1884] . 
The Light of Fire Flies. 
595 
VII. THE LIGHT OF FIRE-FLIES. 
S HE light emitted by luminous inserts has often been 
the subject of observation and experiment. Recently 
MM. Aubert and Dubois have obtained some highly 
interesting results in this direction with a Pyrophorus, which 
arrived in a living state at Havre in a cargo of wood. 
The author first submitted the light of the insect to a 
spectroscopic examination. 
The spectroscope used was an ordinary one with a flint- 
glass prism of high refractive power and with a micrometer. 
The insect was fixed opposite the slit, which was illuminated 
by one of the luminous organs of the prothorax. It is, of 
course, well known to many of our readers that the Pyro- 
phori have three light-organs ; one on the ventral side and 
the two others on the upper part of the prothorax. The 
latter, which are always visible, have been used in the 
experiments in question. The light which they throw off 
takes a divergent direction to each side of the animal, so 
that one and the same point cannot be simultaneously 
illuminated by both organs. Only one of them was, there- 
fore, utilised. The surface to be illuminated was placed 
perpendicularly to the principal direction of the rays, which 
make an angle of about 45° with the plane of symmetry of 
the insect. 
The spectrum of the light is very beautiful, but con- 
tinuous, having neither dark nor brilliant rays. 
The spectrum occupied about 75 divisions of the micro- 
meter, extending on the red side up to the middle of the 
interval which separates the rays A and B in the solar 
spectrum. On the blue side it reaches a little beyond the 
ray F. 
When the intensity of the light varies its composition 
changes also in a remarkable manner. When the bright- 
ness decreases the red and the range disappear completely, 
and the spectrum consists merely of the green with a little 
yellow and blue. The green rays remain longest. The 
reverse takes place when the insect begins to emit light. 
The green appears first ; then the spectrum extends a little 
on the blue side and considerably towards the red. The 
least refrangible rays are, therefore, the last to be emitted. 
No other source of light is known to behave in a similar 
manner. 
