# Popular Science Monthly/Volume 77/August 1910/Physiologic Light

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* PHYSIOLOGIC LIGHT
By F. ALEX. McDERMOTT
HYGIENIC LABORATORY, U. S. PUBLIC HEALTH AND MARINE HOSPITAL SERVICE
THERE are probably but few if any of the readers of this magazine who have not seen and admired at least one of the many manifestations of "physiologic light" of which the most common to us is the firefly. Indeed, from the earliest times the phenomenon of the emission of light by animals and plants has attracted man's attention, and a large amount of scientific work has been done upon the subject. An attempt to compile a complete bibliography of the subject has resulted in the remarkable discovery that there are over seven hundred references to the literature bearing on the emission of light by organized bodies, and "the end is not yet." The work has embraced the physical, chemical, physiologic, histologic and entomologic sides, and much valuable information and many interesting facts have been secured. Among the names of the early writers who refer to some phase of this phenomenon are Aristotle, Pliny the Younger and Josephus; the more recent names include those of Robert Boyle, Sir Humphry Davy, Faraday, Pasteur, Kölliker, Dubois and the late S. P. Langley, and indeed a host of others whose names are more or less widely known. Several extensive treatises on the subject have appeared, some of which are really quite good, though regrettably they are for the most part out of date at this time. For the benefit of those who may care to read further, the names of a few of these are given below.
The phenomenon of physiologic light has been variously termed "phosphorescence," "luminosity," "photogenic function," etc., by different authors. As these are, for the most part, interchangeable in meaning, they will be used in this paper to refer to the same thing. The term "phosphorescence" is unfortunate, since it implies that the light is due to the presence of the element phosphorus—which it is not—and has become still more objectionable recently owing to its application by physicists and chemists to another totally different phenomenon of light emission.
It was my good fortune during the summer of 1909 to be associated with Professor J. H. Kastle, of the "University of Virginia (then chief ​of the Division of Chemistry of the Hygienic Laboratory, of the U. S. Public Health and Marine Hospital Service), in a study of the effects of various chemical agents on the emission of light by the common firefly of the country around Washington, Photinus pyralis K. In the progress of this work we had occasion to review the available literature quite thoroughly, and were struck with the lack of acquaintance of people generally with the theories which had been advanced to explain the phenomenon, and with the work which had already been done upon it. The results of this investigation will be published at an early date. In spite of this great amount of work which has been done, the firefly still preserves its secret of "the cheapest form of light," and seems likely to do so for some time yet.
Although the most common and brilliant manifestations of physiologic light are exhibited by the fireflies, this property is by no means confined to the animal kingdom. Various vegetable forms, from the lowest to the highest, have been reported as producing light. There are many varieties of luminous bacteria and molds, whose activity is seen in the luminous decay of fish and wood. Certain agarics and other of the higher fungi are luminous, and the light given by the underground rhizomorphs of fungus growths is among the first of these phenomena to be reported in scientific literature. Of the higher plants, the marigold, the nasturtium and other garden and wild flowers have been said to emit flashes of light—a circumstance attributed by Phipson to electricity. But, for the most part, the light of vegetable forms seems to be pale and often hard to discern, as compared with the brilliancy and glitter of the firefly and other animal forms.
To those not living on the sea-coast, the most common manifestation of the photogenic function is that produced by some variety of the firefly; but there are a large number of marine forms of varying degrees of organization which possess this property, and some of these are common on certain coasts. For the purpose of discussion, the animal forms will be grouped as marine and land forms.
The simplest marine form which emits light is the "Noctiluca" (Noctiluca miliaris), a tiny globule of protoplasm scarcely a millimeter in diameter, which when present—as it usually is—to the extent of millions upon millions, produces the appearance known as the "milky sea" or "phosphorescent sea." Many interesting studies have been made on this little organism, the principal importance of which lies in the fact that it seems to give practically the same reactions as other more highly organized luminous forms. Besides the Noctiluca, certain Beroe and other Ctenophores are often present in immense numbers, and give rise to the same appearance of the milky sea. Higher still, there are a number of Salpæ, and other marine forms which give light, and interesting studies upon them have been made ​by Panceri, Quatrefages and other scientific men. But perhaps the most remarkable luminous marine organism is the bivalve, Pholas dactylus, known to the French as the "Pholade," and to the Germans as the "Bohrmuschel." This creature has definite luminous organs, whose tissue and secretions are strongly photogenic. It has been the subject of interesting researches by Dubois, and has been shown to react in a manner similar to that of other luminous forms. More recently, certain peculiar organs possessed by deep-sea fish have been determined to be light organs, and thus it appears that in the depths of the sea they need "artificial" light, when the sun's light fails to penetrate, just as on land when the sun is hid.
By far the most brilliant and most commonly known form of physiologic light is that given by the so-called fire-flies; this term embraces a large number of species of insects, mostly Coleoptera (beetles) of two or three genera. Besides these Coleoptera, there are a few luminous forms distributed among the other insects, together with certain myriapods, worms and other occasional forms. In a very few instances luminosity of more highly organized forms has been reported, but for the most part these appear questionable at least. Of the non-coleopterous insects, Diptera (Chironomus) and Hemiptera (Fulgoridæ) are said to be luminous; the hills of the South American termites (Neuropteræ) have also been observed to be luminous.
The majority of the insects commonly called fireflies belong to the genus Lampyridæ, including the Italian luciole (Luciola italica), the English and continental glow-worm (Lampyris noctiluca), the continental firefly (Lampyris splendidula), the American fireflies and "lightning-bugs" (Photinus pyralis, Photuris pennsylvanica, etc.), and a vast number of other luminous insects. Further south, as in Cuba, Mexico and Brazil, the more brilliant insects belong to the genus Elateridæ,-and embrace the cucuyo (Pyrophorus noctilucus and the cucuyana (Pyrophorus physoderus). In India there is said to be a luminous buprestid beetle.
Thus it will be seen that, so far from being a rare phenomenon, the emission of physiologic light is one of well-nigh universal distribution, and appears to be an important function in the life of those organisms possessing photogenic activity.
While most of the facts here given apply primarily to the fireflies, they may, in great part, be taken as true for the entire phenomenon of physiologic light. Different forms may show variations in color, intensity and mode of emission of the light, but basically it all seems to revert to the same cause—a cause as yet, however, unknown.
The light given by luminous insects is usually stated by authors to be greenish or yellowish; a few have claimed to observe insects to emit a reddish or bluish light, and marine forms have been reported to emit ​a large variety of colors,—red, blue, violet, green, etc.—but the colors are in most cases pale and dim.
Perhaps a dozen investigators have submitted some form of physiologic light to analysis by the spectroscope, and with a few exceptions the results have agreed very well. The best known of these spectroscopic investigations was that of Langley and Very, in 1890. These authors worked with the Cuban cucuyo; briefly, they found that the prism of their spectroscope resolved the light into a narrow band in the yellow and green region of the spectrum, ending somewhat abruptly and showing few red or blue rays; they were unable to find that the light was accompanied by any evolution of heat, such as we ordinarily associate with light produced by combustion or by electric heating, and hence they called the paper presenting their results "The Cheapest Form of Light." This valuable research has recently been confirmed by Drs. Ives and Coblentz, working in the National Bureau of Standards, in Washington, and using more sensitive instruments than were available to Professor Langley and his coworker. Ives and Coblentz found that the light of the common firefly (Photinus pyralis K.), was resolved by the spectroscope into "an unsymmetrical, structureless band" in the red, yellow and green, but not extending further than wave length $0.67\mu$ toward the red end of the spectrum, nor than wave length $0.51\mu$ toward the violet end. From the facts at hand it seems extremely unlikely that the spectrum could be discontinuous and renewed in the infra-red or ultra-violet non-visible portions of the solar spectrum.
The remarkable fact which these researches bring out is the extremely high luminous or radiant efficiency of the light. This was estimated by Langley and Very at 100 per cent., and has been shown by Ives and Coblentz to be about 96 per cent. In other words, 96 per cent, of the total energy radiated by the firefly is exclusively illuminating radiation, and does not embrace heat or other subordinate effects. This is the more remarkable when it is considered that the best artificial illuminant has a luminous efficiency of only 4 per cent., and most of them run less than 1 per cent. Of course, this does not mean that the mechanical or chemical processes resulting in the production of the light have an equally high efficiency—that is quite another matter. But it does mean that for a given amount of radiation, the firefly produces the greatest amount of luminous radiation.
But even if we should discover the means by which the firefly produces its light, we should hardly care to use it in our homes. The insect has indeed reached the highest possible radiant efficiency, but it has been accomplished at a sacrifice of color variety that makes the light worse for color effects than even the ghastly green of the mercury vapor arc. Anything not within a very limited range of yellow and green tones would appear black.
​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 ​stimulants to light production; examples of this class are mononitrobenzene, carbon disulfide and carbon tetrachloride; (2) those substances which are neutral in their action, neither provoking luminescence nor inhibiting it; examples of this class are hydrogen and nitrogen; (3) substances which poison the tissue and permanently prevent the production of light; examples of this class are bromine, sulfur dioxide and iodine cyanide. Strychnine and other alkaloids cause the production of light, as do also certain poisons; oxygen appears to activate the production of light somewhat.
Probably the most interesting fact so far developed by the chemical study of this phenomenon is that when photogenic tissues have been dried out, the dry tissue glows again when moistened with water in the presence of air. Carradori mentioned this fact in 1808, and quoted Spallanzani and Eeaumur as having made the same observation at earlier dates. Carus reported the same observation in 1864, and Dubois confirmed it some twenty years later. Professor Kastle and the writer have been able to perform the same experiment with the American firefly; it is indeed a fact that the photogenic tissue of this insect may be dried, the dry material powdered, and the dry powder kept for some time away from access of moisture, and it will, when moistened in the presence of air or oxygen, glow again; indeed, by careful redrying, the same result may be obtained two or three times on the same specimen of the dry material. Moreover, this dried tissue gives, when moistened, many of the same phenomena with chemical reagents as do the living insect and its freshly detached luminous organ. The property of thus glowing upon moistening after having been dried, does not appear to be confined to the luminous organ of the firefly, but appears to be a constant characteristic of luminous tissue as a class. The main deduction from this fact is that at least three factors are necessary for the production of light by photogenic forms—water, oxygen and some material, as yet unknown, whose oxidation in the presence of water produces light.
Several theories have been advanced from time to time to account for the production of physiologic light. Probably the earliest view was that it was due to the presence of the element phosphorus. That this is not the case is best evidenced by the fact that there are only traces of this element present in the luminous tissues, and that which is present is in the form of phosphates. Yet this is the commonly accepted view of the cause of the phenomenon, and even as recently as 1880, Jousset de Bellesme suggested that the light might be produced by the spontaneous combustion of phosphine. Carradori assumed that the luciole was capable of absorbing from the air or from its food, the "material of light," and of then emanating it again at pleasure.
The fact that the light is unaccompanied by the evolution of ​measurable amounts of heat certainly shows that if it is indeed a combustion, it is a most remarkable one and one which differs from any analogous process known to us. The view that the light might be the result of oxidation has, however, long been held. Robert Boyle made experiments on this point in 1667, and concluded that the light produced by shining wood and fish was not affected by the absence of air, and was therefore not what we now call a combustion or oxidation. Spallanzani, as the result of his studies on luminous sea forms, came to the opposite conclusion, in which he was opposed by Macartney and Carradori. More recently this phase of the subject has been studied by Dubois, Watasé and Townsend, all of whom have published very interesting observations. As a result of these several observations the conclusion must be drawn that oxygen is essential to the process of the production of physiologic light, and that we have in this phenomenon a true but remarkable form of combustion. Of the mechanism of this process we are still very ignorant. Dubois's theory is that the light is produced as the result of the action of an "oxidase" (oxidizing ferment), to which he has given the name "Luciferase," upon a substance of unknown composition, which he calls "Luciferin," the latter being oxidized by the atmospheric oxygen through the agency of the ferment. It is a little early to accept this hypothesis finally, although it certainly presents some analogy to known processes—for example, the production of the black pigment melanin through the action of the oxidase tyrosinase upon the organic compound tyrosin. Phipson had already described a substance he called "Noctilucin" as the active principle of physiologic light; it seems possible that Phipson isolated and analyzed a culture of photogenic bacteria.
In this connection the structure of the light organs of various animal forms has been given special attention. In general, the results of studies on those forms having special photogenic organs have been essentially similar. Briefly, the luminous organs appear to be masses of cells of some special kind, possibly a fat-derivative, or according to Macaire and Kölliker, an albuminous substance penetrated by a network of trachea (tracheoles), and as the result of some chemical action, apparently oxidation, taking place in these tissues, the light is produced. Whether these tracheoles are in life filled with air or with a liquid seems doubtful; the evidence is contradictory so far as given, but it seems quite probable that they convey air.
What is the purpose of this production of light? Of what value is it to the forms which possess it? This is another side of the "secret of the firefly," which has yet to be solved. Quite probably the function bears some relation to the reproductive life of the insect. The females of the local species (Photinus pyralis K.) give a very much less bright light than, and are quite rare as compared with, the males; one female ​to from seven to fourteen males seems to be about the proportion. The same condition appears to hold with other species of Lampyridæ also. King states that the female of the Texan form Pleotomus pallens is much more luminous, and rather less active than the male. In addition to the photogenic power, the common firefly is possessed of a strong and characteristic odor; Carradori also notes that the Italian luciole has an odor like that of garlic. Many insects indeed possess odors, but that of the Lampyridæ appears to be especially characteristic of the group.
In conclusion, we may say that while a vast amount of work has already been done on this interesting problem, the production of physiologic light still presents many mysteries which science has yet to explain. Nature keeps her secrets well, but this one seems well worthy of solution; the immediate practical and economic importance may not be so great as has been sometimes assumed, but it is a problem of interest alike for the physicist, the chemist, the biologist and the entomologist, and the scientific world awaits its solution with much curiosity.