 1937) described his 

 studies on the periodicity and diurnal rhythm in the firefly, Photinus 

 pyralis: 



In nature, males of P. pyralis ordinarily flash every evening between 7 and 

 9 p. m. The time of flashing is correlated with temperature and light intensity. 

 Males which have been in strong light long flash immediately if the intensity is 

 sufficiently reduced, regardless of the time of the day, showing that the regular 

 evening flashing period can be modified. Males kept continuously in darkness 

 do not flash. If, however, they are exposed to weak light they flash (regardless 

 of the time of the day) provided they have previously been in darkness 24, 48, 

 72, or 96 hours but not if they have been in darkness 12, 36, 60, or 84 hours. If 

 males which have been in darkness less than 24 hours are exposed to weak light 

 and left in it they do not flash until the sum of the time spent in darkness and the 

 time spent in weak light is equal approximately to 24 hours. It is thus apparent 

 that there exists in the firefly an inherent diurnal periodicity which is manifested 

 by periods of flashing which recur at 24-hour intervals and which persist for at 

 least four days in the uniform environment of the dark-room. It is also clear 

 that, although in the field this periodicity coincides with the diurnal changes in 

 light intensity, it is not actually linked to any specific hour of the day. 



By "inherent" Buck does not mean "inherited" but implies that 

 the 24-hour rhythm is produced by internal stimuli. A similar con- 

 dition had been noted in female larvae and female adults of Lam- 

 pyris noctiluca (Perkins, 1931). The animals exposed continuously to 

 light stopped glowing about the third day, while those receiving 

 diffuse light by day, and those in continued darkness continued to 

 glow at normal hours (10 p. m. to 4 a. m.) for about 2 weeks. 



It has been noted that when the eggs of the stick-insect, Dixippus 

 (Schleip, 1914, 1920), are placed under constant light conditions the 

 adults no longer exhibit any periodicity in the migration of hypoder- 

 mal pigment granules. Similar experiments are necessary in order to 

 determine whether the internal stimulus producing the periodical 

 flashing in fireflies is inherited. The occurrence of diurnal rhythms in 

 general is the subject of a recent review by Welsh (1938). 



