M r • Macartney's Observations 
common interstitial substance, after the season for giving light 
is past. 
The segments of the abdomen, behind which this peculiar 
substance is situated, are thin and transparent, in order to 
expose the internal illumination. 
The number of luminous rings varies in different species of 
lampyris, and as it would seem at different periods in the 
same individual. 
Besides the luminous substance above described, I have 
discovered in the common glow worm, on the inner side of 
the last abdominal ring, two bodies, which to the naked eye 
appear more minute than the head of the smallest pin. They 
are lodged in two slight depressions, formed in the shell of 
the ring, which is at these points particularly transparent. On 
examining these bodies under the microscope, I found that 
they were sacs containing a soft yellow substance, of a more 
close and homogeneous texture, than that which lines the 
inner surface of the rings. The membrane forming the sacs, 
appeared to be of two layers, each of which is composed by a 
transparent silvery fibre, in the same manner as the internal 
membrane of the respiratory tubes of insects, except that in 
this case, the fibre passes in a spiral, instead of a circular di- 
rection. This membrane, although so delicately constructed, 
is so elastic as to preserve its form, after the sac is ruptured, 
and the contents discharged. 
The light that proceeds from these sacs, is less under the con- 
troul of the insect, than that of the luminous substance spread 
on the rings : it is rarely ever entirely extinguished in the season 
that the glow worm gives light, even during the day ; and when 
all the other rings are dark, these sacs often shine brightly. 
