Among the Ilemipterous insects, the Fulgora is next conspicuous for its light. It is not known how many of the species possess that quality, but it has been observed in the Lanternaria, the Candelaria, and the Pyrrarmchus. The first, or the lantern-fly, is a native of South America. In this tribe the luminous matter is contained in a trans parent snout, or projection of the head, which is differently shaped in the different species. The lantern-fly is two or three inches in length, and the luminous organ is so large, that the light which it produces is very considerable; quite sufficient for reading the smallest print. In the Scolopen dra, the Elcctrica and Phosphorea are both luminous, and the former is a common insect in our gardens. It is sus pected that other species in this genus have also the same property. In the genus Pausus, which we mention ed, the SpInerocerus is known to be luminous, and the light, which is feeble, proceeds front the antennx. Other genera besides those which we first mentioned, also ex hibit these faint partial lights ; as the Noctua psi, the Bombyx cossus, and some other moths, and in these it is produced in or about the eye. It is also suspected by Lamark, that the Chiroscelis bifenestrata has a luminous organ ; and it has further been observed, that the Ocelli, in the Elytra of Buprestis ocellatx, are luminous. The same is suspected to be the case in the Gryllotalpa vulga ris ; and we have little doubt that it will be found in many others where it has not yet been observed.
The observations on the nature of the luminous matter are very scanty, and we are indebted for some of the principal ones to Mr. Nlacartney. That it is in some cases a permanent substance, independent of the will of the in sect, is ascertained by the fact, that the boys in the West Indies rub their faces with the dead body of a fire-fly to make them shine in the dark. In both the glow-worms, the Elater noctilucus and the ignitus, Mr. Macartney found that the light proceeded from a substance resembling the common fat, or interstitial matter of insects, but of a yel lower colour. Besides this, he found under the last ab dominal segment two minute oval sacs, formed of an elas tic spiral fibre similar to that of the tracheae, containing a soft yellow substance, of a closer texture, like that which lines the adjoining region, and affording a more permanent and brilliant light. This light is less under the control of the insect than that from the adjoining luminous substance, which it has the power of voluntarily extinguishing, not by retracting it under a membrane, as Carradori suppos ed, but in some other manner which could not he ascer tained. It is further suspected that, in this animal, the whole interstitial matter• is luminous; and De Geer ob served, that there are two luminous patches under the elytra, and that the intervals between the abdominal seg ments gave light when the animal was stretched.
M. Macaire's'observations on the luminous organ do not differ very materially from those of Mr. Macartncy, but are more minute, and apparently more accurate. The in ner surface of the three last abdominal segments is cover ed with a pale yellowish substance, which appears semi transparent. But the microscope discovers it to be form
ed of numerous minute fibres, ramifying in a complicated manner, and phosphorescent. When this substance has been removed, the luminous appearance of the tail ceases; and it is owing to the transparency of the segments under which it lies that the light is at all visible. In the two posterior luminous points the same fibrous substance is found, but of a more compact texture.
The immediate cause of the light has not been ascertain ed in the Fulgoras, nor in the Pausus. But in the Scolopen.
dra electrica there is a luminous matter secreted over the surface, which may be taken off on the hand, where it continues to shine for a few seconds. The chemical cause has rather been guessed at than ascertained. It is known that phosphoric acid is an ingredient in animal substances, and therefore it has been supposed that this luminous matter was phosphorus. This is doubtful.
It is a secreted substance, in the first place, because when extracted from the receptacles, it was restored in two clays. Being dried, it becomes glossy like gum, and loses its luminous property, but recovers it again on being moistened. When kept moist after extraction, it con tinues luminous for some time. By some naturalists it is said to have its brilliancy increased by heat, and by being placed in oxygen Bass, and to be extinguished by cold, and by hydrogen and carbonic acid gases. Hence Spallanzani, and others following him, concluded that the phenomenon arose from the slow combustion of phospho rus. But Carradori, on the contrary, observed that the luminous matter of the Lampyris Italica shone alike in vacuo, in oil, and in water ; a fact which is sufficient to set aside this hypothesis. Mr. Macartney's experiments coincide with Carradori's, and as he further found that the luminous substance was not susceptible of inflammation by applying flame, or a hot iron to it, and that it had no effect on the thermometer, it is very certain that it is not phosphorus, and that the light is not produced by any combustion.
Thus far former observers ; but we must now notice the most recent observations made by M. Macaire. On immersing a glow-worm alive into warm water, of the temperature of 11° Reaumur, it became restless ; and when the heat was raised to 22° it began to give out light. At 33° it became still brighter, but shortly after this the animal died ; yet the light was not extinguished. But at it ceased altogether. The same experiment, fre quently repeated, gave mean results, from which it was deduced that the light appeared at a heat varying from 20° to 25°, and ceased at one varying from 47' to 50°. When the heat of the water was maintained at 28°, the phosphorescence continued, although the animal was dead : and when it was suffered to subside below the light went out. When the insect was thrown into water heated to 35°, or 40°, it died instantly, but shone very bright : but, on raising the heat ten degrees, the light was extinguished, and nothing could revive it again.