ANIMAL LUMINOUSNESS. 201
the glow-worm continue to shine in carbonic
acid gas. Immersion in oils of all kinds de¬
stroys the light-giving property in most of the
insects endowed with it ; but in Lampyris
italien, Carradori found that the light conti¬
nued to be emitted when the luminous part
of the body was plunged into oil.
3. Pressure of their bodies.—It has been
observed that shortly after the death of the in¬
sect, the light-giving organs of dater emit light
freely when the body is bruised, and in general
mechanical irritations of all kinds cause a cer¬
tain degree of increase in the intensity of the
light given out. Some animals, as pennatula,
seem to emit their light rarely, excepting in
such circumstances.
4. Removal of the luminous organs, and mu¬
tilation of these and of other organs.—The
luminous organs may be cut out from the
bodies of glow-worms and fire-flies without the
peculiar property of the organs being imme¬
diately destroyed. The emission of light can
for some time be re-excited by slight me¬
chanical irritations ; as by touching the organs
with the point of a pin. Those of the glow¬
worm have been seen to shine for two or three
days after excision, when slightly moistened
with water, heated or electrified. In experi¬
menting on the same insect, Todd found that
the light was extinguished within six minutes
after the head was cut off ; as also when the
luminous rings were cut into, but was renew¬
able by the application of heat. Sheppard
removed the luminous matter from a glow-
worm ; the wounds healed within two days,
and the body became again filled with new
light-giving substance.*
5. Exposure to various degrees of heat and
moisture.—Light-giving insects in general do
not shine at any temperature below that of
53° Fahr. Macaire took some glow-worms
that had been kept for some time at a tem¬
perature of 50° Fahr., plunged them into water
at 55°, and gradually raised the temperature.
Light was emitted for the first time at 77°,
and increased in intensity until the water was
at 105°. At this temperature the animals died,
but the light continued until the temperature
had reached 134° 5, when it wholly disap¬
peared. When glow-worms are thrown alive
into water heated to 110° and upwards, they
die instantly, but at the moment emit a brilliant
light. When they are exposed to an artificial
cold suddenly, they perish at any degree below
the freezing point of water ; but the light may
be partially restored by a temperature of 70°,
although the animals shew no other sign of
vitality. When the insects are dried artificially,
the light is extinguished, but it may be restored
by their being again moistened.
6. Immersion in vacuo.—When glow-worms
are placed in vacuo, their light fades, but re¬
appears on admission of air.
7. Removal from allforeign sources of light.
—If luminous insects be confined in a dark
place, they shine little in the early part of the
day, but long before night they begin to do so ;
* Kirby and Spence’s Entomology, ii, 426.
although generally, in their native situations,
they do not emit light until the twilight. If
the confinement in a dark place be protracted,
they do not shine so brightly as after having
seen the sun during the day.
IV. Seat of luminousness in different animals.
—In most of the luminous animals that inhabit
the ocean, a great part of their surface seems
to b*3 endowed with the property of forming,
and pouring out, a mucous fluid, which contains
the luminous matter, and is frequently mis¬
cible with water and other fluids. This some¬
times so entirely covers the whole animal as
to cause it to emit light from every point of its
surface ; but more generally when the animal
is swimming, the light is seen to proceed only
from certain regions. Some of the medusae,
even of the largest size, emit light from a very
small point, particularly when the luminous
organ is placed in the central parts of the body.
When the light is vivid, it seems to be larger
than it really is, from the refracting power of
the gelatinous tissues through which it passes.
Occasionally the luminous point has not a
diameter equal to the l-200th of that of the
animal itself. In cydippe pileus and Oceania
pileata of the Baltic, Ehrenberg finds that the
light issues solely from the vicinity of the
ovaries, and in Oceania hemispherica, from the
bases of the cirri. Pholas dactylus gives out
light most strongly from the internal surface
of its respiratory tubes. The luminous mucus
is sometimes poured out even by very small
animals in such quantity as to leave a lumi¬
nous wake behind them, as in an instance
mentioned by Quoy and Gaimard. These“ ob¬
servers saw such luminous lines formed in the
paths of certain extremely small creatures, so
transparent that their forms could not be dis¬
tinctly made out. The positions of their bodies
were marked in the water by bright spots,
which were followed in their course by lumi¬
nous wakes, at first about an inch in breadth,
but afterwards by the movements of the water
spread out to the breadth of two or three
inches. This luminous mucus is supposed to
be the seat also of the remarkable stinging
property possessed by many of the acalephce.
It retains its luminousness in some instances
for a day or two after being emitted by the
animal, but loses it whenever putrefaction
commences.
But although this luminous mucus be so ge¬
nerally secreted and emitted by marine ani¬
mals, it is evident that the light given out by
many of them has its seat in certain organs
more or less internal, whence it proceeds in
gleams and momentary flashes that seem to
depend only on the movements of some im¬
ponderable agent. The exact position and re¬
lations of these organs can seldom be satis¬
factorily discovered, but in some crabs and mi¬
nute crustaceous animals that emit light, it is
observed to proceed from the central organs of
the nervous system. In other Crustacea the
whole body seems to be full of light, which is
emitted, as at so many windows, through the
translucent membranes interposed between the
segments of the crust. Dr. Macculloch con-
