GLOW-FLIES. 
107 
read without moving it. The under part of the thorax 
has a singular appearance when the tubercles are fully 
lighted up ; for the horny coat of skin being some- 
what pellucid, displays the light within redly and 
dimly, as if the whole thorax were red-hot, particu- 
larly at the edges, immediately beneath the tubercles. 
When left alone, the insect soon relapses into still- 
ness, and the tubercles presently fade into darkness, 
either total, or redeemed only by a spark scarcely 
perceptible. 
I had been familiar with this Glow-fly for some 
weeks, and had made the above observations on it, 
without being aware that it possessed any other 
source of light than the thoracic tubercles. I had 
indeed remarked that when flying at liberty the light 
which it diffused was of a rich ruddy glow, and yet 
these individual insects, if captured and held in the 
hand, showed only green light. I much wondered at 
this, but knew not how to account for it, until a 
friend explained it, illustrating his remarks by expe- 
riment. On the ventral surface, when the abdomen is 
extended, there is seen, between its first segment and 
the metathorax, an oval transverse space, covered 
with thin membrane, which glows with orange- 
coloured light ; totally concealed, however, when the 
abdomen is relaxed, by the overlapping of the meta- 
thorax. When the insect is placed on its back it 
throws itself into the air like other Elaters ; but if it 
be made to repeat this many times it appears to be- 
come weary, and endeavours to raise itself by bending 
the head and the abdomen back, so as to rest on the 
extremities, in hope to roll over. It is when thus 
