he whole of the
arch; on the third the luminous part is much less and consists simply
of two small crescent-shaped markings, or rather two spots which shine
through to the back and are visible both above and below the animal.
Belts and spots emit a glorious white light, delicately tinged with
blue. The general lighting of the Glow-worm thus comprises two groups:
first, the wide belts of the two segments preceding the last; secondly,
the two spots of the final segments. The two belts, the exclusive
attribute of the marriageable female, are the parts richest in light:
to glorify her wedding, the future mother dons her brightest gauds; she
lights her two resplendent scarves. But, before that, from the time of
the hatching, she had only the modest rush-light of the stern. This
efflorescence of light is the equivalent of the final metamorphosis,
which is usually represented by the gift of wings and flight. Its
brilliance heralds the pairing-time. Wings and flight there will be
none: the female retains her humble larval form, but she kindles her
blazing beacon.

The male, on his side, is fully transformed, changes his shape,
acquires wings and wing-cases; nevertheless, like the female, he
possesses, from the time when he is hatched, the pale lamp of the end
segment. This luminous aspect of the stern is characteristic of the
entire Glow-worm tribe, independently of sex and season. It appears
upon the budding grub and continues throughout life unchanged. And we
must not forget to add that it is visible on the dorsal as well as on
the ventral surface, whereas the two large belts peculiar to the female
shine only under the abdomen.

My hand is not so steady nor my sight so good as once they were; but,
as far as they allow me, I consult anatomy for the structure of the
luminous organs. I take a scrap of the epidermis and manage to separate
pretty nearly half of one of the shining belts. I place my preparation
under the microscope. On the skin a sort of white-wash lies spread,
formed of a very fine, granular substance. This is certainly the
light-producing matter. To examine this white layer more closely is
beyond the power of my weary eyes. Just beside it is a curious
air-tube, whose short and remarkably wide stem branches suddenly into a
sort of bushy tuft of very delicate ramifications. These creep over the
luminous sheet, or even dip into it. That is all.

The luminescence, therefore, is controlled by the respiratory organs