4 56 THE POPULAR SCIENCE MONTHLY. 



numbers of these organisms are present in the sea, the water, especial- 

 ly 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 con- 

 tain 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 phos- 

 phorescence 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 Lampyridoe and the Elaterklce. We may take the com- 

 mon English glow-worm as a type of the former, and the famous fire- 

 flies, 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 simi- 

 lar. 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 sub- 

 stance of the insect. A multitude of minute air-tubes surround and 

 ramify through the sacs, terminating in a larger tube and a spiracu- 

 lum, 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 glow- 

 worm's lamp by the power which insects enjoy of closing their spira- 

 cula at will. But the Lampyris can in reality only partially extin- 

 guish 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 w^hich 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 diftusion 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 Eng- 

 lish black beetle in size, but it is dark-brown in color, and the divis- 

 ions of its body are less deeply marked. The light is sufiiciently 

 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 



