34 A PHYSICAL STUDY OF THE FIREFLY. 
everything going smoothly, variations would suddenly be introduced. For 
example, the two couples would be brought close together, so as to touch a 
single segment, luminous or dark, when no effect would be observed—the 
reader at the galvanometer not being aware of the change. 
Turning the insect on its back and proceeding in the above-described 
manner, it was found that the region above the luminous segments was 
hotter than the region of the dark segments, just as was observed for the 
ventral side of the insect; but the difference in temperature between the 
region of the luminous segments and the dark segments was Jess than when 
measured on the ventral side. 
Placing one junction on a luminous segment and the other junction on the 
dorsal part of the same segment (dorsal-ventral temperature) the measure- 
ments were subject to greater fluctuations, hence not very conclusive. 
There was some evidence that the region directly over the luminous organs 
is hotter than the dorsal part of the same segment. ‘This test was applied 
only to the Photinus pyralis, the Photuris being too active for any measure- 
ments of this kind. In one very concordant series of measurements on the 
Photinus, of the difference in temperature between the dark and the lumi- 
nous segments, it was observed that on the dorsal side the temperature dif- 
ference was only about half as great as on the ventral side, where the lumi- 
nous organs are close to the surface. Comparing different species, it was 
found that the Photinus pyralis seemed hotter (the temperature difference 
between the bright and dark segments) than the Photuris pennsylvanica. 
Dry specimens had no effect; but in dead specimens in which the lumi- 
nous organs had begun to glow (putrefaction) there appeared to be a small 
heating. A freshly killed specimen seemed to come to room temperature 
(no cooling) very quickly; but the glowing luminous segments were always 
hotter than the dark segments. After a time this heating effect decreased, 
but on stimulating the luminous organs to a bright glow the usual (“‘off the 
scale”) heating effect became more nearly normal. 
In the female Photinus pyralis the luminous organ is a small circular spot 
on the third abdominal segment from the end (see Fig. 1). On placing one 
junction on the luminous spot and the other junction on the second segment 
from the end, the galvanometer showed that the luminous spot was much 
hotter than the other segments. In fact, in the first trial, the indication 
was so decided that the galvanometer reading was thrown entirely off the 
scale, requiring considerable time to bring it back into adjustment. 
Both the male and female of Photinus pyralis have two small glowing 
luminous points, Fig. 1, on the next to the last abdominal segment. ‘The 
thermo-couple test on one of these glowing points, on the female just men- 
tioned, indicated a higher temperature than the adjacent dark segment. 
The complete thermo-couple test comprised four distinct series of meas- 
urements made on as many different days, those on the Photinus pyralis 
being made some 26 days later than the measurements on the Photuris. Of 
the Photuris, two males and three females, and of the Photinus pyralis, eight 
males and one female were subjected to thorough tests. Numerous other 
specimens were discarded before completing a test, because of accidents or 
inactivity. 
These experiments are described in such detail because of their novelty, 
