The light of some species is intense, while that of others is very feeble. By placing detached parts of the luminous organs upon a page, I have been able to make out the type; and, if numbers of living lightning-bugs are confined, they can be utilized as a lamp,--rather a dull one, it must be confessed, unless the numbers are greatly augmented. The _larvæ_, as well as the _imagos_, are often luminous; even the eggs of some emit light. An examination of the luminous organs during the daytime shows them to be yellowish or whitish patches on the various segments. If the hand is held over them, the light is seen, and in complete darkness they present a magnificent spectacle,--the light dying away, then growing intense, about the spot, so that it appears to be fairly trembling with heat, as if some chemical action was periodically asserting itself, causing the tissues to become suffused with a fiery glow; yet, if the most delicate thermometer is placed against the luminous organs of a large number of these insects, there is not the slightest elevation to show the presence of heat. If now we kill the insect, and remove the luminous matter, it resembles a bit of starch with luminous spots; and pressure, which admits more oxygen, causes a temporary increase in the light. The luminous organs are similar in structure to the fat body of the insect, and are made up of light-emitting cells (Plate X., Fig. 8), surrounded by a maze of _tracheæ_, or air-tubes. In explanation of the light, it has been suggested that the cells secrete phosphuretted hydrogen, which becomes luminous upon contact with oxygen which reaches it through the minute air-tubes. Regarding _Luciola Italica_, Professor Emery says that the male _Luciolæ_ gave out light in two distinct modes: in the night, when they are brisk and fly about, the light increases and decreases at short, regular intervals, so that it seems to twinkle. If one of them is caught flying, or disturbed in its rest by day, it shines less than at the maximum of its intensity when on the wing, but without intermission. It is remarked, however, that the luminous plates do not shine uniformly over their whole extent; but that sometimes one spot, and sometimes another, glows more strongly. If such a specimen is examined under the microscope, we perceive, on a dark background, bright, luminous rings, which are not, however, uniformly brilliant, but display certain more intense points, which flash up, and again disappear, or continue to shine on faintly for a time, re-appearing afterward in full splendor. These changes take place without any regular succession.[38]