LETTERS ON ENT03IOLOGY. 141 
ignitus, proceeds from masses of a substance not 
generally differing, except in its yellow colour, 
from the rest of the body, closely applied under 
those transparent parts of the skin where the 
light is seen. In the glow-worm, besides the 
last-mentioned substance (which, when the sea- 
son for giving light is passed, is absorbed and 
replaced by the common substance), Mr. Ma- 
cartney observed, on the inner side of the last 
abdominal segment, two small oval sacs, formed 
of an elastic spirally wound fibre, similar to that 
of the tracheae, containing a soft yellow substance 
of a closer texture, and affording a more brilliant 
light, less under the control of the insect than 
the other luminous parts, which it has the power 
of voluntarily extinguishing, not by retraction 
under a membrane, as Carradori imagined, but 
by some inscrutable change dependent upon its 
will. There have been various and contradictory 
opinions upon the immediate cause of this lu- 
minous property, and many experiments made 
with very opposite results ; so we must conclude 
it uncertain, and be content with admiring the 
effect. As I have nothing more to say on this 
subject, my dear friend, I will conclude for the 
present ; but it will not be long before I write 
again. 
