The luminous proboscis or snout of the fulgora is hollow, and has a free communica tion with the external air by a narrow slit situated near the base of the organ. Its cavity is lined with a fine membrane, between which and the outer translucent corneous crust, there is interposed a soft tissue of a pale reddish colour, arranged in lines longitudinally, which is supposed to be the seat of luminousness in this insect.
In several instances it has been found that the light-giving substance has continued to shine for a considerable time after being re moved from the body of the insect. In such cases it has been observed that there appeared to he no diminution either in the weight or the bulk of the luminous organs, excepting what was obviously produced by evaporation of the fluids.
VI. Geographical distribution of luminous animals.—ln almost all seas, in every latitude from 60° S. to 80° N., have light-giving ani mals been seen; but they are more abundant, and shine with greater brilliancy, in the tropical than in the colder climates. In general, it is observed by voyagers, the luminous mollusca and acalephm occur in greatest numbers not far from land ; and that they are particularly plen tiful in the seas surrounding groups of small islands within the tropics.
The luminous insects are met with chiefly in the warmer climates of the temperate zones and within the tropics. We are not aware of any having been met with beyond the latitude of 58°.
VII. Theories of animal luminousness.— Very numerous have been the theories formed by philosophers with regard to the nature of the luminous matter which produces the phe nomena now under review. From the facts stated above, it appears that we are not yet in a position to determine with certainty whether or not animal luminousness has its source in the operations of any agent already known. At least it appears to us that facts enough have been accumulated to set aside the assumption of most of the theories hitherto promulgated. The following are some of these.
1. That light is imbibed from the sun's rays by luminous animals and given out in the dark. (Beccaria, Mayer, &c.) 2. That the light is owing to a kind of com bustion maintained by the oxygen of the air. (Spallanzani.) 3. That light is swallowed with the food, and disengaged by peculiar organs. (Brugna telli.) 4. That the light-giving matter is composed of phosphorus and albumen ; and that the variations in the intensity of the light depend on the more or less complete coagulation of the albumen, by some internal means placed under the control of the animal's will. (Macaire.) 5. That a fluid containing phosphorus is secreted by the luminous organs, and shines on its being exposed to the oxygen of the air introduced by respiration. (Darwin, H. Davy,
Heinrich, Trevimnus, and Burmeister.) 6. That the luminous organs conce,trate and modify the nervous influence so as to form it into light ; so that, according to this theory, animal luminousness is an effect solely of vital power. (Macartney and Todd.) 7. Tiedemann thus expresses his opinion : " Animal luminousness would seem to depend on a matter, the product of the changes of composition accompanying life, and, to all ap pearance, secreted from the mass of humours by particular organs. This liquid probably contains phosphorus or an analogous combus tible substance, which combines with the oxy gen of the air or of aerated water at a medium temperature, and thus produces the disengage ment of light. The preparation and secretion of this substance are acts of life, change, augment, or decrease by the influence of ex ternal stimulants, whose action on the animals modifies their manifestations of life. But the phosphorescence itself depends on the com position of the secreted matter and cannot be regarded as a vital act ; because, on certain occasions, it continues for whole days and even after the death of the animal."* This opinion seems to coincide pretty nearly with that of Darwin, Heinrich, and others, stated above (5); and we must admit that it ap pears to harmonize the facts better than any of the other theories that have been propounded. But, while it seems satisfactorily to assimilate many of the phenomena to others more familiar to us, and more within the reach of our investigations, and thus appears to furnish future inquirers with a key to the elucidation of what yet re mains obscure, it is obvious that it leaves some of the phenomena unexplained, and that se veral of these seem to be quite irreconcilable with the theory of the phosphorescence being essentially dependent on the composition of secreted fluids. In some of the extremely delicate acalepha, for instance, from which the brightest radiance is so frequently emitted in momentary fitful flashes, it is difficult to con ceive why, if the light mainly depended on the nature of some matter poured out by cer tain organs at the instant of the flash, the light should not continue for at least a few seconds. The circumstance of its doing so in some in stances, and even mixing gradually with the surrounding sea-water, certainly proves that there is such a fluid as this theory supposes in certain animals; but does not remove the dif ficulty presented by the facts we have al luded to.