The Encyclopedia Americana (1920)/Luminosity of Animals
LUMINOSITY OF ANIMALS is most familiar to us in the fire-fly and glow-worm, on land, and in what is called phosphorescence, at sea; with regard to the latter popular term, arising naturally from the resemblance to the wavering light of phosphorus, it may be said that the phenomenon has nothing to do with phosphorus. It appears in plants, especially in the mycelium of fungi and certain agarics, often causing rotten wood, decaying vegetables, etc., to glow in the dark. Bacteria on such objects are another source of “phosphorescence,” causing the shining appearance of putrid fish. There appears to be no essential difference between the luminosity of plants and of animals, and it is believed by some that the power to produce it belongs to all creatures, although visible in only a few, mostly living in the sea. On land some myriapods and worms, and various insects or their larvæ and in the ocean a great number and variety of invertebrates, including mollusks (cephalopods), crustaceans and many fishes, manifest luminosity. “On a dark night,” says a writer, “the crest of every wave often seems to break in a pale glow, the wake of the vessel is a trail of light, and an oar dipped in the water seems on fire.” The narratives of voyagers abound in descriptions of such phenomena seen at their best in the tropics, but observable even in Arctic waters. The origin and nature of this light are not clearly known, although it has been extensively studied. It appears to be dependent on the presence of oxygen in an alkaline medium. This luminescence is never manifested by fresh water, although so common among marine fishes, which are in contact with sodium chloride or calcium chloride. It is thought by some to be the product of the chemical action of oxygen on fatty constitutions in the cells, by others of two special substances, one an enzyme, the other an element of the blood that betrays itself as light when it flows into a luminous organ. Watasé considers the light-giving material a cell-secretion, not a product of a gland. Still another view is that the light is the effect of chemical action resulting in an electrical manifestation. The spectrum of the fire-fly shows greatest intensity in the central (green) part, and disappears before reaching either end of the solar spectrum — that is, it exhibits neither heat nor chemical energy. Langley and Very, who devoted much effort to its investigation, called it the “most economical light known,” and very near to the ideal of light without heat. The manifestation of such light may occur in three ways. In the minute protozoans (Noctiluca, etc.), which sometimes are met with in warm seas in such dense masses that the water is like a glowing broth, the light issues from a myriad of points, a diffuse illumination along the muscle fibres.
In a class that includes such mollusks as the pholads, and also the copepod crustaceans, the photogenic material is ejected as a liquid (slime) from the body, according to Watasé, and becomes luminous only by contact with the water, or, more strictly, with the oxygen mixed with it; and the same is true of luminous terrestrial worms and annelids, but here the material emanates as fine granules that glow when they encounter the air. A third class includes the more highly organized insects, squids, fishes, etc., which possess definite light-producing organs.
The luminous insects with few exceptions are beetles of the pentamerous families Lampyridæ and Elateridæ. The former family is well represented in North America, and furnishes us with several species of fire-flies. The females of some species in this group are wingless, or nearly so, creep about among the herbage, and are called glow-worms; in other species it is the larvæ that are glow-worms. Their luminous organs are situated on the head or the abdominal segments or both, in positions varying with the species, and consist of cells just beneath the cuticle, reached by nerves and surrounded by a wrapping of tracheal filaments, supplying air — the necessary oxidizing agent. Their light is usually given out in intermittent flashes, but in some species continuously, and in all, apparently, it is under nervous control. The most famous of the fire-flies is the cucuyo of the American tropics (Pyrophorus noctilucus), one of the Elatendæ; but several other species equal its powers. This beetle is an inch and a half or more long, dull in color, and has on each side of its thorax an oval, whitish “lantern,” from which at will streams a bright light. Gosse (‘Naturalist's Sojourn in Jamaica,’ 1851) says that when this beetle was handled these spots would ignite gradually, “the centre of each tubercle first showing a point of light which in a moment spreads to the circumference and increases in intensity until it blazes with a luster almost dazzling.
” The color of the thoracic light is a rich yellow-green. In a pitch-dark room this insect gives so much illumination as to cast a definite shadow of any object on the opposite wall, and when held two inches from a book the whole line may be read.”
Gosse and the many others acquainted with these phosphoric beetles describe another light on their abdomen, just behind the legs. “When fully illuminated,” the Beebes (‘Search for a Wilderness,’ 1910) tell us, “this area was brilliant and of a figure-of-eight shape. The light, however, was radically different from that of the thorax, being yellowish and candle-like, giving an illusive impression of an opening from the incandescent interior of the insect. In flight the abdominal searchlight comes into play, burning brightly with a strong yellowish glare.” In other species the glare is orange or reddish, and the insect races along like a railroad train, with two green head-lights and a red tail-light. Beebe found that two or three in a glass tube answered the purpose of an electric flash-light. These and similar insects are worn in the hair, and otherwise adapted to the purpose of ornament by the Mexicans and Brazilians.
One of the surprises of the early explorers of the depths of the ocean was that many of the tropical pelagic fishes had luminous organs; and for a long time it was believed that all of these came from the abysses where no particle of sunlight penetrated, and that thus the stygian depths were illuminated. Later investigations have modified this view. The luminous fishes belong mainly to the families Stomiatidæ, Sternoptychidæ, Scopelidæ and Ceratidæ. All these are pelagic families and have species ranging from the surface to the deepest bottoms (see Deep Sea Exploration). The extensive explorations of the ocean depths carried on in the Valdivia, the Michael Sars and other vessels in recent times show that light-organs are “specially characteristic of fishes belonging to the upper 500 meters in warm oceanic waters.” This is true of the crustaceans and cephalopods, in the latter case squids, living at intermediate depths. The light-organs in these animals are distributed in various parts of the body, and are highly complicated, eye-like glands, each having not only a lens to increase its power, but a layer of black pigment in the rear, to act as a reflector. There is no doubt that this structure enables the fishes and squids to project the light in definite directions. The function and importance of these organs have been much discussed; and the purpose is still in question. “Is it in order to illuminate the surrounding water, to avoid foes, or to recognize their own kind?” asks Hjort (‘Depths of the Ocean,’ 1912). “The answers,” he replies, “would probably tend to show that the many different kinds of light-organs serve different purposes.” Brauer finds that their position is precisely the same in all individuals of any one species, and concludes that they replace, in the darkness of the depths, the specific color-marks or “recognition-marks” of surface or terrestrial animals. As it seems certain that this emission of light is intended to be seen, the relation between them and the eyes of these fishes and other oceanic animals must be studied; but the data for this are defective and confusing. It appears that a remarkable coincidence exists between the development of light-organs and eyes in pelagic fishes. The Scopelidæ, Sternoptychidæ and Stomiatidæ, which live above 500 meters, possess well-developed light-organs and eyes, while from 500 meters downward light-organs and eyes both decrease in size. Along the sea-bottom, however, the fishes have only eyes and no light-organs.
" The eyes of the bottom fishes (Macrurus) are large, as if to admit as much as possible of a scanty supply of light. In those depths all the invertebrate animals are aglow, clothed in a copious mucus that emits light; and it is believed that this illumination is sufficient to enable the big large-eyed bottom fishes to see what is about them. The situation is, nevertheless, full of puzzling contradictions. Consult books mentioned under Deep Sea Exploration; and for the most recent discussion of the matter consult Murray and Hjort's ‘Depths of the Ocean’ (London 1912).