FACTS AND THOUGHTS ABOUT LIGHT-EMITTING ANIMALS. 239 
male has them, and the elytra, in a rudimentary condition. Both 
are slightly luminous. The last metamorphosis develops the per- 
fect males and females, the last being apterous, the former being 
able to fly. Both, and not only the females, as has been popu- 
larly believed, are light-emitting, but the lady has greatly the 
advantage in brilliancy and in the extent of her photogenic 
apparatus. In her (fig. 7) they consist of six separate thin sacs 
of a white colour, each one occupying most of the width of the 
underside of a segment of the body. They are situate imme- 
diately beneath the skin of the ventral surface of the three 
segments which precede the last but one ; and in the male they are 
on the penultimate and antepenultimate segments only (fig. 8). 
In the female the sacs on the fourth and fifth segments from the 
end are rectangular and large and the others are smaller. A 
thin expanse of the common soft integument covers them, and 
they are in contact with the last two nervous ganglia, many 
large air-tubes, and, in the female, with the sexual organs. 
They are exposed and hidden by the expansion and contraction 
of the abdomen, and their light is visible under the first condi- 
tion ; but when in full vigour, the luminous appearance may 
diminish, but not be quite lost under the second. This has some- 
thing to do with the glowing. In all the grades of develop- 
ment the sacs are more worthy of the name of layers or laminae, 
and they consist of a mass of large cells with nuclei, and re- 
fractive granules. These are aggregated without order, in the 
larva, and covered with an investing tissue, in which tracheae 
(air-tubes) and minute nerves ramify, the tracheae entering 
within and coming in contact with the cells joining on to their 
walls (fig. 10). In the female, the lamina is made up of a number 
of these cell-aggregates or organs, and there is a yellowish tinge in 
the part nearest the outer skin, and the back part is crowded 
with the refractive granules, and has a white and opaque tint. 
It is said, and one would like it more satisfactorily proved, that 
the refractive granules contain uric acid ; and, on the other hand, 
it is by no means certain that the whole is not closely allied 
to that very recondite and unstable organic compound, wax. 
Many entomologists are disposed to connect these highly 
fatty, light-emitting organs, so well provided with air- tubes 
and nerves, and so close to those organs where the most rapid 
structural changes progress in some periods of insect life, with the 
great mass of body and inter-muscle fat. This fat, however, 
diminishes with the advance of the sexual organs, and we know 
that in some insects a positive development of immature young 
takes place in it ; but the luminous organ is present in the 
larva, and is most developed in the perfect state. Hence more 
knowledge is required before these views can receive universal 
acknowledgment. 
