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).