200 



ANIMAL LUMINOUSNESS. 



sparkling of the minute medusa and other 

 animals, swimming on the surface of the sea, 

 when they are dashed against the sides of a 

 ship, struck by an oar, or tossed on the foamy 

 crests of the waves ; and this even while no 

 other light is seen excepting just at the points 

 where the water is agitated. In experimenting 

 witli Medu.tee, Macartney found that, when 

 kept in a glass vessel in a state of perfect rest, 

 they gave out no light, but that, on the slightest 

 movement of the vessel, a brilliant flash was 

 emitted, which was brightest when the animals 

 swam near the surface. Macculloch remarks, 

 " Very often we have found the water crowded, 

 even with the largest ineduste, yet scarcely be- 

 traying themselves by an occasional twinkle, 

 when the dash of an oar or any accidental agi- 

 tation was sufficient to involve the whole water 

 in a blaze of light." 



5. Lund noises. When any loud noise is 

 made near a luminous insect while shining, it 

 frequently ceases to give out its light. 



6. Internal movements of the animals them- 

 selves, will, fyc. With regard to insects, we 

 have many concurrent testimonies to the fact 

 that more light is emitted during the season of 

 procreation by most of the species than at other 

 times. So strikingly is this the case in the Lam- 

 pyrides, that the light given out by the female 

 has been generally regarded, (although without 

 sufficient reasons,) as destined only to attract 

 the attention of her mate. After the eggs are 

 deposited, the light gradually decreases in in- 

 tensity. 



While it is obvious that, for the most part, 

 the emission of light is altogether independent 

 of any voluntary effort on the part of the animal 

 itself, yet it appears probable that, through 

 some means or other, the animal has the power 

 of varying the intensity of the light at pleasure. 

 We cannot, for instance, imagine that sound 

 can have any direct effect on the light-giving 

 organs themselves, so as to cause them to shine 

 less brightly when loud noises are made near 

 them. Such effect must be communicated 

 through the animal's sensorium. It is sup- 

 posed by some physiologists that variations in 

 the intensity of the light given out by insects 

 depend on the quantities of air admitted through 

 the trachece in respiration, over which quantities 

 the animal's will seems to exercise some con- 

 trol. In observing the luminousness of the 

 elater, Spix concluded that this control is so 

 peifect, as to admit of the light being wholly 

 extinguished by the animal's preventing the ad- 

 mission of air ; and this view is adopted also 

 by Treviranus. These changes, however, are 

 explained by others, (as by Miiller and Mur- 

 ray,) by supposing that, when the light seems 

 to fi.de, the organs are merely withdrawn be- 

 hind opaque parts, or, as it were, veiled by a 

 curtain. 



In general the light is increased when the 

 animal is in motion; and in insects, parti- 

 cularly during flight. Macartney observed of 

 the bero'i, 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 tluit 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 many 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 shew 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 vacua; 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 vacuo, 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 Jluid 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 H. 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 



