ON THE LIGHT ORGAN OF FIRE=FLIES. 
By Mrs. A. B. Howard. 
( Read before the Transvaal Biological Society.) 
Some of the most interesting problems connected with the light of 
the hre-fLy belong to physics and chemistry rather than to biology. 
These are the problems connected with the nature and source of the 
light. The physical properties have been studied by Dubois, Langley, 
Very, Young, Watose, and others, with essentially the same results. 
The spectrum of the light given by the Lampyridae is perfectly 
continuous, without any trace of lines, either dark or bright. It 
lies almost wholly within that portion of the spectrum which is 
visible to the human eye, although the presence of actinic rays lias 
been demonstrated by photography. 
The most remarkable fact about the light is that it is practically, 
if not absolutely, a “ light without heat ’ 9 . In 1902 the late Professor 
Langley, of Washington, told me that after thorough experiment 
with the most delicate thermal apparatus known to science he could 
authorize the statement that not more than one-thousandth, if any at 
all, of the energy expended in the flash of the fire-fly is lost in heat. 
When one considers that with our ordinary artificial lights less than 
one per cent, of the energy is sometimes available for light, and even 
with the electric arc only ten per cent., the wonderful economy of this 
“ cheapest form of light ” becomes very apparent. 
Were it not for the small size of the light organs, and the minute 
quantity of photogenic substance obtainable for analysis, doubtless 
more would long since have been done by the chemist toward 
determining its nature. If this substance could be analysed, pro- 
duced artifically, and used for lighting purposes conveniently, its 
economic importance could scarcely be estimated. 
While organic light has always been a source of interest to 
scientists, little has been done beyond the investigation of its physical 
properties. The generally accepted theory as to the origin of the light 
is that it results from the oxidation of some substance secreted by the 
cells of the light organ. The purpose of my investigation, begun at 
Cornell University in June, 1901, has been to study the structure of 
the light organs with the hope that the knowledge gained would be 
of value in determining the nature of the photogenic process. In 
other words, I wished to see whether the tracheal structures were such 
as to presuppose some special demand for oxygen in that portion of the 
body. 
In spite of the number of species and wide distribuiton of fire-flies 
in America, I found that practically no histological work had ever 
been attempted. The very limited literature on the entomological 
side of the question was in German and French, and was confined to 
a very few writers. Comparisons of structure were, therefore, possible 
