E. Newton Harvey

The production of light is the converse of the detection of light. In the first case chemical energy is converted into radiant energy; in the second case radiant energy is converted into chemical energy. The lantern of the firefly is an organ of chemi-photic change; the eye is an organ of photo-chemical change. While it is theoretically probable that all reactions which proceed in one direction under the influence of light, will proceed in the opposite direction with the evolution of light, the formation of luciferin from oxyluciferin (described in Chapter VI) is the only one definitely known. Perhaps we may place in this category also the instances of photoluminescence, but the chemical reaction involved cannot be pointed out.

We know of no animal whose eyes, the organs, par excellence, of photochemical change, give off light in the dark. All cases of luminous eyes have been conclusively shown to be purely reflection phenomena. The eyes of a cat only glow if some stray light is present which may enter and be reflected out again. Photochemical reactions and chemiluminescent reactions do have this in common, however, that they are largely but not exclusively oxidations. Whether all photochemical changes in the eyes in animals require oxygen or not, is unknown, but all animal light-producing reactions, without exception, are oxidations, and light is only produced if oxygen is present. Some material is oxidized.

In general, we may divide luminous organisms into two great classes according as the oxidizable material is burned within the cell where it is formed or is secreted to the exterior and is burned outside—intracellular and extracellular luminescence. Many animals with intracellular luminescence have quite complicated luminous organs. It is an interesting fact that a great similarity may be observed between the evolution of the complex organs of vision and of these complicated organs. In the simplest unicellular forms certain structures within the cell serve as the photochemical detectors of light, while in luminous protozoa, similarly, granules scattered throughout the cell are oxidized with light production. In the higher forms the eye contains groups of photosensitive cells connected with afferent nerves, lenses, and accessory structures for properly adjusting the light, while luminous organs contain groups of photogenic cells in connection with efferent nerves, lenses, and accessory structures for properly directing the light. It is interesting to note that in the two groups where the eye has attained its highest development, the cephalopods and vertebrates, here also the luminous organ is found in greatest complexity and perfection. In intermediate stages of evolution the eye and luminous organ so closely approach each other in structure that it is still a mooted question whether certain organs found in worms and crustacea are intended for receiving or producing light.

We may also divide luminous forms into two groups according as the oxidation of luminous material goes on continuously, independently of any stimulation of the organism; or is intermittent, oxidation and luminescence occurring only as a result of stimulation, using the word "stimulation" in the same sense in which it is used in connection with nerve or muscle tissue. Bacteria, fungi, and a few fish produce light continuously and independently of stimulation. Its intensity varies only over long periods of time and is dependent on the nature of the nutrient medium or general physiological condition of the organism. All other forms give off no light until they are stimulated. Stimulation may of course come from the inside (nerves) or outside. Only under unfavorable conditions, such as will eventually lead to the destruction of the luminous cells, do these forms give off a continuous light. This has often been spoken of as the "death glow," and is to be compared with rigor in muscle tissue.

Some of the fish which produce a continuous light possess a movable screen similar to an eyelid which can be drawn across the organ, thus shutting off the light, so that the animal appears to belong to the group which flashes on stimulation. This is true of Photoblepharon, while Anomalops can rotate the light organ itself downward, so as to bring the lighting surface against the body wall and thus cut off the light (Steche, 1909). Other fish (Monocentris) are unable to "turn off" their light.

Animals which flash spontaneously on stimulation through nerves from within, possess a very varied rhythm. The different species of fireflies can be distinguished by the character of their flashing (McDermott, 1910-17; Mast, 1912). Fig. 16 shows the method of flashing of some common eastern North America species. The glowworm light lasts for many seconds and then dies out. This interval of darkness persists for some minutes and is then followed by another period of glowing. Some fireflies have a light which may be described as partially intermittent. It lasts for hours, but may become more dim or be intensified on stimulation.

Fig. 16.—Chart showing relative intensities and durations of flashes of American fireflies (after McDermott). One cm. vertically = approximately 0.02 candle power; one cm. horizontally = approximately one second. The flash of the males (?) is at the left; that of females (?) at right of chart.

Some forms only produce light at certain seasons of the year. According to Giesbrecht (1895) this is true of the copepods, which only light in summer and autumn, and according to Greene (1899) in the toad-fish; Porichthys, which can only be stimulated to luminesce during the spawning season in spring and early summer.

Some animals possess a periodicity of luminescence. They only luminesce at night and fail to respond to stimulation or are difficult to stimulate during the day. Bright light has an inhibiting effect. Perhaps correlated with this is the fact that most luminous forms are strongly negatively heliotropic. Fireflies lie hidden in the day, to appear about dusk and the ostracod crustacean, Cypridina, is difficult to obtain on moonlight nights.

The Ctenophores were the first forms in which the inhibiting effect of light was noticed. This was described by Allman (1862) and has been confirmed by a number of observers, especially Peters (1905). Massart found that Noctiluca was difficult to stimulate during the day and Ceratium, according to both Zacharias (1905) and Moore (1908), only luminesces at night, or if kept in darkness, for some little time. Crozier[4] finds a persistent day-night rhythm of light production when Ptychodera, a balanoglossid, is maintained for eight days in continued darkness. The animal is difficult to stimulate during the period which corresponds to day and luminesces brilliantly and at the slightest touch during the period which corresponds to night.

On the other hand, a great many forms are able to luminesce quite independently of previous illumination. According to Crozier[4] ChÆtopterus luminescence is not affected by an exposure to 3000 metre-candles for six hours.

In the case of animals with extracellular luminescence we may speak of luminous secretions and true luminous glands. A large number of forms possess luminous glands or gland cells, including some of the medusÆ, the hydroids (probably), the pennatulids (?), the molluscs (Pholas and PhyllirhoË) (probably), some cephalopods (Heteroteuthis and Sepietta), most annelids, ostracods, copepods, some schizopods (Gnathophausia) and decapod (Heterocarpus and Aristeus) crustaceans, all myriapods, and the balanoglossids. The remaining organisms burn their material within the cell. These include the bacteria, fungi, protozoa, some medusÆ (?), ctenophores (probably), most cephalopods, a few annelids (Tomopterus (?)), ophiuroids (?), some schizopod (Nyctiphanes, Euphasia, Nematocelis, Stylochiron) and decapod (Sergestes) crustacea, all(?) insects, Pyrosoma, and fishes (selachians and teleosts). It is among this latter type that the most complicated luminous organs have been developed. While a description of all the types of luminous organs and luminous structures cannot be attempted here (excellent descriptions have been given by Dahlgren and Mangold) it is necessary to understand the structural conditions in a few of the forms whose physiology has attracted most attention.

Luminous bacteria are so small that the light from a single individual cannot be seen. It is almost impossible to make out structural differences within the cell and we cannot definitely state in just what special region, if any, the luminescence is produced. We do know that the light is intracellular and that filtration of the bacteria from their culture medium gives a dark sterile filtrate absolutely free from any luminous secretion.

Among protozoa, in certain forms at least, it is easy to observe that luminescence is connected with globules or granules which were considered by the earlier observers to be oil droplets. Thus, in Noctiluca (Figs. 17 and 18), when the animal is violently stimulated or in the presence of reagents which slowly kill it, the whole interior appears a mass of starry points of light which can be traced to minute granules along the strands of protoplasm (Quatrefages, 1850).

Turning to the multicellular forms, we find the simplest development of luminosity in those animals which possess gland cells producing a luminous secretion. These cells may be scattered over the surface of the animal as in ChÆtopterus (Fig. 19) or Cavernularia, or restricted to certain areas [Pholas, (Fig. 19),] or more definitely localized to form an isolated group of gland cells as in Cypridina. True multicellular glands also occur. In every case, however, we find that the luminosity of these uni- or multicellular glands is connected with the presence of granules. They are often spoken of as luciferine granules, although it is not certain whether they are made up of luciferin or luciferase (see Chapter IV) or both. They are most similar to the zymogen granules found so abundantly in gland cells and thought to be the precursors of various enzymes. According to Dahlgren (1915), the luciferine granules stain blue-black by iron hÆmatoxylon after fixation at the boiling point, and photogenic cells can be detected by this method of selective staining. Dubois (1914, book), who regards them as examples of bioprotein, comparable to the chondriosomes and handed on from one generation to another, gives them the name of vacuolides or macrozymases. In some forms he has described their transformation into crystals and believed at one time that animal light was a crystalloluminescence. His figures of the crystal transformation are not very convincing. Pierantoni (1915) has considered the granules to be symbiotic luminous bacteria, but this is certainly not the case.

The light of ChÆtopterus comes from a material mixed with a mucous secretion formed over almost the whole body surfaces of the animal. A section of the epithelium shows large mucous-producing cells and smaller granule-containing light cells (Fig. 20). These appear to be under nervous control, as a strong stimulation in one part of the body causes luminescence which spreads over the whole surface of the worm. The animal becomes fatigued rather readily, however. In the pennatulids, such as Cavernularia, we have also the formation of a luminous secretion over the whole surface of the body and the individual animals in this colonial form are also connected with nerves. A stimulation in any local region, as Panceri (1872) first showed (Fig. 21), will cause a wave of luminosity to spread from this point until it extends over the whole surface of the colony. In Pennatula the rate of this luminous wave is about 5 cm. per second.

In many luminous animals the light secretion formed over the surface of the body is small in amount and adheres to the animal because it is embedded in the mucous skin secretions. In those forms which possess a true localized light gland the luminous secretion when expelled into the sea water (if the animal be a marine form) may persist as a luminous streak for some time and exhibit diffusion and convection movements. The most beautiful examples of luminous secretions are found among the ostracod crustacea.

In Cypridina hilgendorfii the luminous gland is situated on the upper lip near the mouth. It is made up of elongate (some 0.7 mm. in length), spindle-shaped cells, each one of which opens by a separate pore with a kind of valve. The openings are arranged on five protuberances. Muscle fibres pass between the gland cells in such a way that by contracting the secretion can be forced out. In the sea water the secretion luminesces brilliantly and the Japanese call these forms umi hotaru, or marine fireflies. Fig. 22 is a diagram showing the structure. Watanabe (1897), who first studied this form, and also Yatsu (1917) have described two kinds of granule-containing cells, one with large yellow globules, 4-10µ in diameter (Fig. 23), the other with small colorless granules 0.5, in diameter. I have observed in the living form these two types and also large colorless globules of the same size as the yellow globules. All dissolve when extruded into the sea water. Dahlgren[5] has described from sections four types of cells containing (1) large globules, (2) small granules, (3) a fat-like material, (4) a mucous material. Just what the significance and nature of these types of substance is cannot be stated at present. At least one, probably two, are concerned in light production. The others may possibly form digestive fluids which act on the food of the animal.

Turning now to the animals possessing light cells with intracellular luminescence we find in general that such light cells are localized to form definite light organs and that these may be single, as in the common fireflies, paired, as the prothoracic light organs of Pyrophorus, or scattered over the surface of the body, as in so many shrimps, cephalopods and fishes, when they are often called photophores. The light cells proper are often associated with reflectors, lenses, opaque screens and color screens.

The insects possess the simplest types of intracellular light organs, a mass of photogenic cells, which, in the common firefly (a lampyrid beetle) of Eastern North America, has probably been developed from the fat body, while in the New Zealand glowworm, the larva of a tipulid fly (Bolitophila luminosa), part of the Malpighian tubule cells have acquired photogenic power (Wheeler and Williams, 1915). This is illustrated in Fig. 24.

The photogenic organ of the firefly is made up of two kinds of cells, a dorsal mass of small cells several layers deep, the reflector layer, and a ventral mass of large cells with indistinct boundaries, the photogenic layer (Fig. 25). The photogenic cells contain a mass of granules, spherical in the male and short rods in the female. The photogenic cells are divided into groups by large tracheal trunks which pass into the light organ and branch to form tracheoles connected with tracheal end cells. The exact distribution varies in different species, but in all the arrangement is such as to give a very abundant oxygen supply. Each group of photogenic cells is surrounded by a clear ectoplasm containing no granules. The tracheoles pass through this and either end openly within the photogenic cells or anastomose with tracheoles from neighboring tracheÆ. Nerves, but no blood-vessels—which are absent in insects—enter the organ. It is difficult to determine if the nerves supply the tracheal end cells or the photogenic cells.

The dorsal reflecting layer is made up of cells containing numerous minute crystals of some purin base, either xanthin or urates, or both. They have a white milky appearance and while they are certainly not good reflectors in the optical sense, they do act as a white background, scatter incident light, and partially prevent its penetration to the internal organs of the firefly. Although a few crystals similar to those of the reflector layer are found in the photogenic cells and in other cells of the body, it is known that the photogenic cells are not transformed into the reflector cells. The two layers are distinct and permanent from an early stage in development.

Curiously enough, the light organ of the larva of the firefly (glowworm) is quite distinct from that of the adult. Like so many other structures in insects, the adult organ is developed anew from potential photogenic cells during the pupal period. Even the egg of the firefly is luminous and glows with a steady light, and during the pupal period light may sometimes be seen coming from the thoracic region.

In the firefly there is no true lens, the light merely shining through the cuticle which is transparent over the light organ, whereas over the rest of the body it is dark and pigmented. In the deep sea shrimp, Acanthephyra debelis, with light organs scattered over the surface of the body, the cuticle covering the light organ forms a concavo-convex lens, behind which are the photogenic cells (Kemp, 1910). As may be seen from Fig. 26, the lens is made up of three layers which suggests that it may be corrected for chromatic aberration—a veritable "achromatic triplet." In an allied form, Sergestes (Fig. 27), the lens is of two layers and double convex. Optical studies of these lanterns have been made by Trojan (1907). The course of the light rays is shown in Fig. 28. The lens of these organs is also bluish in color which suggests that they may serve also as color filters. Behind the photogenic cells is a mass of connective tissues through which enters the nerve, for the light of these organs is under the control of the animal and may be flashed "at will."

All gradations in complexity of light organs may be found from the condition in the shrimp just described to that found among the squid and fish. Figs. 29 and 30 are sections of two of the more complicated types found in squid. The explanation given to the various structures is that of Chun (1903) to whom we are indebted for a careful histological investigation of these forms. It will be noted that in addition to photogenic and lens tissues there are various types of reflector cells and a line of pigment about the whole inner surface of the organ to effectively screen the animal's tissues from the light. In one form (Fig. 30) chromatophores are found about the region where the light is emitted and these no doubt serve as color filters. There are also an abundant blood supply and nerves passing to the organ. Figs. 30 and 31 are sections through light organs of fishes.

We thus see that light organs may be very simple and also very complicated. The latter must have evolved from the former, although it is not always possible to point out the intermediate stages. It is not within the scope of this book to discuss bioluminescence in its evolutionary aspects. It may be worth while, however, to point out briefly what is known concerning the use of the light to the animal. There are four possibilities.

This appears to be the case in the luminous bacteria and fungi and perhaps the great majority of forms which make up the marine plankton, Noctiluca, dinoflagellates, jelly-fish, ctenophores and even the sessile sea pens.

We know that luminous bacteria occasionally lose the power of lighting and that on certain culture media they develop as non-luminous forms. Luminescence is not indispensable to them. The same is true of some of the fungi but Noctiluca and other animals are not known in a non-luminous condition, although we can see no definite value to the organism of this power of luminescence.

(3) The light may serve as a means of recognition or a sex signal to bring the sexes together for mating. It would seem from the work of Mast and of McDermott that this is the case in the common fireflies and it may be the case in the toad-fish, Poricthys, which is only luminous in the spawning season and in the worm, Odontosyllis, of Bermuda, which is brilliantly luminous while swarming when the eggs and sperm are shed. It is non-luminous at other times (Galloway and Welch, 1911.)

(4) Finally, it is possible that animals with complex luminous organs, such as squid, fish and shrimp, actually use these as lanterns. It is significant that most of them are deep sea forms, living in a region of perpetual darkness, and it is perfectly logical to suppose that they make use of their light organs for illuminating purposes.

The whole problem of the use and purpose of luminous organs is an exceedingly complex and difficult one. We have, perhaps, said enough to indicate this and may add that in most cases, so far as opinion is based on actual evidence and observation, that of the layman is of as great value as that of the scientist.