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," caus ing the shining appearance of putrid fish. There appears to he 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 \whits in sects or their larva 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 is a trail of light, and an oar dipped in the water seems on fire." The narratives of voyagers abound in descriptions of such phe nomena seen at their best in the tropics, but okervable 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 lumi nescence is never manifested by fresh water, al though so common among marine fishes, which are in contact with sodium chloride or calcium chloride. It is thought by some to be the prod uct 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. Watase considers the light-giving material a cell-secre tion, 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 in tensity in the central (green) part, and disap pears before reaching either end of the solar spectrum — that is, it exhibits neither heat nor chemical energy. Langley and Very, who de voted 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 Watase, 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 ter restrial worms and annelids, but here the ma terial 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 Lampy rida and Elaterida. 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 larva that are glow-worms. Their luminous organs are situated on the head or the ab dominal segments or both, in positions varying with the species, and consist of cells just be neath the cuticle, reached by nerves and sur rounded by a wrapping of trachealfilaments, supplying air—the necessary oxidizing agent. Their light is usually ON en 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 Elaterada; 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 ( > The color of the thoracic light is a rich yellow-green. In a pitch-dark room this insect gives so much illu mination 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. fully illuminated,° the Beebes 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, i 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 rail road 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 pur pose of ornament by the Mexicans and Bra zilians.