Popular Science Monthly/Volume 7/August 1875/Animal Phosphorescence
AMONG the marvels which excite the admiration of the student of Nature, not the least strange is the group of phenomena known under the name of Animal Phosphorescence. We are so accustomed to associate light with heat, and to consider that fire of some kind is necessary to its production, that the imagination is appealed to with unusual force, when we find light proceeding from the body of a living animal Yet, it is well known that the emission of light is not an uncommon characteristic among the members of the invertebrate divisions of the animal kingdom. Travelers have often expatiated on the beauty of the scenes which they have witnessed in the tropics, when the seas or forests have seemed to be illuminated by innumerable sparks of fire; and recent discoveries have shown that the luminous quality is even more common than was previously supposed. During the dredging expeditions of H. M. S. Porcupine in the years 1869 and 1870, so many of the deep-sea animals were found to be phosphorescent, that Prof. Thomson has suggested that the light necessary to the development of the eyesight which some of the specimens possessed may have had its origin in that source. In animal phosphorescence, as in all her works, Nature exhibits an immense variety in the forms in which she displays her power; in one case, the luminosity will be visible in a fluid secretion; in another, it will manifest itself through the action of a minute and complicated organ; one species of animal will shine with a yellow light; a second, with brilliant green; a third, with pale lilac; and we are acquainted with one instance in which the light changes successively to the chief colors of the solar spectrum. The causes which produce these phenomena are still very obscure. Although many forms of life are known in which the luminous quality is present, scientific men are not yet agreed on what the quality depends; and the purposes which the light serves in the animal economy are also little understood. But the phenomena themselves are often very remarkable.
Some strange theories were advanced to account for the phosphorescence of the sea, before the real cause was discovered. In 1686, an ecclesiastic, named Tachard, suggested that the ocean absorbed the sun's light by day, and emitted it again at night. About the same time, a better-known philosopher, Robert Boyle, endeavored to account for the same phenomenon by the friction which, he supposed, the rotation of the earth upon its axis caused between the water and the atmosphere. The problem was finally solved in 1749, by the discovery of luminous animalcules in the water of the Adriatic Sea; and a large proportion of the lower classes of marine animals are now known to be phosphorescent to a greater or less degree. Let us take the invertebrate divisions of the animal kingdom in their regular order, and briefly glance at one or two examples in each. Beginning with the simplest forms of life, the Protozoa, we find, in addition to certain Infusoria, the little jelly-like organism to which naturalists have given the name of Noctiluca, the phosphorescence of which is largely demonstrated around our coasts.
The radiated class of sea-animals possess high phosphorescent qualities. Star-fish, sea-pens, jelly-fish, sea-fans, sea-rushes, may be mentioned as cases in which the luminous quality is present among the radiata. We will take our examples from among the specimens captured during the expeditions of the Porcupine. On some occasions when the dredge was hauled up late in the evening, the hempen tangles which were attached to it came up sprinkled over with stars of the most brilliant uranium green; little stars, for the phosphorescent light was much more vivid in the younger and smaller specimens. The light was not constant, nor continuous all over the star, but sometimes it struck out a line of fire all round the disk, flashing, or one might rather say glowing, up to the centre; then that would fade, and a defined patch, a centimetre or so long, break out in the middle of an arm, and travel slowly out to the point, or the whole five rays would light up at the ends and spread the fire inward. Doubtless, in a sea swarming with active and predaceous crustaceans, with great bright eyes, phosphorescence must be a very fatal gift. On one occasion the dredge came up tangled with the long pink stems of a kind of sea-pen, which were resplendent with a pale lilac phosphorescence like the flame of cyanogen gas; not scintillating like the green light of the star-fish, but almost constant, sometimes flashing out at one point more brightly, and then dying into comparative dimness, but always sufficiently bright to make every portion of a stem caught in the tangles or sticking to the ropes distinctly visible. In some places, nearly everything brought up seemed to emit light, and the mud itself was perfectly full of luminous sparks. The sea-rushes, the sea-fans, and usually the seapens, shone with a lambent, white light, so bright that it showed distinctly the hour on a watch. In the neighborhood of the Madeiras, jelly-fish have been taken which emitted light in flashes, and the same phenomenon has been noticed in other parts, both in respect to jelly-fish, and, as we shall see, in respect to other animals.
Some of the most beautiful, luminous phenomena of the ocean are caused by animals belonging to the molluscous sub-kingdom, which is nearly as prolific in light-giving species as the Radiata. There is a shell-less mollusk which inhabits the Atlantic, in the neighborhood of the equator, and resembles a tiny cylinder of incandescent matter. It is microscopic in size, but prodigious numbers adhere together, until a tube from five or six to fourteen inches in length is formed, and the sea sometimes presents the appearance of a sheet of molten lava, from the number of these tubes which are floating in it. Moreover, a singular phenomenon is connected with this form of phosphorescence: the color of the light is constantly varying, passing instantaneously from red to brilliant crimson, to orange, to greenish, to blue, and finally to opaline yellow. Another highly phosphorescent species of Mollusca belongs to the family of the Salpidæ, which abounds in the Mediterranean and the warmer parts of the ocean. These individuals also swim adhering together in vast numbers, and produce the effect of long ribbons of fire, sometimes drawn straight in the direction of the currents, sometimes twisted and almost doubled by the action of the waves. In the Mediterranean their phosphorescence often resembles the light of the moon, giving rise to what the French term une mer de lait.
Luminosity is not so frequent a characteristic of the marine Articulata; nevertheless, it is exhibited by certain worm-like animals belonging to the class Annelida, and by a large number of the smaller Crustacea. In many instances the light takes the form of vivid scintillations similar to those emitted by the Medusæ, or jelly-fish, already mentioned. The appearance is sometimes very brilliant, when great numbers of these organisms are present in the sea, the water, especially where it is agitated, being illuminated by sparks of light, varying in size from that of a pin's-head to that of a pea, and vanishing and reappearing in countless myriads. The late Prof. E. Forbes recorded instances in which he found individuals of a species of mollusk, whose visceral cavities had been deprived of their natural contents, to contain multitudes of minute crustaceans which emitted bright and rapid flashes.
If we now leave the marine world, and pursue our investigations among the inhabitants of dry land, we shall find the examples of phosphorescence much reduced in number. With few exceptions, the Articulata alone among land-animals possess this characteristic, and the class Insects furnishes us with by far the largest number of light-giving species. Thus, naturalists enumerate between two and three hundred kinds of luminous beetles, which are nearly restricted to two families, the Lampyridæ and the Elateridæ. We may take the common English glow-worm as a type of the former, and the famous fireflies, said to serve the West Indian belles instead of jewels, as a type of the latter. In both, the organs which emit the light are very similar. Dissecting the abdomen of the glow-worm, two minute sacs of cellular tissue are seen, lying along the sides just under the skin. The cells are filled with a substance which, under the microscope, looks like soft, yellow grease. When the season for giving light is past, this yellow matter is absorbed, and replaced by the ordinary substance of the insect. A multitude of minute air-tubes surround and ramify through the sacs, terminating in a larger tube and a spiraculum, or air-opening in the skin. Free communication with the outer air is essential to the emission of the light of these two sacs, and we are thus able to account for the frequent disappearance of the glowworm's lamp by the power which insects enjoy of closing their spiracula at will. But the Lampyris can in reality only partially extinguish its light; beneath the last segmentary ring of the abdomen a second pair of still more minute sacs are placed, likewise filled with yellow, greasy matter, and the light of these is not entirely under the insect's control. It may always be seen if the glow-worm be closely examined. The most curious feature connected with the organ has still to be mentioned; each of the points at which the light is visible is covered by a transparent, horny cap, divided into little hexagonal facets, and exactly similar in principle to an apparatus invented by man for increasing the diffusion of light.
The best known species of fire-fly, the cocuja of Spanish America and the West Indies, is an insect which resembles the common English black beetle in size, but it is dark-brown in color, and the divisions of its body are less deeply marked. The light is sufficiently strong to be of use to the inhabitants of the countries in which it is found. By inclosing three or four of the beetles in a glass bottle, a lamp is obtained sufficient for ordinary household purposes, and travelers are said to fasten the insects to the toes of their boots, in order to illuminate the pathways at night. The light proceeds from four yellow spots upon the thorax, two of which are hidden by the wing-covers, unless the insect be in flight, when the brightness of the light is also increased by the quicker respiration caused by the motion. The luminous matter is more largely distributed than in the glowworm, and, if the segmentary rings of the abdomen be gently pulled asunder, the light may be seen shining through the semi-transparent skin of the interstices.
Two East-Indian species of luminous beetles are especially worthy of mention. In the island of Singapore, a Lampyris is found which shines with an intermittent light. The insects cluster among the foliage of trees where the ground is damp and swampy, and, in accordance with some strange instinct, flash out their lanterns simultaneously. At one moment the tree will be dotted with bright sparks, which a moment later will have disappeared, excepting two or three. The intervals of darkness are about a second in duration. At these times the insects appear to be settled upon the leaves, and, if they are disturbed, they dart out at random, flashing their lights irregularly, and at shorter intervals. Borneo produces a species of glow-worm which shines with a triple row of lamps. It has been found crawling among dead-wood and leaves, the first row of lights being placed along the back, and the second and third rows along the sides.
Turning to another class among the land Articulata, we may briefly mention the phosphorescence of the centipede and that of the earthworm. Both phenomena may be seen in England, but are more common on the Continent. The centipede, which is tawny brown in color, and scarcely exceeds the tenth of an inch in diameter, is about an inch and a half in length. It frequents out-houses and arbors, where it may sometimes be found crawling along the ground, and searching for the insects on which it feeds. The phosphorescent property resides in a fluid which it secretes, and with which it can moisten the whole of its body. The light becomes more brilliant when the animal is irritated, and, if the fluid be received upon the hand, it will continue luminous for some seconds. M. Audouin, a French naturalist, residing near Paris, was witness of a remarkable appearance which was caused by luminous centipedes. He was informed that there were "earthworms" in a field near his house, glowing like red-hot coals. On going to the place to see, he found merely a few luminous streaks here and there upon the ground; but, when a spade was brought, and some of the earth thrown up, a beautiful spectacle presented itself. Great numbers of centipedes, which had collected together for some purpose, were unearthed, and the soil shone with the light which they emitted, the streaks remaining visible for many seconds, if the clods were crushed beneath the foot. Similarly, Prof. Moquin-Tandon has recorded a case of the phosphorescence of earth-worms, which lie noticed on a garden-walk at Toulouse. The worms were about an inch and a half in length, and looked like little rods of white-hot iron.
It would be out of place in the pages of this journal to discuss the merits of theories which have been advanced to account for animal phosphorescence. As we have already said, Science has not pronounced any final decision on the matter. Some philosophers look upon the light as the result of the slow combustion of some combination of phosphorus contained in the animal secretions; others believe it to be a direct manifestation of vital force, acting through special organs, much in the way that electricity is produced in the torpedo or gymnotus. No doubt the problem will ultimately be solved as we advance in the study of comparative anatomy, and, in the mean time, many experiments have been made, in the hope of assisting the solution. It has been found that the luminous matter will communicate its peculiar property to liquids or solids with which it may come in contact. The light is extinguished by a cold or boiling temperature, or by strong stimulants; it also disappears in vacuo but becomes visible again on the admission of the air; and it is increased by moderate heat, and by gentle stimulants. In respect to the glow-worm, the two smaller sacs of yellow matter which we described possess the curious property of shining uninterruptedly for several hours, after they have been removed from the living body, the light from other parts being extinguished immediately under similar circumstances. A simple galvanic current passed through water containing Noctilucæ produced no effect; but an electro-magnetic current, on the other hand, caused, after a short interval, a continuous and steady glow to issue from the water. The light disappeared at the end of a quarter of an hour, and could not be reproduced, the animalcules being evidently dead.—Chambers's Journal.