I. Circumstances essentially connected with the state of nature in which the animals are placed when they give out light.
II. Circumstances artificially produced af fecting the emission of light.
I. Natural circumstances in which light is emitted by living animals. The luminousness of animals in their natural state is affected by, 1. Changes in the state of the medium in which they live, whether air or water, in regard to its temperature and electricity. 2. By solar light. 3. By abrupt collision with other bodies. 4. By loud noises. 5. By the internal move ments of the animals themselves, amongst which may be included the exercise of the ani mal's will.
1. Temperature, 4•c.—By far the greater number of luminous animals with which we are acquainted are natives of warm climates ; but those inhabiting the ocean are seen in almost all latitudes, even in the coldest ; al though in these they are not so numerous, and give less light. No aerial insects give out light, in ordinary circumstances, excepting at a temperature of about Fahr. and upwards ; and the higher the natural temperature, the brighter is the light emitted.
In temperate climates the Lampyridte shine only in summer and autumn. L. noctiluca appears in this country between June and Sep tember; L. splendidala, in Germany, is lumi nous in May; and L. hemiptera so early as in the end of April.
The light of pholas dactylus is strongest in summer; and that of marine animals in ge neral is increased before storms.
2. Solar light.—Tt is said that Scolopendra does not shine at night excepting it has been exposed during the day to solar light. A short time of exposure to the sun's rays seems to be sufficient to refresh its luminous power, as (like all other light-giving animals) it secretes itself as much as possible during the day. It is stated by Burmeister,* with regard to the Lam pyris Italica, that if it be kept some days in the dark it entirely loses its luminousness, but regains it on being again placed in the sunshine.
3. Lunar light.—Macartney remarked that luminous medusa generally retreat from the surface of the water at moon-rise.
4. Abrupt collision with other bodies.— Marine luminous animals very readily emi their light on being struck by any moving body; so that one of the most commonly observed phenomena connected with this subject is the 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
with Medusa, Macariney 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 medusa, 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. Loud noises.—When any loud noise is made near a luminous insect while shining, it frequently ceases to give out its light.
ti. Internal movements the animals them selves,—will, 4c.—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 deAtined wily to attract the attention of her mate. After the eggs are deposited, the light gradually decreases in in tensity.
NV hile 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 trachem in respiration, over which quantities the animal's will seems to exercise some con trol. In observing tile luminousness of the elater, Spix concluded that this control is so perfect, 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 Muller and Mur ray,) by supposing that, when the light seems to fade, the organs are merely withdrawn be hind opaque parts, or, as it were, veiled by a curtain.