386 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



him no matter where he may be located; and the male, in response 

 to a flash of light produced by the female, turns and flies or walks 

 directly toward her no matter where she may be located." (See also 

 Osten-Sacken, 1861; and McDermott, 1912, 1917.) 



It remained for Buck (1935, 1937a) to determine how the male 

 distinguishes the flash of a female from that of another male. Thus, 

 at nightfall the female firefly (Photinus pyralis) climbs up on the 

 herbage and the male flies about, flashing at fairly regular intervals 

 of about 5.8 seconds at 24° C. The female responds by flashing 

 2.1 seconds after the male flash. The length of the response interval 

 also varies inversely with the temperature. 



The male responds to flashes of artificial light and to flashes produced by other 

 males if they occur about 2 seconds after his flash. This shows that no possible 

 difference in the color, duration, or intensity of the light produced by the two 

 sexes, or difference in the relative motion or form of the luminous area, is of any 

 importance in enabling the male to distinguish between flashes emitted by male 

 and female. The essential factor in the ability of the male to distinguish between 

 flashes produced by the female and those produced by other males is the fact 

 that the female invariably responds to the flash of the male at a nearly constant 

 level of 2 seconds. 



The synchronous flashing of masses of fireflies has beeD explained 

 by an application of the above facts (Buck, 1935a, 1937c, 1938). 

 Thus, other males may join in precise unison even though they had 

 been flashing asynchronously originally. This involves a readjustment 

 of the phase but not of the frequency. Similarly, other females in 

 the vicinity may act accordingly — i. e., a single pair may cause a 

 swarm of synchronous flashings. In other cases, however, swarms 

 may exhibit no synchronism (e. g., Photinus pallens, a Jamaican firefly). 

 Sheer photopositivity is the probable explanation for the huge swarm 

 of P. pallens, since swarm-nuclei of this species may be artificially 

 formed by flashlight. In such cases, mating "appears to be due 

 entirely to accidental contact of the sexes during their peregrinations 

 on the branches. The aggregation habit thus seems to take the 

 place of the accurate systems of flashing signals which serve to bring 

 male and female together ..." (Buck, 1937c). 



CHEMICAL PHENOMENA 



1. Luminescence an oxidative process. — In 1667, Boyle, by means of 

 a modified von Guericke (1634, 1672) air pump, discovered that air is 

 necessary for the existence of animals and for the luminescence of 

 bacteria (although, of course, he did not recognize the bacteria as such) 

 and "glowworms" {Lampyris sp. larva or the wingless luminous ad alt 

 English female). Following the discovery of oxygen by Lavoisier, 

 Spallanzani (1796) was evidently the first to indicate that biolumines- 

 cence is an oxidative piocess. Murray (1826) showed that when oxy- 



