In general the light is increased when the animal is in motion; and in insects, parti cularly during flight. Macartney observed of the beroi, that when it swam slowly near the surface of the water, its whole body became occasionally illuminated in a slight degree; but that, during its contractions, a stronger light issued froM the ribs, and that when a sudden shock was communicated to the water in which it was swimming, a vivid flash was given out.
That the luminous function is in niany ani mals directly under the control of their will, seems to be proved by the fact, that while under any sudden irritation calculated to alarm them, they, at first, emit light strongly, yet on the frequent repetition or continuance of the same kind of irritation, they extinguish their light, and cannot be excited to spew it again for a considerable time.
II. Artificial circumstances in which light is emitted by living animals, or by which the emission of it is affected.
Light-giving animals being removed from their natural situations, and subjected to arti ficial processes and agents, are found to have their luminousness affected by being exposed to, 1. the effects of accumulated electricity and electrical currents ; 2. immersion in va rious fluid and gaseous media; 3. pressure of their bodies; 4. removal of their luminous organs, and mutilation of these and of other organs ; 5. exposure to various degrees of heat and moisture ; 6. immersion in vacuo ; 7. re moval from all foreign sources of light.
1. The effects of accumulated electricity and electrical currents. In experimenting on ma rine luminous animals, Macartney passed a shock through water in which they were swim ming; immediately their light was extinguished for an instant, but afterwards became brighter than before. In reporting the result of a si milar experiment, Humboldt merely says that the luminousness of the animals was increased after the shock. Macaire subjected glow worms to the action of galvanism, and found that when one wire was forced through the body of the insect as far as the luminous organs, while the other was applied to the sur face slightly moistened, the light became bril liant. One galvanic pole produced no effect; but when insects not shining at the time were placed in a galvanic circle they always began to give out light. This result was not ob tained in vacua, but whenever the air was admitted, the light reappeared. No effect what ever seemed to be produced by common elec tricity, howsoever applied.
2. Immersion in various fluid and gaseous media.—Luminous marine animals, when re moved from their native element, and plunged into fresh water, give out their light for a time more vividly and more steadily, but afterwards it gradually fades and becomes extinct. Mineral and vegetable acids, alcohol, potassa, and solu tions of corrosive sublimate, and the salts, all produce nearly the same effect ; only that by these the light-giving property is more speedily destroyed. Observers differ in their accounts of the effects produced by immersion in va rious gases. Most of those who have experi
mented in this way have seen the light of the glow-worm very rapidly extinguished in hy drogen gas; also in sulphuretted and car buretted hydrogen, carbonic acid, chlorine and nitrogen gases; but Sir II. Davy found that hydrogen gas produced little or no change in the state of the light ; the same was the result of Murray's experiments, who also found 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 italica, 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 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 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.