# Popular Science Monthly/Volume 79/December 1911/Why Do Certain Living Forms Produce Light?

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* WHY DO CERTAIN LIVING FORMS PRODUCE LIGHT?
By F. ALEX. McDERMOTT
WASHINGTON, D. C.
TO every observer of natural phenomena, the query must some time come, "Why do certain living creatures produce light?" The luminous molds of decaying wood, the photogenic bacteria of the sea-water, the fire-flies and lightning bugs, the deep-sea fish and other mysterious forms "that move in the waters"—why should some of them be endowed with the property of producing light? The question is undoubtedly one of fundamental biologic importance. The production of light by living forms is really no more wonderful than the production of heat, motion or electricity, but the production of heat and of motion is so common and so well known that but little attention is paid to them, while the forms which produce electricity are relatively so scarce that they are little known outside of the scientific world. Between these classes are the forms possessing the photogenic function—sufficiently common to be well known almost everywhere, and yet sufficiently scattered among the creatures of the earth to excite wonder and admiration at the novelty of the property. We can, of course, beg the question by replying, "These creatures have the power of producing within themselves some chemical substance which, under certain circumstances, produces light, probably as the result of oxidation," but this or equivalent statements leave us very little nearer satisfaction than at first.
The matter presents also another question which is difficult to answer: Why should one creature be endowed with the photogenic function, and yet some other form, closely related to the first, be unprovided therewith? Did all creatures originally possess the power to produce light and have all but the few we know lost this power, or have the few that possess photogenicity acquired the power as a result of the development of certain habits or conditions of life? It seems probable that both explanations may be advanced for different forms—i. e., that in some cases related existing forms, some of which possess the photogenic function and others of which do not, are descended from a common photogenic ancestor, while in others the function has been developed during the history of the species.
There are certainly three reasons for the existence of the photogenic function some one of which is applicable to the great majority of luminous creatures above the unicellular and very lowly organized ​forms. The first of these is that it is a secondary sexual character; the second, it is protective in purpose; third, it is a lure for prey. Other less general or less obvious uses may be shown, but in the great majority of cases one or the other of these explanations of the use of the light-producing function will be found adequate, and sometimes more than one will apply to a given species.
The simplest luminous forms are the bacteria. It is certainly difficult to see that photogenicity can be of any especial service to these unicellular organisms. Most, if not all, of the photogenic bacteria are cf marine origin, and it is possible that some of them may be pathogenic to certain marine creatures when ingested, and the light thus serves to warn the bacterivorous plankton, etc., that these bacteria are dangerous food. Some species have been found to be pathogenic to Talitrus, a crustacean, which may serve as a case in point.
Luminous fungi are quite common, and here again we must leave the question with at most only a poor attempt at explanation. Protective or warning it may be; but the author has never heard of luminosity in the most poisonous of all fungi, the Amanitas, and certainly the luminous fungi he has seen did not appear to form any exception to the usual fate of fungi in general of being abundantly attacked by various species of insects.
In both of these cases, the photogenicity may be like the fluorescence of extracts of the common firefly, Photinus pyralis, and of cultures of Bacillus fluorescens liquefaciens—merely a property of some chemical substance elaborated by the life processes of the organism, and having no bearing upon its economy.
In simple marine forms like Noctiluca miliaris, the Pyrocystæ, Pyrosoma, etc., it is possible, though unlikely, that the luminosity has a warning significance and it is obviously not a sexual character. These creatures exist in swarms, or in the form of communities—compound, or rather composite animals—and it would appear very probable that in them the possession of the power to produce light finds its usefulness in the fact that by its means they are enabled to communicate in such ways as their low state of organization may require.
It is perhaps a digression, but a few words may not be amiss as to why the emission of light would be more useful to marine creatures than some other modes of communication in use among land forms. The sea-water is full of currents, ever changing and wavering; it varies in density slightly in different portions, owing to slight variation in concentration and in temperature. Therefore, the emission of a substance to produce olfactory or gustatory sensations would be of little use as a means of communication, especially to a creature capable of motion, as many luminous forms are; they would have far more control over their own movements than over that of any emission. The ​production of a sound would be better so far as transmission is concerned, for water is an excellent transmitter of sound; but to produce a sound, especially under water, a not inconsiderable amount of power is required—as may easily be demonstrated by trying to clap one's bands under water—and this amount of power is far and away beyond any possessed by the luminous marine creatures. True, some fish emit sounds—e. g., the drum-fish—but those marine creatures which do so are of considerable power and a quite high degree of organization. All living creatures, probably without exception, produce a certain amount of heat through their life-processes; but heat is obviously of no value so far as the purposes of communication are concerned, especially in an immense body of liquid of high specific heat. Variations in pressure more especially vibrations of longer interval than those of sound, may, of course, be produced and transmitted very effectively, but here again much power is required. As a matter of fact, some species of fishes have been found to possess along their lateral line, organs susceptible to vibration-frequencies approaching six per second in the water. Electricity is a possibility, but in all cases of electrical tissues so far studied, considerable masses of muscular tissue have been found as the site of the electrogenic phenomena, again a matter out of the question for a simple organism. With light the problem is different: All that is necessary is the elaboration by the cell through its vital processes of a substance which, when in contact with the oxygen dissolved in the seawater, will produce light. Since certain bacteria can produce such a material from a compound as comparatively simple as asparagin (aminosuccinamidic acid, ${\ce {CONH2 . CHNH2 . CH2 . CO2H}}$, this is a matter of comparative ease and requiring nothing more than the metabolic processes which might be ordinarily expected. After synthesis, the substance when brought into contact with the sea-water would be oxidized with the evolution of light. Light knows little of water currents, and but little more of differences in concentration; it would spread in all directions from the point of emission, and to the delicate structures of the ocean fauna and flora, would, however weak it might appear to human eyes, be sensible for considerable distances. Hence light is an ideal method of communication for marine forms of low organization and indeed for many of those of higher organization.
Returning again to a discussion of the various forms: Perhaps the first case, in the upward scale, where we may apply with any degree of certainty one of the uses mentioned before, is that of certain marine worms, the Annelids. Professor W. T. Galloway has recently shown the use of photogenicity in a species of Odontosyllid as a mating adaptation, with evidence which leaves little ground for doubt. In this case propagation of the species appears to be entirely dependent upon a periodic photogenicity limited to certain more or less definite ​portions of the year, of the month, and of certain hours of the early evening, during which the male is attracted from below to the luminous female at the surface. Various species of earth worms have also been reported to be photogenic, and it seems probable that the usefulness of the luminosity in this case is somewhat the same as in the Odontosyllid mentioned above.
The bivalve Pholas dactylus presents another anomaly, however, for it lives a rather sedentary life, and is certainly not poisonous, at least to man. Yet it possesses definite photogenic organs. Although it is possible that in this case the luminosity is protective, we probably have here one of the cases of the limited use of photogenicity, not yet discovered.
Among the Crustacea there are several interesting cases of photogenicity, and in regard to them, and indeed to the whole subject of the use of "phosphorescence" in sea-forms, Alcock's interesting book "A Naturalist in Indian Seas," is well worth reading. Certain shrimplike crustaceans throw out from glands, corresponding to kidneys, a substance which in contact with sea-water produces clouds of bluish light. There seems but little doubt that this is defensive in nature, and acts in much the same manner as the "ink" of the cuttle-fish. Some of these prawns are provided with enormous eyes, others with only rudimentary ones, and some with none at all. Alcock also mentions a large spider-crab, which, although completely blind, "shone like a star." Here we may readily conceive the light is alluring in function, serving to attract the creatures on which the crab feeds.
Among the insects we find the most widely known cases of photogenicity, and probably, also, the greatest field of usefulness. "With the true fireflies, the Lampyridæ, the evidence that has been collected tends to show that the possession of the photogenic function is primarily a secondary sexual character. It has long been known that if the female of the European glowworm (Lampyris noctiluca) were exposed by night, a male would shortly come to it. The use of the photogenic function as a secondary sexual character has also been shown for the Italian luciola, and for certain of the fireflies common in the eastern United States (e. g., Photinus pyralis), and it appears probable that the same thing applies to the entire family. Curiously enough, the true "lightning bugs" show but little tendency to come to ordinary lights, though in Photinus pyralis either sex will respond to a small electric bulb operated in imitation of the light of the opposite sex.
Among the Pyrophores, the Elaterid fireflies of the tropics, such as the cucuyo of Cuba, the luminosity very probably plays the same role as in the Lampyrids. These insects give a light which is continuous, though of varying intensity, instead of a flashing light as is emitted by ​the Lampyrids generally, and it is of interest to note that they may be attracted by an ordinary light.
To be sure, a great many insects which are not luminous themselves are attracted to light, and there is some evidence that this is something which the insects themselves can not help. The phenomenon of attraction to light among luminous insects, however, must be regarded as of particular interest, and as being most probably voluntary.
That the photogenic function in insects may also be protective or warning in significance may scarcely be doubted. The Lampyridæ, as a group, are soft, easily crushed insects, slow of motion, and often, in the females, apterous, or of but slow and labored flight. It has indeed been observed that the flash of these insects has a tendency to discourage pursuers, perhaps frightening them in some cases, but probably more often warning them that the light-bearer is inedible. The elaters are better protected by their hard, external chitin, than the soft-bodied Lampyrids, and hence it is unlikely that their luminosity has much protective purpose. So far as its possible use as a lure for prey is concerned, this is out of the question for the elaters, and also for many of the adult winged forms of the Lampyrids; the larvæ and probably some of the larviform females of Lampyrids, however, are carnivorous, and in them it is possible, though hardly probable, that the alluring significance for the photogenic function may hold good.
A few species of myriapods are known to be luminous; perhaps more species than are now known to possess this function actually possess it for short periods during the year, probably during the height of the mating season. In some myriapods, the luminosity seems to be developed in a secretion which is ejected from pre-anal glands, while in others it is located in organs on the body of the creature, as in the insects. Mrs. Thomas has shown the almost undoubted defensive character of the first class of luminosity in myriapods, while the observations of Bruner suggest rather the sexual significance in the second class.
Among the Cephalopods we find a very peculiar class of luminous organs, occurring immediately upon or just beside the eyeball, which in these creatures is often relatively enormous. Here the significance of the luminosity seems to be interpreted by the situation of the organs as an adjunct to the function of sight, and such it very probably is. But very similar organs are found on various other portions of the body and in situations where they can not very greatly aid in vision, or illuminate the creature's path through the water. Much the same thing is true of some fish, which possess one or more large photogenic organs situated near the eye, and rows of smaller organs along the sides or abdomen; here the organs near the eye are naturally considered as an aid to vision, while the others can not possibly be so considered. In ​both fish and cuttlefish the explanation of these body-organs is probably the same. In the depths of the sea colors are practically indistinguishable. The dim light of the stationary photogenic forms there would be insufficient to differentiate colors. But in the more than semidarkness of these depths, a creature with a row of luminous dots along its sides would show up like an electric sign in a dark street. Tbe arrangement of the light-giving spots could very easily be followed, even for considerable distances through the water. It is probable, therefore, that these rows of photogenic organs on the bodies of these creatures serve the purpose of plumage and pigmentation on land, a welcome for friend and a warning for foe. No doubt one species of fish, seeing a luminous streak some distance away through the water, could readily tell whether its pattern of light—its "electric sign "—spelled the same as those upon its own body, or the legend of a foe. Many of the fish that possess these rows of luminous organs, while insignificant in size, are obviously raptatorial, and are armed with vicious teeth.
Of course another expression of the luminosity of deep-sea as well as surface fish is that it is alluring. This is especially true in those species which are provided with luminous barbels, or baits, like those of the angler-fishes. Alcock mentions a blind angler-fish with a luminous barbel, in which the alluring significance is scarcely to be doubted.
An objection which seems to have been urged against the "pattern" theory of photogenicity in marine forms is that different specimens of the same species occasionally show variations in the number and distribution of their lights. It seems probable, however, that the reason for this variation may be the age of the specimens of the fish in question, the number or position of the spots varying with age. Almost any of the various books and papers on the deep-sea fish will illustrate the effect of this "pattern" arrangement as seen, for example, in Cyclothone and Astronesthes.
Still another type of organ of light-production among fishes is illustrated in Carl Chun's book "Aus den Tief en des Weltmeeres"; in this case the luminous apparatus is set in a pit in the foremost part of the head, before the eyes. Here there can be little doubt, again, that the usefulness to the animal consists in the illumination of the "road ahead," as the searchlight does for the automobile.
A large number of other forms might be mentioned, which emit light in more or less characteristic ways, but what has gone before will serve to illustrate the majority of the points in interest. Some considerations as to the phenomenon in the fireflies may, however, be of interest, especially as deductions therefrom will doubtless hold true for many other luminous forms also.
It has been observed that the various species of fireflies (Lampyrids) emit lights of slightly differing tone, and in decidedly different man ​manners. For instance, the observer would soon notice that the lights of Photinus pyralis and Photinus scintillans were decidedly more yellowish than those of Photuris pennsylvanica and Lecontea lucifera, and that the latter were distinctly greenish in tone. A little further observation would soon enable one to distinguish these two latter insects from the two former ones also through the different method of lightemission; the flickering flashes usually given by Photuris and Lecontea differing markedly from the long flash of the Photini. Thus it Incomes probable that different species can recognize their own kind through the color and manner of emission of the light.
Another interesting circumstance is that the majority of Lampyrids have their luminous apparatus on the ventral side, the greater part of the light being directed downwards and sideways, and but very little passing upward. The effect of this ventral arrangement so far as the sexes is concerned is that a female resting on a leaf or on the ground illuminates by her flash a considerable portion of the supporting surface, and a male flying above her would see not merely a flash, but a silhouette of his mate against an illuminated background. The green color of the light would, of course, be of special advantage on foliage. Moreover, the flash of the flying male would illuminate most particularly an area immediately below him and ahead of him, as these insects fly with the body inclined, the head being highest. In species where the male is non-luminous, or but slightly so, this last service does not exist, while in those like Photuris, where both sexes are about equally active, the manner of applying the luminous property may be entirely different; it is perhaps significant in this latter group, that the light is easily visible from the dorsal side between the elytra. The cucuyo (Pyrophorus noctilucus) in which both sexes are equally active, have lights both above and below.
We have been considering the purpose of biophotogenicity so far as its application to the creatures possessing the function is concerned, and to a more limited extent, to their enemies. A few words may well be given regarding its relation to man. The use of the cucuyos as decorations and as night-lamps in the tropical countries is quite well known, and a number of instances have been recorded where travelers have owed their safe passage and even their lives to the light given by a collection of these Elaterid beetles. Several naturalists have written and read by the light of vessels filled with Noctiluca and other sea-organisms, and Chun has photographed a Cephalopod by means of its own light. The luminous bacteria have been put to a number of uses, mainly in the laboratory; flasks coated on the interior with fresh cultures of some of these organisms give light which appears of considerable intensity when the eye becomes accustomed to it, and Dubois and Molisch have taken quite a number of photographs by bacterial light. It has ​been suggested that "living lamps" made from these bacteria could be used to advantage in coal mines and powder magazines, since as they emit no appreciable heat, they would be absolutely without danger of producing an explosion. They would certainly furnish a nice cool light for use in summer; the author would very much like to have one here in his little study now, in place of the Welsbach which is engaged in turning some 98 per cent, of its expended energy into heat instead of light, after a day that has shown 90° F. However, such lamps would probably not do for anything like general illumination, even if the intensity were great enough, for in their light any color effects beyond a very limited range would he impossible. Color considerations would not be a considerable factor in mines and mills, however, so this does not interfere with this application.
It is more reasonable, however, to consider that these luminous forms merely point to what is possible in the way of efficient light—to serve as the goal to which all effort in the improvement of light-efficiency must strive. This must be the ideal—light without heat.