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 substance 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 sensitive to irritants, whether mechanical, electrical or chemical; in other words, these tissues are very irritable. Almost any schoolboy is familiar 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, etc., 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 stimulus; 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 produce the evolution of light, and which may therefore be classed as
