Page:Popular Science Monthly Volume 7.djvu/472

456 numbers of these organisms are present in the sea, the water, especially where it is agitated, being illuminated by sparks of light, varying in size from that of a pin's-head to that of a pea, and vanishing and reappearing in countless myriads. The late Prof. E. Forbes recorded instances in which he found individuals of a species of mollusk, whose visceral cavities had been deprived of their natural contents, to contain multitudes of minute crustaceans which emitted bright and rapid flashes.

If we now leave the marine world, and pursue our investigations among the inhabitants of dry land, we shall find the examples of phosphorescence much reduced in number. With few exceptions, the Articulata alone among land-animals possess this characteristic, and the class Insects furnishes us with by far the largest number of light-giving species. Thus, naturalists enumerate between two and three hundred kinds of luminous beetles, which are nearly restricted to two families, the Lampyridæ and the Elateridæ. We may take the common English glow-worm as a type of the former, and the famous fireflies, said to serve the West Indian belles instead of jewels, as a type of the latter. In both, the organs which emit the light are very similar. Dissecting the abdomen of the glow-worm, two minute sacs of cellular tissue are seen, lying along the sides just under the skin. The cells are filled with a substance which, under the microscope, looks like soft, yellow grease. When the season for giving light is past, this yellow matter is absorbed, and replaced by the ordinary substance of the insect. A multitude of minute air-tubes surround and ramify through the sacs, terminating in a larger tube and a spiraculum, or air-opening in the skin. Free communication with the outer air is essential to the emission of the light of these two sacs, and we are thus able to account for the frequent disappearance of the glowworm's lamp by the power which insects enjoy of closing their spiracula at will. But the Lampyris can in reality only partially extinguish its light; beneath the last segmentary ring of the abdomen a second pair of still more minute sacs are placed, likewise filled with yellow, greasy matter, and the light of these is not entirely under the insect's control. It may always be seen if the glow-worm be closely examined. The most curious feature connected with the organ has still to be mentioned; each of the points at which the light is visible is covered by a transparent, horny cap, divided into little hexagonal facets, and exactly similar in principle to an apparatus invented by man for increasing the diffusion of light.

The best known species of fire-fly, the cocuja of Spanish America and the West Indies, is an insect which resembles the common English black beetle in size, but it is dark-brown in color, and the divisions of its body are less deeply marked. The light is sufficiently strong to be of use to the inhabitants of the countries in which it is found. By inclosing three or four of the beetles in a glass bottle, a