118 THE POPULAR SCIENCE MONTHLY 
The spectrum of the light of some of the organisms which have 
been reported to give reddish, bluish or other variously colored lights, 
is said to differ from that of the firefly. 
Another very interesting fact brought out by these observers (Ives 
and Coblentz) is that there may be extracted from the common firefly 
(Photinus pyralis K.) a substance which is fluorescent in certain 
lights, and that the spectrum of the bluish fluorescent light of this sub- 
stance is complementary to that of the light emitted by the insect itself 
—that is, the spectrum of this fluorescent light occupies that portion 
of the spectrum lying between the green and the violet. The presence 
of this fluorescent substance may, of course, be merely a coincidence; 
these same authors found a similar substance in a non-luminous species 
of the same genus, and various observers have extracted fluorescent 
substances from different organisms; but if it is a coincidence, it is 
certainly a remarkable one. Dubois has also discovered a fluorescent 
substance in the blood of the cucuyo (Pyrophorus noctilucus). 
Luminous animals and their photogenic tissues are extremely sen- 
sitive to irritants, whether mechanical, electrical or chemical; in other 
words, these tissues are very irritable. Almost any schoolboy is fa- 
miliar with the fact that pinching a firefly will result in the production 
of light from its luminous organ. Any other mechanical irritation, 
such as scratching or pricking with a pin, light taps or blows with a 
splinter of wood, ete., will produce a similar effect, and this is true not 
only of the live insect, but also of the luminous organ immediately 
after removal from the body of the insect; as it dries, however, the 
luminous organ gradually loses its sensitiveness, and when completely 
dry it will not respond to mechanical stimuli. 
The electric current acts as a stimulus to light production. The 
passage of the current through the body of a firefly causes it to flash, 
and sea water containing the Noctiluca shows luminous activity during 
the passage of a current. Light may also act as an irritant or stimu- 
lus; Henneguy records that the admission of light to the darkened 
cabinet wherein were some Noctiluca in sea water, caused the evolution 
of light from these infusoria, and the local firefly has been known to 
flash following the turning on of an electric light in a darkened room 
where the insects were confined, the phenomenon being repeated several 
times. 
The most extensive observations upon the irritability of photogenic 
tissue, however, have been made with chemical substances. These have 
included a large number of gases and vapors, acids, alkalis and salts, 
alkaloids, and a vast number of miscellaneous compounds. In general, 
chemical substances may be divided into three classes with reference to 
their action upon the photogenic tissue: (1) Those which tend to pro- 
duce the evolution of light, and which may therefore be classed as 
